Study of the Impact of Indoor Environmental Quality in Romanian Schools through an Extensive Experimental Campaign

Abstract

Decentralized ventilation systems in schools are becoming more important due to the focus on indoor air quality and energy economy. The research aims to explore how these technologies affect classroom air quality, thermal comfort, and noise. The study examined four decentralized ventilation systems in a real-world school using field measurements and data analysis. This included measuring the CO₂, temperature, noise, and thermal comfort using the Predicted Mean Vote (PMV) index. All systems greatly improved the air quality, keeping CO₂ levels within suggested limits. They failed to control indoor humidity, often lowering it to below optimal levels. Noise surpassed the 35 dB(A) criteria at maximum operation but was acceptable at lower airflows. Noise and air drafts did not bother residents. The study found that decentralized ventilation systems improve air quality and are easy to adapt to, although they need humidity control and noise management at higher operational levels.

1. Introduction

In the last decades, many studies have been performed to investigate the indoor environmental quality (IEQ) in educational buildings, such as primary and secondary schools or even colleges and universities. All of these studies outlined the importance of a good IEQ, in order to protect pupil’s health and academic performance, knowing the fact that children spend 8 h per day or more during workdays in these types of buildings.

The idea of Indoor Environmental Quality (IEQ), as described in the scientific literature for various indoor environments, becomes more significant when considering schools due to evidence showing that children are more susceptible than adults to indoor conditions [1,2]. Some authors linked the IEQ concept to the TAIL concept, meaning the following: T—Thermal comfort, A—Acoustics, I—Indoor air quality, and L—Lighting [3,4], the four main criteria that influence the health and wellbeing of classroom pupils and educational staff. For the design stage of the HVAC systems for all buildings, including the educational ones, the indoor environmental parameters, with respect to the TAIL concept, are defined in the European standard EN 16798-1 [5].

Catalina et al. [6] investigated the IEQ concept in eight Romanian schools, five located in urban regions and three in rural ones, by means of extended surveys among pupils and academic staff. It was shown that the proximity to intense road traffic led to increased indoor air concentrations of carbon dioxide (CO₂) and suspended particles (PMs) in the classrooms from urban areas, compared to the schools located in a rural environment.

2. Literature Review

In relation to the assessment of indoor pollutant levels in schools, there is still significant variability among the available studies. However, Tran et al. [2] recently conducted a comprehensive review to assist future researchers in developing a standardized rating system and measurement protocols for the consistent evaluation of indoor environmental quality (IEQ) in educational facilities.

While the thermal comfort in schools has been investigated frequently by using the well-known PMV-PPD model proposed by Fanger in the early 80s, the indoor air quality in schools concept remains a challenge today, due to different influencing factors, such as the type of indoor pollutants to be investigated, the school location (in a rural or urban environment), the most appropriate ventilation system, or the local climate.

Galicic et al. [7] investigated the indoor air quality factors and natural ventilation in 454 primary Slovenian schools, by means of extended surveys, performed for one winter month (February 2020). The authors outlined the importance of the school micro location, such as the proximity to main roads or other polluting sources (industrial areas), on the IAQ level in classrooms.

Concerning the importance of the natural ventilation, it has been shown that the lack of outdoor filtration and the strong dependence of the ventilation airflow on the meteorological factors (wind speed and direction and outdoor temperatures) make this system unreliable, and a mechanical ventilation system should be mandatory.

Regarding the indoor air pollutants in educational buildings, the majority of scientists assumed that some of them could be considered less harmful than others. While the carbon dioxide (CO₂) is considered a marker of human presence and a maximal indoor average concentration of 1000 ppm could be considered a threshold for “un-ventilated” air [8,9,10], larger concentrations (around 2000 ppm), frequently seen in poorly ventilated spaces, including classrooms, lead to a decrease in the cognitive and academic performance of pupils, as shown in a recent study [11].

Apart from CO₂, there are many other indoor pollutants in schools that could be harmful at elevated concentrations and with long-term exposure, taking into account that pupils spend around 8 to 12 h in the indoor school environments. It has been shown that TVOC (Total Volatile Organic Compounds), a category of pollutants released indoors by building materials, furnishings, acrylic paints, permanent markers, and some cleaning products [12], have measured concentrations markedly elevated compared to those in the outdoor air and lead to severe adverse health effects among school children [13,14,15,16]. A toxic influence on pupils’ health is also present as PAHs (Polycyclic Aromatic Hydrocarbons), transported to the indoor school environments via ventilation airflow, when schools are in the proximity of heavy road traffic [17].

Concerning airborne suspended particles, already known in the scientific literature as Particulate Matters (PMs), their harmful effect on children’s health could be important, especially for the smaller ones (PM_2.5 and PM₁, with the down index corresponding to the largest particle diameter of this class, in μm), which could penetrate the inner parts of the pulmonary tissue after inhalation.

Consequently, numerous studies have examined the sources of particulate matter (PM) in schools, whether they originate from indoors or outdoors, and if they are influenced by factors, such as the school location and local climate (e.g., the built environment). These studies have also explored the harmful effects of PM on students’ health and proposed ventilation solutions to mitigate the concentration of PM in the classroom air [18,19,20,21] A recent systematic literature review on the IAQ related to aerosol dispersion in indoor spaces with a high occupancy degree (schools included) showed the necessity to establish a robust optimization tool to assess the correlation between several influencing factors, such as ventilation, infection risk, people’sbehavior, and design comfort parameters [22]. The authors of this research concluded that such an optimized tool is lacking at present and should be clearly defined based on a more holistic approach. Regarding the influence of the traffic air pollution nearby the schools, Gartland et al. [23] showed the major impact of elevated PMs (notably PM₁₀ and PM_2.5) and nitrogen oxide (NO_x) outdoor concentrations on the corresponding indoor air concentrations, which directly affect the executive function and academic performance of primary-school-aged children.

An important contribution to the ventilation strategies to be adopted for schools to reduce the risk of airborne viral infections was presented by Almaimani et al. [24], which searched an optimal ventilation strategy to minimize the air transmission of biological pathogens in classrooms, with a particular focus of the SARS-CoV-2 virus, responsible for the COVID-19 infection [25]. In their study, the authors demonstrated that a 2 ACH (air changes per hour) ventilation rate, combined with a 2-meter physical distance between two persons in the class should be an optimal strategy to reduce the POI (Probability Of Infection) with SARS-CoV-2, at a decent energy cost. The increase in the ventilation rate above this value was found to be more effective than the increase in the social distance, in terms of a POI reduction, but with higher cooling energy costs.

