Influence of the Heating System on the Indoor Environmental Quality—Case Study

Abstract

This aim of this paper is to explore the specific indoor environmental quality factors under different heating conditions in a meeting room of an administrate building located in Kosice. In terms of thermal comfort, a system with radiant ceiling heating provides more favorable results. Low relative humidity was recorded for both heating systems, which could be due to insufficient air conditioning settings. The results of measuring CO₂ concentrations were almost identical for both systems and did not exceed the recommended limit value of 1000 ppm. The increase in CO₂ concentrations was mainly related to the presence of employees in the monitored room. On none of the monitoring days, whether in the case of a mechanical heating system or a radiant ceiling heating system, the average 24 h concentration of PM₁₀ did not exceed the legally permissible limit of 50 µg/m³. The presence of selected volatile organic compounds in the room has not been demonstrated due to effective ventilation by air conditioning. The results of the evaluation were comparable and smaller fluctuations in values can be attributed to other factors, such as the presence of persons in the monitoring room or the overall heating as well as ventilation and air conditioning (HVAC) systems.

1. Introduction

Over the years, indoor environmental quality (IEQ) has been widely studied from a variety of perspectives: from the first experiments to assess the physiological response of individuals, it has become a human-centered concept [1]. IEQ is a broader concept that includes many factors, such as thermal conditions, indoor air quality (IAQ), lighting, and acoustics inside a building, which affect the health and well-being and productivity of occupants. IAQ quantifies the minimum acceptable air quality that promotes human health, comfort, and productivity. This includes the removal of harmful pollutants entering the living space from the outside or those that form inside the space. The level of IEQ depends on many complex interconnected parameters [2,3] and reflects the performance of the building in relation to the health and well-being of its occupants [4]. It is well-known that IEQ can be influenced by residents and their activities in the indoor environment [5]. It is very important to develop better control techniques for HVAC systems to ensure occupant-controlled energy and comfort management, given the importance of energy savings in buildings [6]. The relationship between the level of satisfaction of the population with the conditions of the indoor environment, and their productivity in the workplace is well established. However, the relationship between building renovation decisions and the level of satisfaction of the population with the IEQ is insufficiently explored [7]. User comfort and power consumption become two opposing entities, leading to pareto-optimal control lines when designing HVAC systems [8]. The difference in the energy performance of buildings is a well-known phenomenon [9]. Sustainable building design should be in line with even more urgent energy-saving requirements—as in nearly zero-energy buildings (NZEB)—and a high level of IEQ [10]. Addressing the two common challenges to building performance—reducing the carbon footprint associated with providing a comfortable indoor environment and improving the health and well-being of residents—requires a more comprehensive understanding of how the indoor environment of buildings works [11]. To understand the future of sustainable buildings, it is important to recognize that services such as HVAC and lighting are provided to create suitably comfortable conditions for staff to be productive [12]. The COVID-19 (SARS-CoV-2) pandemic has significantly affected our daily lives [13]. Its rapid spread and subsequent deaths worldwide led to the declaration of a pandemic situation in the world at the end of December 2019 [14]. According to a study [15], people generally spend more than 60% of their time at home [16] and the rest of their time at work, school, and/or commuting, leading to about 90% or more of their time indoors. Because of the occurrence of the COVID-19 pandemic, it can be stated that almost 100% of the time people spend indoors [17]. Concerns about the spread of the virus in confined spaces due to insufficient ventilation have gradually raised the need to improve IEQ which depends significantly on the performance of the building’s outlets, lighting, and HVAC systems [18]. As people spend more time indoors, it is important to identify the relationship between IEQ and health of the building’s occupants [19]. Public health strategies to reduce indoor transmission, such as ventilation and centralized insulation, will be beneficial for the prevention and control of COVID-19 [20]. The design of a modern working environment must take into account a high level of spatial and technological change by providing responsive heating and air quality systems. Residents of the building will require internal conditions to support computer-intensive activities as well as paperwork [21]. Occupancy is a key input variable for HVAC sizing in buildings. However, HVAC designers usually estimate occupancy data based on assumptions that rarely reflect actual situations. As a result, these assumptions can lead to undersized or oversized HVAC systems that either provide too low or too high peak loads or ventilation airflows than is actually necessary to meet IEQ requirements during building operation [22]. According to a study by Borgstein et al. [23], buildings do not regularly operate at an optimal level and often do not meet project forecasts. These failures affect energy efficiency and ensure adequate IEQ and user satisfaction. This study points out that several of the buildings failed on a basic level to provide basic conditions of IEQ, as demonstrated by the 13 different failure modes such as thermal comfort not met by undersized HVAC systems; no external air supply (or insufficient external air); unbalanced air distribution for cooling and so on. Only five of the 33 buildings studied did not report a single issue related to environmental quality. Most issues were related to the HVAC systems, which either do not provide thermal comfort or are not properly controlled. A study also emphasizes that the most common problem of unbalanced air distribution for cooling was found almost exclusively in large buildings. In terms of IEQ and building service performance, user satisfaction with greenery is significantly higher than in terms of thermal comfort, IAQ, equipment, operation, and maintenance. In addition, factors influencing the energy consumption of buildings are analyzed in order to provide guidance on further improving the performance of green buildings during the design and operation phases [24]. The results of another study [25] suggest that building managers could pay less attention to people living in energy saving measures, while paying more attention to communicating with ordinary people in order to raise their awareness of energy savings. The results of this study also revealed that wasting residents have the greatest potential to narrow gaps, which can be realized by combining communications and HVAC system equipment with zone control. According to a study by Leyten and Kurvers [26], the robustness of an office building or HVAC system can be defined as the degree to which a building or system fulfills its design purpose in a real situation. Insufficient robustness can be caused by hypersensitivity to deviations from design assumptions, unrealistic maintenance requirements, integration of heating and ventilation, regulation of supply air volumes, and lack of transparency for residents and building management. As a study states [27], assessment tools for office buildings are very important. The work done in the study [27] could lead to the development of an IEQ model that reflects the user’s opinion. The use of variables used in the calculation of a building’s energy performance to calculate IEQ is an important step in the development of IEQ methodologies, if they are to be compared between energy consumption and the comfort of the occupants. The importance of the office environment to the comfort, productivity, and health of workers cannot be overstated [28]. The results of a Polish study [29] show that the importance of indoor air quality should not be forgotten when considering energy-saving strategies. Their simulations prove that for instance a garage attached to a house with more air tightness can be harmful to human health. The results of this study encourage research into different impacts strategies to increase energy efficiency and maintain appropriate indoor air quality inside residential buildings. The results of another study [30] prove that the location of the furnace influences contaminant accumulation and migration. Such simulations can be an important tool when designing a ventilation system with respect to the furnace to improve the removal of dangerous substances.