It appears obvious, as a result of numerous studies investigating the IAQ in schools, that ventilation is the main factor influencing the magnitude of indoor pollutant concentrations. Moreover, mechanical ventilation is much more appropriate to ensure a good IAQ (e.g., concentrations below threshold values given by reference regulations), because it does not depend on the variability of meteorological factors (wind and outdoor temperature) or on the occupants behavior (windows opening periods). Regarding the necessary ventilation airflow for the dilution of indoor pollutants, an interesting comparison between the values recommended by the American norm (ASHRAE standard 62.1 [26]) and European norm (EN standard 16798-1 [5]) was conducted by Kuramochi et al. [27]. The authors showed that, for a classroom with 10–12 years old pupils, the ventilation airflow recommended by the EN 16798-1, equal to 9.8 l/s*person, is larger than the corresponding airflow recommended by ASHRAE 62.1 (e.g., 6.71 l/s*person). Moreover, Kuramochi et al. [27], performed in this study, a meta-analysis judging the effect of ventilation on intellectual productivity in schools, based on a large survey with 3679 participants, by varying ventilation airflows. They demonstrated that an airflow of 10.7 l/s*person would be an ideal choice for an optimum IAQ in classrooms, when searching for high intellectual productivity of pupils. This value is closer to the ventilation airflow recommended by the European norm than to the corresponding value recommended by the American norm.

In the literature, many other studies relying on the IAQ in schools and ventilation strategies for improving IAQ have been performed. Jendrossek et al. [28] analyzed the influence of the ventilation solutions on the airborne risk of infection in schools. Calama-Gonzalez et al. [29] compared different ventilation scenarios (constant ventilation based on reference airflow by person, constant ventilation based on CO₂ concentration limitations, and demand-controlled ventilation), while Chang et al. [30] investigated the air-conditioning operation strategies for thermal comfort and IAQ in Taiwan’s elementary schools.

Not all the studies found in the literature lead to the same conclusions regarding the optimum ventilation strategy for a school building, due to different influencing factors, such as the building architecture and the corresponding constraints, the local climate, and the proximity to a polluted environment (road traffic, industry, or other). We can conclude that every case study implying an educational building needs a particular focus and a preliminary experimental campaign for assessing IAQ, which should be mandatory, in order to judge the optimal ventilation strategy, with respect to energy savings for the system that is possible to implement. Moreover, the building owner, in conjunction with the architect, should agree with the proposed solution, in order to preserve the shape and the unicity of the school. The best choice should be, therefore, the decentralized and compact ventilation systems rather than the centralized ones, which could affect the esthetical image of the building. Recent studies on the decentralized ventilation systems for classrooms could be found in the literature, the great majority of them presenting compact systems using air-to-air heat exchangers [31,32,33].

3. Indoor Air Quality Solutions

A decentralized ventilation system is one where individual ventilation units are placed in the rooms or areas to be ventilated, unlike a traditional ventilation system, which includes a centralized system from which air is distributed through ducts from a single central unit.

Decentralized ventilation systems require regular maintenance to achieve their efficiency and prevent the accumulation of dust and dirt in the ventilation units. The filter must be changed regularly. Additionally, they may not be as effective as centralized systems for ventilating large areas or very tall buildings. This constraint does not apply to our practical case because the classrooms are of a reasonable volume.

Decentralized ventilation systems can be controlled separately, depending on the room temperature, humidity level, or CO₂ concentration of the indoor air. They can be equipped with sensors that automatically adjust the ventilation rate based on these parameters.

Implementing single-flow controlled mechanical ventilation (CMV) enables the ventilation of a space while effectively addressing humidity levels. A single-flow CMV system can result in the wasteful consumption of energy due to the extraction vents creating negative pressure in the ventilated area. This negative pressure causes outside air to be drawn in, often through window grilles located at the top of the frame. The heating must then operate more to compensate for the intake of cold air into the ventilated space due to the infiltration of cold air, particularly in winter or in cold countries. These energy losses can lead to the overconsumption of around 20% of heating.

Decentralized double-flow ventilation keeps the same principle as classic double-flow CMV, except that the heat exchanger and the ventilation are in a single box, which is installed at the level of a facade through the wall or through from a window recess. Just like a classic double-flow CMV, the heat exchanger is equipped with filters to filter pollutants and allergens. It is therefore recommended to change these same filters regularly.

There are two principles of ventilation operation inside the decentralized ventilation box:

• The single fan alternates the direction of rotation in cycles of 50 to 120 s to reverse the flow, for extraction, then for blowing;
• The box is equipped with two fans and two air networks to ensure continuous ventilation.

Finally, a humidity-controlled CMV is a controlled mechanical ventilation solution that adjusts the air flow according to the relative humidity of the ambient air. Humidity-controlled ventilation systems are not suitable for use in schools for the main reason that the occupancy of the premises requires a constant minimum air flow to guarantee a suitable CO₂ level. This parameter is not linked to the relative humidity level in the air.

In Romania, there is NP 010–2022 (regulation on the design, construction, and operation of buildings for schools and high schools) [34] with a binding character, recently updated, which provides clarifications regarding the ventilation of spaces in educational buildings and the quality of indoor air in classrooms.

All occupied spaces in schools must be mechanically ventilated, locally or centrally, with ventilation systems equipped with heat recovery units that carry out the heat exchange between the exhausted air and the incoming air. In the classrooms, a flow of fresh air will be ensured according to the requirements of the technical Romanian norm I5 [35] to comply with the air quality category IDA1 (indoor air).

The incoming air is filtered with ePM efficiency filters in correlation with the ODA outdoor air quality class (minimum F7 to F9 is recommended).

It is recommended to use installations with variable air flow, which operate in a controlled manner according to the difference in the CO₂ concentration between indoor and outdoor air. Thus, a maximum CO₂ concentration difference of 400 ppm is allowed in classrooms.

Building materials and furniture items that do not contain or emit formaldehyde or other volatile organic compounds will be used. Radon from building materials and soil must not exceed the concentration of 200 Bq/m³ on average per year.

If the ventilation units are located directly in the classroom, then the project will be accompanied by an acoustic impact study. The air ducts used in common spaces are made of non-combustible materials.

For air distribution inside the rooms occupied by students, the ventilation system is used by mixing or by displacement, with air vents specific to each chosen ventilation system. Textile air ducts can also be used. The air vents are made so that the air speed in the occupied area does not exceed the limits indicated for the average air movement speeds in the rooms in the occupied area, correlated with the IDA 1 environment category.

It is recommended to respect the values of the air changes per hour depending on the destination of the room, for example for 6–8 h⁻¹, laboratories 8–10 h⁻¹, sport halls 2–3 h⁻¹, canteens, university restaurants 8–12 h⁻¹, kitchens 5–8 h⁻¹ (the air change rate is defined as the ratio between the total air flow rate introduced into the room and the air volume of the room).

In laboratories or other spaces where local exhaust devices are provided, the general ventilation system includes special measures to organize the introduction of compensation air.

In the case of centralized ventilation installations, it is recommended that the mechanical ventilation installation be made so that it can also be used to exhaust smoke and hot gases in the case of fire.

Inside the occupied spaces, the principles of user comfort are respected according to EN 16798-1/2019 [5], technical regulations I5 [36] and I 13 [37]. In the classrooms, at least the criteria of the ambience category II (IEQ2), in terms of thermal and acoustic comfort, and the criteria of the air quality category IDA1 are respected. Ambient category I (IEQ I) and air quality category IDA1 are recommended for these.

The indoor air temperature is set according to the purpose of the rooms, in the warm season (classrooms, laboratories 23–25 °C, libraries 24–27 °C, dining areas 23–27 °C, gyms 20–26 °C) and in the cold season (classrooms, laboratories 18 °C, libraries 20 °C, dining areas 18 °C, gyms 18 °C). For cooling, the indoor air temperature is chosen according to the specified values, provided that the difference between the outdoor and indoor temperature does not exceed 10 °C.

Regardless of the heating source, the maximum allowable temperature of the inlet pipe will be 70 °C. In situations where this requirement cannot be ensured, the heaters will be provided with protective grills to avoid accidents.

From the point of view of energy performance, whether they are newly built or rehabilitated buildings, these buildings must have an energy consumption almost equal to zero, by creating a suitable envelope, by providing high-performance technical systems, and by covering the energy requirement with energy from renewable sources in a proportion of at least 30%.

In

Table 1. Comparison of different ventilation solutions.

Ventilation System	Single-Flow CMV	Decentralized Double-Flow CMV	Classic Double-Flow Ventilation
Advantages	Very affordable price, between 400 and 800 Euros. Simple to install and maintain, can be carried out by an individual. Very affordable price, between 400 and 800 Euros.	Appropriation of the machine locally by class users, mainly teachers. Allows equipment to be managed separately, class by class, according to the specific needs of each. This modulation allows for significant energy savings. Allows for investment room by room. Renovation projects or projects with small budgets will therefore be able to benefit more easily.	These are technologies that are already a few years old. Maintenance work is centralized in a single location, at the heart of the installation. Purifies the air.
Disadvantages	The system creates a depression in the room, causing cold air to enter and excess heating consumption.	Maintenance must be carried out on each piece of equipment. It is still a very recent technology, making the question of repairability difficult to assess. Requires multiple openings in the facade to supply the groups.	These are complex installations that require careful management. Schools often call on outside companies for maintenance because they do not want to take on this responsibility, which represents significant costs. The air duct system requires a lot of space. Groups and ducts that are located outdoors quickly perish due to bad weather. Higher consumption than simple CMV due to the motors. Possibility of noise in the ducts.
	Cold drafts in rooms.

, a short comparison between these systems is presented.

The study team proceeded on a comprehensive investigation to investigate several forms of decentralized ventilation systems, acknowledging the current lack of sufficient studies in this area. The unique aspect of our research paper is its examination of different decentralized ventilation models through a comparative analysis. This particular aspect has not been widely investigated in previous scholarly works, thus making a valuable and original contribution to the area.

4. Methodology

To provide a comprehensive comparison of ventilation systems according to the aforementioned quality standards, it was necessary to devise a standardized process that ensures uniformity across all systems. This protocol was implemented to obtain the most precise and reliable data for analysis. The ventilation systems, specifically System#1, System#2, System#3, and System#4, underwent testing in a primry school located in Bucharest. The installation of all equipment took place on the ground floor of the building, namely in classrooms that have comparable capacities. A multitude of sensors was deployed within these rooms. In order to conduct data analysis, our attention will be directed towards the sensors located in the central area of each classroom, specifically in close proximity to the video projector. Other CO₂ sensors were placed on the walls and in the back of the room at normal height (approx. 1.5 m). For the measurements, the ARANET CO₂ monitors have been used as an advanced and accurate device specifically engineered for the purpose of detecting carbon dioxide concentrations in diverse settings. The device has a large measuring range ranging from 0 to 9999 ppm, rendering it appropriate for a diverse range of applications, encompassing educational settings, as well as industrial situations. The monitor’s elevated resolution of 1 part per million (ppm) enables the precise and comprehensive monitoring of carbon dioxide (CO₂) levels. The device exhibits a level of precision with regards to accuracy, adhering to a standard of ±30 ppm or ±3% of the measured value, thereby guaranteeing the dependability of the collected data. The time constant τ, which represents 63% of the total response time, is set at 100 s. This number represents the sensor’s capacity to rapidly detect and communicate changes in CO₂ levels, hence guaranteeing timely and precise information about air quality. Furthermore, the ARANET CO₂ monitor is equipped with the ability to measure temperature, including a range from 0 to 50 °C and boasting a precise resolution of 0.1 °C. The temperature readings exhibit a high level of precision, with a margin of error of ±0.3 °C. The device is capable of quantifying relative humidity within a specified range of 0–85%. It achieves this with a resolution of 1% and an accuracy of ±3%. To evaluate the efficacy of the equipment, an additional sensor was set up within a classroom without ventilation, alongside an external sensor to quantify the ambient air quality surrounding the building. The present study will examine the variations in humidity, temperature, and CO₂ levels over a duration of one month. This period coincides with the presence of children in the classroom for five consecutive days each week, while they remain at rest during the weekends. We shall do a comparative analysis of the data both internally and in relation to the external sensor.