The main goal of this study is an investigation of IEQ under different heating conditions in the meeting room of the ABC KLIMA office building in Košice in Eastern Slovakia. In Slovakia, there is not enough measurement of IEQ to assess the extent to which building users are exposed to high concentrations. The second aim of the study is to show that the central air handling unit by using the filter technology with high-efficiency (more than 99%) has low indoor/outdoor concentration ratios (I/O ratios). Therefore, outdoor PM concentrations do not affect the level of PM occurrence in the indoor environment.

2. Materials and Methods

Measurements of air temperature, relative humidity, air velocity, and CO₂ concentrations were performed using a Testo 435-4 measuring instrument with appropriate probes (Testo 435, Testo AG, Lenzkirch; Germany). All measuring instruments were placed approximately in the middle of the room on the table. The high of the table was 0.75 m, and this height represented the breathing zone of the sitting person. The measured values were recorded in 1 min intervals.

Surface temperature measurements were taken at six locations—on the floor, on the ceiling, on the internal wall separating the room from the corridor, on the external wall, on the internal wall adjacent to the office, and on the external wall close to the fan coil. The individual sensors were located approximately in the middle of the monitored surfaces. The measured values from the sensors were collected by the data logger Almemo 5690-2M09TG8. The data collection of all listed parameters was performed at 30 min intervals. The total measurement time was, therefore, 14 days, the first 7 days with an active radiant ceiling heating system, and the remaining 7 days with an active mechanical heating system.

Operative temperature values were determined by calculation according to Equation (1):

$$\theta o=\frac{\theta a+\theta r,m}{2},$$

(1)

where θ_a is the air temperature (°C) and θ_r,m means the radiant temperature (°C).