To assess the level of comfort in a room and consequently the level of contentment among occupants, a subsequent set of measurements was carried out on 15 February 2023 in the morning. This was performed in four ventilated rooms utilizing a Testo440 sensor along with its corresponding probes. Additionally, this sensor was capable of measuring several environmental parameters, such as temperature, air velocity, relative humidity, and concentrations of carbon dioxide (CO₂). One advantage of this approach is the availability of data regarding the number of students in each room and their respective activities. This information enables us to make more accurate estimations of the carbon dioxide (CO₂) levels in the air, considering the influence of human occupancy.

5. Case Study: Cuza School Pilot School

The school building is in Bucharestand has a height regime of Basement + Ground floor + 2 Floors. From the information received from the manager of the building, it appears that it was built in 1967, the project being one that was often replicated during that communist period. The building has a rectangular shape with sides of 18.80 m and 45.50 m, respectively, and a footprint of approximately 765 m². The level heights are as follows: 1.80 m for the technical basement (partially) and 3.40 m for the ground floor, the 1st floor, and the 2nd floor, while the constructed area is 2435 m².

From a functional point of view, the building includes 17 classrooms, laboratories for chemistry, biology and informatics, chancellery, library, bathrooms, administrative spaces (director’s office), a medical office, technical spaces, annexes, and storages, and it is considered as a typical school for Romania.

The structure of the outer envelope is made of solid brick walls (fired clay elements) with a thickness of 37.5 cm. Inside the building are structural brick walls 25 cm thick. The exterior PVC joinery and double glazing is partially degraded. The building has a gable roof with a wooden frame. The construction was designed according to the seismic design norm P13 from 1963.

The building is heated by steel radiators, a panel type, mounted on the walls with the help of support brackets and protected with masks to prevent injury to children by touching hot surfaces. The radiators are connected to the cogeneration system of the city of Bucharest, operating with a thermal agent with parameters of 80/60 °C, in quantitative regulation according to the outside temperature. The distribution of the thermal agent for heating and hot water for consumption is performed through the technical basement of the building and through columns to the floors.

The lighting of the classrooms is performed with lighting fixtures equipped with compact fluorescent lamps, depending on the purpose of the rooms.

The building is not equipped with mechanical ventilation and air cooling systems; thus, we expected high indoor pollution.

In the period 2010–2011, thermal rehabilitation works took place with the aim of increasing the energy performance of the educational unit, respectively reducing the energy consumption for space heating, under the conditions of ensuring and maintaining indoor thermal comfort. Figure 1 illustrates the exterior façade and a typical classroom from this school.

The thermal rehabilitation works consisted of the following:

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Thermal insulation of the external walls with fireproof expanded polystyrene of a 10 cm thickness.

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Replacement of the pipes for the distribution of the thermal heating agent with polypropylene pipes, in the areas where failures have occurred.

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Restoring the thermal insulation of the distribution pipes in the basement or replacing them entirely.

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All the sanitary groups were equipped with exhaust fans, and to avoid overpressure, transfer grids are installed in the doors; the exhaust air flow from each compartment of the sanitary group is 100–120 m³/h.

The first ventilation system was installed in Room (1) and it is heat recovery ventilation equipment that is usually mounted in a false ceiling or in a space adjacent to the classroom, thus allowing the floor to remain free; the module is connected to the piping necessary for the supply of fresh air and the exhaust of stale air to the outside, but also to the network of ventilation ducts for the supply/exhaust of air from the classroom.

It is also used in the free-cooling version when the outside temperature is lower than the inside temperature (for example, during the night). It can be used as an individual ventilation unit or can be integrated into air conditioning systems (air-refrigerant type). There is a varied range of units, for an air flow between 150 and 2000 m³/h, but for this specific application, the model with 800 m³/h was selected.

Medium and fine dust filters (M6, F7, F8) are optional for this system, to meet customer requirements or applicable legislation. The installation time was short due to the easy adjustment of the nominal air flow—in our case a 1 day installation; therefore, fewer air dampers are required than in the case of traditional installations. It can work in conditions of overpressure and depression.

The system is installed with two filters, G3 + F7, with automatic switching of heat-recovery units to a free cooling mode in the event of high heat, and has a cost of 3400 €/unit (without VAT) + €147 for filters and has operating temperatures from −15 to 50 °C. The introduction of fresh air was performed using textile piping (see Figure 2), while the extraction was performed using steel classic ventilation ducts.

The second system (see Figure 3) is also a decentralized ventilation unit with a high heat-recovery efficiency (up to 93% according to technical specifications), low noise levels, low installed power consumption, and minimal installation requirements. The equipment is equipped with an aMotion control module to operate all the necessary functions and consists of flexibly mounted inlet and suction fans, a countercurrent heat-recovery exchanger, a sliding supply air filter, a bypass flap for the heat exchanger, autonomously operated closing flaps, and a control box. The condensation tray (no drain) is heated using an electric cell with an automatic switching function. The upper section is equipped with noise attenuators with separation, grilles for the exhaust and intake of air, a filter for the extracted air, and an external CO₂ concentration sensor. The lower part of the unit has a spacer frame made of anti-vibration rubber. This system was also installed in one day and was connected to Wifi for control. Compared to the previous solution, it has the disadvantage that it is occupying around 1 m² in the back of the room, but on the other hand, it is well fixed to the wall and in case of earthquakes (Bucharest has a high risk), the system is secured with no danger for the children.

The third system is a decentralized mechanical ventilation system consisting of two HRV ventilation units and one ERV ventilation unit installed in a classroom number (3)—see Figure 4. The systems are smaller and are decentralized ventilation systems, including a heat-recovery system from the exhausted air, thus reducing energy consumption for heating or cooling the outside air. Integrated CO₂ sensors enable the continuous monitoring of carbon dioxide concentrations in the classroom. When the CO₂ concentration exceeds a preset level, the ventilation system automatically activates, ensuring a constant supply of fresh air. The outside air introduced is preheated by means of an electric resistance and reheated with the help of a battery fed with hot thermal water (supplied by the heating network of the city of Bucharest). The systems not only allow for the introduction of fresh filtered air but also act as radiators for the heating. Clearly, its main advantages is the fact that it does not reduce the area and is also fixed to the wall, making it safe.

While two HRV units were placed left and right of the room, in the middle, the ERV unit was installed and equipped with an advanced energy-recovery system, known as enthalpy recovery, which allows for the transfer of not only heat, but also moisture between the exhaust air and the air introduced into the classroom. Through this process, the ERV Decentralized Ventilation System ensures more the efficient regulation of humidity inside the classroom. Thus, the state of thermal comfort is improved, not only by maintaining a constant temperature, but also by controlling the humidity, which has a positive impact on the performance of the occupants. Also, proper humidity management can prevent indoor air quality problems, such as the growth of mold and other microorganisms that prefer moist environments.

The installed ventilation equipment has a nominal air flow of 250 m³/h/unit, so 750 m³/h total, a declared noise level 32.6 dB, an electric battery for frost protection with a power of 0.54 kW, a reheating battery that works with hot water, a heat recovery in countercurrent type HRV or type ERV, depending on the model, and two centrifugal fans.

Moving to the last study system, called System #4, it is a decentralized ventilation system with heat recovery 700 m³/h and is specially designed for classrooms (see Figure 5).

The plastic heat exchanger, with countercurrent hexagonal plates, guarantees, according to the manufacturer, a thermal efficiency of up to 90%. The fans are self-powered with EC motors that save electricity and operate silently. The built-in carbon dioxide (CO₂) sensor can automatically control the unit; when a set CO₂ level is reached in a classroom, the device shuts down. The metal casing contains double walls and insulating material in the space of 25 mm between the panels, which ensures both a high thermal performance and low noise levels. There are also special silencers on both the intake and supply ports to ensure a quiet environment in classrooms.

The ventilation unit uses the ModBus protocol to connect to and communicate with the building management system and to report any malfunctions or periodic maintenance. The ventilation system can be monitored and controlled using a computer or a central system. The unit uses three filters, and the air is filtered both at the inlet and outlet (M5 filter for dust and other pollutants, G4 and F7 filters for particles up to 1 micron and allergens, spores, and pollen). In addition, when the filters are loaded with a certain number of particles, the system issues a warning.

6. Results and Analysis

The graph below shows the evolution of CO₂ levels and temperature in the classroom studied ⑤ without a ventilation system. There are wide variations in CO₂ levels over the course of the day, generally fluctuating between 750 and 1500 ppm, with a peak between 7 and 10 a.m., at above 1500 ppm. As a reminder, the ideal CO₂ level in a room is recommended to be less than 1000 ppm. The levels on this day are not satisfactory and show the need for a ventilation system to evacuate CO₂ air to the outside.

The level of CO₂ in the air can also be used to assess classroom activity. Indeed, we know that CO₂ is released by the people in the room as they breathe. Therefore, the more people there are in a room and the greater their physical activity, the faster the CO₂ levels in the air will rise, saturating the air with CO₂.

As can be seen from Figure 6, the CO₂ levels reach alarming values of more than 3000 ppm, while the air temperature, due to high internal gains, goes up to even 26 °C. We remind the reader that the measurements took place in the middle of March 2023. For the entire set of measurements from 11 March 2023 to 11 April 2023, the average temperature was 23 °C, with a maximum of 26 °C and a minimum of 13.3 °C (during the nighttime on the weekend). For the CO₂, the data showed maximum values of even 7036 ppm (20 March 2023 at 9:23 a.m.) while the average (including nighttime) was 845 ppm.

The data shown in the graph (see Figure 6), which illustrate the change in carbon dioxide (CO₂) levels and temperature within a classroom lacking a ventilation system, provide compelling evidence to support the necessity of conducting thorough investigations regarding the implementation of ventilation in educational settings. The variations in observed carbon dioxide (CO₂) levels, namely the peaks that reach, in most of the cases, 3000 parts per million (ppm), greatly exceed the recommended maximum threshold of 1000 ppm. This divergence not only signifies inadequate air quality but also emphasizes the potential health and cognitive consequences for individuals in educational settings. The presence of values over 3000 ppm, occasionally peaking at 7036 ppm, suggests an environment that has the potential to significantly impede cognitive functions and impair learning efficiency. This is particularly worrisome considering that these measures were obtained in a conventional educational environment during standard school hours.

Elevated temperatures, particularly when combined with elevated amounts of carbon dioxide, have the potential to induce discomfort, impair concentration, and ultimately diminish academic performance. The presence of these conditions in March, a month often associated with lower temperatures, suggests that the severity of the situation may be significantly amplified during periods of higher temperatures.

This highlights the pressing necessity for the implementation of efficient ventilation systems in educational environments, with the aim of guaranteeing the comfort, health, and cognitive well-being of all present. The research presented in this study makes a substantial contribution to the ongoing academic discussion surrounding indoor environmental quality in educational settings, as in this paper, we are focusing on possible solutions.

The study of four distinct ventilation systems inside a classroom environment has yielded noteworthy findings about their efficacy in preserving indoor air quality and controlling the temperature (see Figure 7).

System 1 has shown remarkable efficacy by consistently upholding CO₂ concentrations below 1000 ppm for 97% of the duration, thereby serving as a robust metric for assessing favorable indoor air quality. Nevertheless, it is important to highlight that the temperature within the room increased to 27 °C, which is marginally higher compared to that with the other systems. Nevertheless, the system’s capacity to reliably control the concentration of CO₂ highlights its efficacy.

The findings of System 2 demonstrated even more noteworthy outcomes, as it successfully maintained carbon dioxide (CO₂) levels below 1000 parts per million (ppm) for 99% of the duration of occupancy. This outcome serves as a strong indicator of the system’s exceptional control of air quality. The temperature within the enclosed space reached a maximum of 26 °C, a value that falls within the allowed range for creating appropriate educational settings.

The third system, despite maintaining air quality levels below 1000 ppm for 65% of the time, did not employ its boost mode. The utilization of this function during periods of rest has the potential to significantly improve its effectiveness of removing pollutants. Significantly, this classroom, characterized by the presence of two external walls, showed a lower maximum temperature of 25.2 °C in contrast to the preceding two systems.

System 4 consistently demonstrated exceptional air quality, as seen by its ability to consistently control carbon dioxide (CO₂) levels below 1000 parts per million (ppm) during its operation. The classroom, experiencing reduced solar heat gain, had a temperature of 25.3 °C, which is beneficial for creating a comfortable learning environment.