The mean radiant temperature was determined according to measured surface temperatures.

A Handheld 3016 IAQ measuring device (Lighthouse Worldwide Solutions, Inc., Fremont, CA, USA) was used to measure the particulate matter concentrations (fractions from 0.5 to 10 µm), which used a laser-diode light source and collection optics for particle detection.

The Airlite sampling pump was used to collect selected VOCs from the air. This sampling was performed on the selected day of the week, and it lasted 8 h. The air sample was collected onto an Anasorb CSC sorbent tube and further analyzed in an accredited laboratory by gas chromatography.

All measurements and evaluation of results were performed in accordance with the requirements given by the Decree of the Ministry of Health of the Slovak Republic No. 259/2008 [31]. During the measurements, employees had free access to the room, and forced air exchange was provided by the central air-handling unit. Employees were asked to record their presence in the room on a visit list. According to this documentation, an increase and a decrease of CO₂ concentration could be observed. Characteristics of the measuring instruments are mentioned in

Table 1. Measuring instruments descriptions.

Instrument	Sensor Type	Measuring Range	Accuracy	Resolution
	Temperature—NTC	−20 to +70 °C	±0.3 °C	0.1 °C
	Humidity—capacitive sensor	0 to +100% RH	±2% RH (+2 to +98% RH)	0% RH
TESTO 435-4	Air velocity—hot wire anemometer	0 to +20 m/s	±(0.03 m/s + 4% of m.v.)	0.01 m/s
	IAQ sensor (CO2)	0 to +10,000 ppm CO2	±(75 ppm ± 3% of mv) (0 to +5000 ppm) ±(150 ppm ± 5% of mv) (+5001 to +10,000 ppm)	1 ppm
HANDHELD 3016 IAQ	Laser-diode light source and collection optics for particle detection	0.3 to 10.0 μm	50% at 0.3 μm; 100% for particles with sizes of >0.45 μm (per ISO 21501-4)	-

.

2.1. Location

The building is located in the built-up area of Košice—Nad Jazerom (Figure 1). The building is used for administrative purposes for the company ABC KLÍMA KOŠICE s.r.o. The main entrance to the building is directly from the street. In front of the building is a parking lot for employees. Behind the building is a concrete area, which is used for parking company cars and storing oversized materials. Near the building, there is a grocery store, bakery, and car bazaar. On the other side of the road is a garden area.

2.2. Object

The building for monitoring was chosen due to two methods of heating, with the first method being heating using a ceiling radiant system and the second with a fan coil. The main goal was to compare the difference in IEQ using these two systems of the heating.

The building is L-shaped. One part consists of the main building—administrative, and the other part consists of a warehouse. The administrative building has two floors with a flat impassable roof. The warehouse is single-storey. The built-up area of the administrative part is 470 m². The total built-up area of the building is 931 m². The facade is of two main colors, white and gray. The gray part optically divides the building in half (Figure 2).

2.3. Layout of the Administrative Building

On the first floor after passing through the vestibule, there is a gallery with a staircase on the right side. On the second floor where the monitoring of the indoor environmental quality factors were carried out, offices for employees, directors, designers, and project managers, as well as archives and toilets, are placed. Figure 3 depicts the floor plan of the 2nd floor with marked measured space and orientation to the world side.

2.4. Construction-Technical Design

Below are the described built-in building materials and HVAC systems that can have an impact on the IEQ of the investigated building.

Vertical load-bearing structures consist of reinforced concrete columns of longitudinal frames with dimensions of 300 × 300 mm. The columns are at a distance of 1905 mm to 5450 mm. The plaster is made of YTONG fittings. The bearer has dimensions of 300 × 480 mm. The perimeter walls are insulated with 100 mm-thick contact thermal insulation from facade polystyrene foam.

The ceiling supporting structure consists of a reinforced concrete continuous slab with a thickness of 250 mm. Assemblies of the horizontal structures are described in

Table 2. Horizontal construction.