In general, it can be observed that all four systems demonstrated satisfactory performance by successfully delivering fresh air and ensuring the maintenance of suitable indoor conditions. The notable disparity in air quality and temperature regulation between the ventilated and non-ventilated classroom serves to emphasize the effectiveness of these systems and emphasizes the pressing necessity for their implementation in educational settings. The present investigation not only showcases the efficacy of these systems in upholding ideal learning settings but also provides significant data to the domain of indoor environmental quality in educational structures.

The analysis of carbon dioxide (CO₂) concentrations in different ventilation systems, in comparison to a situation without any ventilation, provides valuable findings regarding their efficacy in controlling indoor air quality (see Figure 8). Without adequate ventilation, carbon dioxide (CO₂) levels experience substantial variations, ranging from a minimum of 440 parts per million (ppm) during nighttime to a maximum of 7036 ppm. Consequently, the average CO₂ concentration calculated only for the occupation period (8:00–18:00) in this situation amounts to 1296 ppm.

System 1 demonstrates a notable enhancement with a more favorable concentration of 697 ppm (46% reduction compared to with no ventilation). System 2 demonstrates superior performance compared to the other systems, as it effectively achieves a harmonious equilibrium between the lowest maximum concentration of 1495 ppm and an appropriate average concentration of 636 ppm (50.9% reduction compared to with no ventilation). Moreover, System 2 constantly maintains outstanding air quality throughout its operation. System 3, however, is less efficient than System 2 and demonstrates a notable reduction in the maximum concentration of CO₂ to 2072 ppm and maintains an average level of 753 ppm (41.8% reduction compared to with no ventilation). Finally, System 4 demonstrates strong performance by maintaining a maximum carbon dioxide (CO₂) level of 3234 parts per million (ppm) (the system shut down at that moment) and an average level of 673 ppm, thereby preserving favorable air quality for a significant duration.

In general, it can be observed that each ventilation system exhibits a notable improvement in air quality when compared to that with the absence of ventilation.

Similar results were obtained in other studies that analyzed decentralized ventilation (e.g., like SYSTEM 2) and we can state that the experimental data are validated with this cross-reference [33] on CO₂ and temperature values. In [6] it was also found that CO₂ levels increase gradually but can be kept to values of around 1200 ppm with decentralized ventilation systems.

The investigation of air temperature in several ventilation systems, including a scenario without ventilation, demonstrates a significant degree of uniformity in maintaining comparable temperature levels among educational spaces. The mean ambient temperature within the enclosed classroom, lacking proper ventilation, was measured as 23.5 °C. When comparing the data, it can be observed that Systems 3 and 4 exhibited a similar average temperature of 23.1 °C, which nearly resembled the temperature of the non-ventilated environment. System 2 exhibited a marginally lower mean temperature of 23.4 °C, but System 1, which was installed in a room featuring two external walls, registered a little higher average temperature of 24.1 °C.

The noteworthy aspect lies in the negligible temperature difference observed across the various scenarios, regardless of the presence or absence of ventilation systems. The findings indicate that the ventilation systems are successfully sustaining a consistent thermal environment, a critical factor for ensuring comfort and cognitive performance in educational environments. The fact that Systems 1 and 3, located in rooms with two external walls, showed no statistically significant temperature fluctuations compared to the other systems is particularly noteworthy. This observation suggests that the insulation and architectural features of the rooms are successfully mitigating the risk of thermal energy transfer across the external walls.

Based on the consistent room volumes and the approximate ratio of inhabitants (about 23 kids and 1 professor), it is apparent that the ventilation systems are not inducing substantial variations in the thermal conditions within the classrooms. The consistency of temperature, irrespective of the type of ventilation system utilized, is advantageous in terms of education, as it guarantees a uniform and pleasant learning atmosphere for both students and educators.

The control of humidity within educational settings is a crucial element in establishing an optimal learning environment, and ventilation systems are vital in facilitating this endeavor. Maintaining humidity levels within the optimal range of 30% to 50% is crucial for ensuring comfort, promoting good health, and supporting cognitive function. Elevated levels of humidity can contribute to the growth and spread of mold and mildew, posing risks to both the physical stability of the building and the well-being of individuals inhabiting it. Such conditions have the potential to induce breathing problems and induce allergic responses. On the contrary, decreased levels of humidity can lead to many forms of discomfort, including the manifestation of dryness in the skin and eyes. Moreover, it can also elevate the probability of contracting respiratory diseases and facilitate the transmission of airborne viruses. Ventilation systems play a crucial role in maintaining equilibrium in interior humidity levels. By facilitating the circulation of new air and eliminating stagnant, damp air, these systems have the capacity to inhibit the accumulation of excessive moisture, thereby reducing the likelihood of mold proliferation and upholding an indoor environment conducive for human health. Moreover, the influence of humidity on thermal comfort is substantial. Despite the implementation of precise temperature regulation, unsuitable levels of humidity might result in individuals perceiving the environment as excessively warm or chilly.

The investigation of the researched ventilation systems, which lack integrated humidity management, uncovers notable difficulty in upholding ideal indoor humidity levels, particularly in winter when the exterior air is cold and dry. The introduction of outside air into classrooms through these systems may result in a notable decrease in interior humidity levels, which could potentially give rise to perceptions of discomfort and health-related concerns. The phenomenon is particularly accentuated when airflows are increased, as ventilation systems have the potential to severely dry the indoor air.

The data obtained from the systems highlight this particular risk (see Figure 9). System 1 shows an average humidity level of 30.87%, which is closely trailed by System 3 at 30.76% and System 2 at 30.43%. The observed results exhibit a proximity to the lower threshold of permissible indoor humidity, suggesting a modest yet constant decrease in moisture levels. System 4 exhibits a little improvement, as it consistently sustains an average humidity level of 33.45%. Although this value is closer to the target range, it still resides towards the lower end. On the other hand, the room without ventilation has a comparatively elevated average humidity level of 35.04%. This can be attributed to the absence of external air circulation and the inherent moisture produced within the space.

The analysis of the ventilated and non-ventilated scenarios indicates that ventilation plays a vital role in maintaining the air quality. However, it is important to note that the implementation of ventilation systems can mistakenly result in decreased humidity levels. The concern is notably apparent in systems lacking the means for controlling humidity levels. To address this issue, the implementation of humidifiers in conjunction with ventilation systems could be a feasible approach, aiding in the maintenance of a balanced and pleasant interior environment. The data unequivocally demonstrate that although ventilation systems are efficient for controlling air quality, they do require supplementary measures, such as humidification, to guarantee comprehensive indoor environmental quality.

The Predicted Mean Vote (PMV) index is a widely recognized method for calculating thermal comfort in indoor environments. Developed by P.O. Fanger in the 1970s, the PMV model estimates the average thermal sensation of a large group of people. It is based on the heat balance of the human body and incorporates six key factors: air temperature, mean radiant temperature, air velocity, humidity, clothing insulation, and metabolic rate. The PMV index serves to forecast the mean rating of a large number of people on a seven-point scale measuring the thermal feeling, which spans from −3 (indicating chilly) to +3 (indicating hot), while a rating of 0 signifies a state of neutral thermal comfort. The inclusion of this scale in the PMV model is crucial as it allows for the measurement of the subjective aspect of thermal comfort, recognizing that different individuals may have varying responses to identical environmental conditions. In our study, assessing the thermal comfort in classrooms equipped with different ventilation systems, we employed the Testo 400 IAQ and Comfort Kit to measure the Predicted Mean Vote (PMV) index. The evaluation of thermal comfort using the Predicted Mean Vote (PMV) index for the four different ventilation systems provides valuable insights regarding their efficacy in sustaining a pleasant indoor environment. System 1 showed a PMV satisfaction percentage of 79.6%, whereas the PMV values ranged from −0.5 to +0.5. This finding suggests that a significant proportion of individuals reported a thermal sensation that was close to neutral, a state that is generally regarded as comfortable in the context of indoor environments (see Figure 10).

System 2 provides a remarkable satisfaction rating of 100%, indicating that all individuals occupying the space had thermal conditions that fell within the ideal Predicted Mean Vote (PMV) range. The observed level of satisfaction is significant and suggests that System 2 is highly proficient in maintaining an optimal thermal environment.

System 3 had a commendable performance, with a satisfaction percentage of 94.1%. The substantial proportion mentioned indicates that a large majority of individuals were situated inside the desirable range of the Predicted Mean Vote (PMV), hence suggesting the efficient management of temperature conditions. In a similar vein, it is worth noting that System 4 demonstrated a satisfaction rate of 92.2%, thereby reinforcing the effectiveness of these ventilation systems in delivering thermal comfort.

In comparison, it can be observed that Systems 3 and 4 exhibit marginally lower satisfaction rates in contrast to System 2, yet they consistently uphold elevated levels of thermal comfort. The observation that the satisfaction rates of all systems, except for System 1, surpassed 90% suggests a notable level of success across the board.

In conclusion, each of the four systems exhibits a notable capacity to effectively maintain optimal thermal comfort, since there have been no documented occurrences of air drafts. The impact of air drafts on perceived thermal comfort is a critical element to consider. The observed high levels of satisfaction across all systems serve to underscore their effectiveness in establishing and sustaining a pleasant and stable indoor thermal environment, a crucial factor in promoting the well-being and productivity of occupants.

The control of noise levels is a crucial feature of the design and operation of decentralized ventilation systems, especially in environments, such as schools or offices. HVAC systems, by their nature, can generate noise, which if not adequately controlled, can become a source of distraction or discomfort. If not effectively managed, this noise can potentially lead to distractions or discomfort. In such circumstances, the prescribed standards for permissible noise levels generally advocate a maximum threshold of 35 dB(A). The purpose of establishing this threshold is to guarantee that the background noise generated by the ventilation system does not disrupt the main functions of the area, such as educational activities in classrooms or concentrated work in offices.

For decentralized systems, their inherent advantages in terms of flexibility and energy economy necessitate meticulous design considerations to mitigate noise. This includes the careful selection of equipment that fulfills low-noise standards, the strategic placement of this equipment to minimize sound transmission, and the potential integration of sound-dampening materials or designs. The challenge is to balance the acoustic comfort with the system’s effectiveness in air circulation and quality.

Surpassing the established standard of 35 dB(A) can result in heightened levels of tension, diminished ability to concentrate, and a general decline in pleasure with the interior environment. Hence, while considering the design and execution of decentralized HVAC systems, it is imperative to recognize that acoustic performance holds equal significance to factors, such as thermal comfort and air quality. It is imperative to maintain the prescribed noise levels to establish an indoor atmosphere that is conducive to comfort, well-being, and productivity. Measurements of acoustic levels were measured for all four systems at different scenarios (e.g., maximum air flow or different air flow stages) using a class 1—sound meter SVANTEK 970. The measurements were realized during the night to minimize the background noise. For all measurements, the background noise was recorded and subtracted for the results. The measurement of data was performed based on the frequency (31.5 Hz to 8000 Hz).

Figure 11 shows that System 1 and 3 are by far the noisiest among all four. For System 1, this is justified as it does not have a sound attenuator. At maximum operation, it emits an average noise level of 64.15 dB(A), which is far too high for a classroom. As a reminder, a background noise level of 65 dB corresponds to the noise heard in a moving car. Even when operating at very low speeds (2/6 for System 3), the system fails to meet the standard, with a noise level of 39.07 dB(A).

System #2 and System #4 achieved similar results, with maximum noise levels of around 45 dB(A) when delivering maximum air flow of 700 m³/h. Although this value is higher than the standard, no complaints were issued during the entire period, thus confirming that even 45 dB(A) is acceptable for the occupants. On the other hand, we note that System #4 operated at 55%, or 385 m³/h, and System #2 at only 40%, or 320 m³/h, with maximum values of 35–36 dB(A), thus respecting the norms.

7. Discussion

In the absence of proper ventilation, the concentration of carbon dioxide (CO₂) surpassed the established threshold of 1500 parts per million (ppm) for a significant duration of 86.5 h. The observed length is notably elevated, suggesting a substantial likelihood of compromised air quality, which has the potential to seriously impact the cognitive abilities and well-being of both children and educators. System 1 exhibits a notable enhancement, as seen by the CO₂ levels exceeding 1500 ppm after a mere duration of 7.95 h. The observed decrease in air quality, when compared to that with the absence of ventilation, serves as compelling evidence for the efficacy of the system in managing air quality. Among the many systems considered, System 2 demonstrates superior efficacy by consistently maintaining carbon dioxide (CO₂) levels below the threshold of 1500 parts per million (ppm) during the entire duration of the month. This finding suggests that the system achieves a high level of effectiveness in maintaining stable indoor air quality, therefore serving as a commendable benchmark for ventilation practices in educational environments. System 3, albeit an improvement compared to the absence of ventilation, resulted in carbon dioxide (CO₂) concentrations surpassing 1500 parts per million (ppm) for a duration of 23.5 h. While this improvement is noteworthy compared to the absence of ventilation, it implies that there is potential for further optimization in this system. System 4 exhibits a moderate level of efficacy in preserving air quality, but is less efficient than Systems 1 and 2, as evidenced by its sustained exposure to carbon dioxide (CO₂) concentrations above 1500 parts per million (ppm) for a duration of 10.66 h.

The analysis of indoor humidity levels in classrooms equipped with different ventilation systems yields a significant observation: although these systems effectively regulate CO₂ levels, they tend to decrease humidity, frequently reaching the minimum permitted threshold. Systems 1–3 have a mean humidity level in the proximity of 30%; however, System 4 has a little superior performance with an average humidity level of 33.45%. Nevertheless, as compared to the average of 35.04% in the non-ventilated space, all other measurements exhibit inferior results. This implies that although ventilation plays a critical role in maintaining air quality, it can also result in decreased humidity levels, which may have implications for both comfort and health. Hence, the incorporation of humidification solutions into these systems may be imperative to maintain an ideal indoor environment.

The space occupied by the system might limit classroom functionality, and unacceptable noise levels could be disruptive, affecting the learning environment and overall student and staff comfort. Addressing these challenges is crucial for evaluating the feasibility and overall effectiveness of decentralized ventilation systems in educational settings.

8. Conclusions and Future Work

The comparative analysis of carbon dioxide (CO₂) levels in classrooms equipped with various ventilation systems, as opposed to a scenario without any ventilation, over a period of one month, offers a distinct signal regarding the efficacy of these systems in upholding satisfactory indoor air quality throughout school hours.

Our study led to a few significant results regarding the assessment of decentralized ventilation systems in educational environments. First and foremost, these systems exhibit a high level of efficacy in improving air quality, successfully controlling CO₂ levels to ensure that they remain below acceptable thresholds. This is of crucial significance for the physical and cognitive wellbeing of both students and staff members. One significant constraint that has been identified is the absence of adequate humidity regulation. In specific instances, this phenomenon has led to a decrease in indoor humidity levels that fall below the optimal range, which may have potential implications for both comfort and health.

In relation to noise levels, it can be observed that the systems do not consistently maintain compliance to the 35 dB(A) noise threshold when operating at their maximum air supply capacity. However, these systems are able to sustain satisfactory acoustic conditions when run within a moderate range of levels, specifically at 55% or between 320 to 380 m³/h. The absence of any complaints from occupants regarding noise or air drafts is a notable characteristic, suggesting a general satisfaction with the ambient conditions facilitated by these technologies.

The systems’ ease of installation and non-invasive characteristics makes them a highly favorable choice when retrofitting existing school structures. The incorporation of this characteristic, alongside with its cost-efficiency, taking into account the significant advantages for the well-being of children and decreased energy usage, puts up a compelling rationale for its implementation. The expenses associated with implementing these systems in classrooms, varying from 4000 to 6500 Euros, can be considered very affordable when considering the positive impact on health and the environment. This is particularly significant as these systems effectively eliminate the necessity for window ventilation.

An effective ventilation strategy that has emerged from our study is the use of boost mode during break times or periods of high background noise. This methodology aims to enhance air quality while minimizing any possible adverse effects on classroom activity. Nevertheless, it is apparent that humidifiers are essential throughout the winter season in order to uphold indoor humidity levels at above 40%, thereby effectively resolving the problem of low humidity induced by these systems.

Similar measurement campaigns in schools could be found in the literature, concerning the evolution of indoor CO₂ concentrations, temperature, and relative humidity. For instance, Lazovic et al. [38] performed an extensive experimental study in four schools from Serbia (three located in urban areas and one in a rural area), with the results showing a good correlation in terms of CO₂ concentration profiles with the results outlined in this paper (Figure 6). The authors showed a strong correlation between the indoor temperature, humidity, and CO₂ concentration for naturally ventilated classrooms, while for the mechanical ventilation case, all these parameters are under control, and this correlation is much weaker. Gladyszewska-Fiedoruk [39] arrived at a similar conclusion in a study of IAQ and thermal comfort performed for a kindergarten case. Similar results were found by Sanchez-Fernandez et al. [8] in a study of the evaluation of different natural ventilation strategies using CO₂ sensors to monitor IAQ in classrooms and by Almaimani et al. [24] when assessing several optimal ventilation strategies for a school in Saudi Arabia, in order to minimize the risk of the transmission of airborne viral infections indoors.

In summary, our study provides significant contributions in terms of understanding the application and constraints of decentralized ventilation systems in the retrofitting process of educational facilities. Although there are certain challenges associated with humidity control and noise management at higher operational levels, the overall advantages of these systems in terms of air quality, installation convenience, occupant satisfaction, and energy efficiency establish them as a feasible and efficient solution for enhancing indoor environmental quality in educational settings.

Limitations of the current study include its focus on a limited number of schools, potentially limiting the generalizability of the findings. Also, the study primarily concentrates on physical measurements of IEQ parameters without delving deeply into subjective perceptions or health outcomes of students and teachers, which could be addressed in future research.

Future studies should consider longitudinal approaches, observing the long-term impacts of decentralized ventilation systems over various seasons and years. This extended timeframe would enable a deeper understanding of the consistency and sustainability of these systems in enhancing IEQ. Expanding the scope to encompass a broader range of schools across different geographical and socio-economic spectrums within Romania is recommended. Such diversity in the study sample would shed light on the influence of external environmental factors and socio-economic conditions on IEQ. Investigating the direct relationship between enhanced IEQ and educational outcomes, such as student academic performance, attendance, and health, is crucial. This would establish a more concrete link between environmental quality within schools and tangible educational benefits. Future research should adopt a more holistic approach to IEQ, examining the combined effects of various IEQ components (air quality, thermal comfort, lighting, and acoustics) and their collective impact on occupants’ health and the learning environment.