Floor Composition on the Second Floor	Roof Composition
tread layer (according to the purpose of the room) with a thickness of 20 mm	SIKAPLAN 15G foil, mechanically anchored in a thickness of 1.5 m
leveling screed thickness of 10 mm	mineral wool NOBASIL SPU with a thickness of 2 × 120 mm
reinforced concrete slab C25/30 (tempered concrete core) with a thickness of 250 mm	gradient layer of polystyrene-concrete PTB 350 with thicknesses of 50–120 mm
	SIKA DS PE vapor barrier
	reinforced concrete slab C25/30 (tempered concrete core) with a thickness of 250 mm

.

Windows and doors are plastic with insulating double glazing. The entrance door to the building (main entrance) is made of aluminum.

2.5. Heating and Cooling

The building is designed for heating and cooling using a ceiling radiant heating system (RHS) and a fan coil (mechanical heating system (MHS)). A wet cooling system is designed in the building, resp. heating and ceiling core tempering. The primary system used is a ceiling radiant system. If this system is not sufficient, it cools and resp. heats up with fan-coil.

A water-to-water heat pump is used as the cooling source. Figure 4 shows technical room scheme.

The measured room is heated and cooled using hot air-heating and air-cooling with the windowsill circulation of fan-coil units. Concurrently, the room has separate fresh air ventilation, which is connected to the central air handling unit. In addition, the room has a ceiling heating radiant system, which enables both the heating and cooling of the space.

The heating system and the cooling system are shown using the functional scheme of the system connection (Figure 4). The system includes a heating water distribution box (6 circuits) for the heating system (left side of the diagram) and an accumulation tank for accumulating the heat produced by the heat pump. The system also includes a cooling water distribution box (3 circuits) for the cooling water system (right side of the diagram). The source of hot and cold water for the system is a water-to-water heat pump. The source of cold water can also be a separate water supply from a well, which allows us to ensure a direct supply of cold water to the cooling system and save the heat pump or in the event of a heat pump failure. The hot water tank is in the functional scheme of the system connection to complete the diagram, but it does not affect the heating and cooling system and of course was not part of the measurements.

2.6. Monitored Meeting Room

The room in which the measurement took place is located on the second floor next to the shared kitchen. It serves for meetings and trainings and as a reception for visitors. It is 8150 mm long and 4400 mm wide. The clear height is 2770 mm. The volume of the room is 99.33 m³. In the middle is a table with chairs, and next to the wall are cabinets. A data projector is hung on the ceiling, which projects the image onto a screen placed on the wall. There are also flowers in the room, and there is a fan coil under the window. The room was designed with a window with a width of 4600 mm and a height of 1570 mm. The room is entered through an all-glass frameless door. Figure 5 is the photo of the monitored room with the measuring instruments.

From 11th February 2020 to 24th February 2020, the indoor environmental quality of the meeting room was monitored at ABC KLIMA operating center in Košice. The measurement was performed twice under the most similar conditions. In the first and second measurements, surface temperatures, CO₂ concentrations, particulate matter concentrations, air temperature, air velocity, and relative humidity were measured. The first measurement (first week) was performed with heating using a ceiling radiant system, and the second measurement (second week) was conducted with heating with a fan-coil. The air supply system was centrally set to 300 m³/h, regardless of the number of users present. The measurements were carried out under these set conditions.

3. Results

The measured data were statistically processed and compared with the requirements for thermal-humidity microclimate and limit values of harmful factors in indoor air given by the Decree of the Ministry of Health of the Slovak Republic No. 259/2008 [31]. The statistical evaluation given in

Table 3. Statistics of the measured data.

	RHS				MHS
	Mean	Min	Max	SD	Mean	Min	Max	SD
Air temperature (°C)	22.89	22.33	23.51	0.32	24.26	23.09	25.24	0.58
Relative humidity (%)	24.50	21.06	27.27	1.40	25.10	22.61	28.8	1.68
Air velocity (m/s)	0.01	0	0.02	0	0.02	0	0.12	0.02
CO2 (ppm)	474	435	680	39.10	487	426	1002	71.75
PM0.5 (µg/m3)	2.52	0.27	7.70	1.80	1.93	0.16	4.88	1.30
PM1.0 (µg/m3)	4.03	0.48	12.39	2.82	3.05	0.27	7.56	2.01
PM2.5 (µg/m3)	4.70	0.66	14.21	3.16	3.62	0.34	8.76	2.28
PM5.0 (µg/m3)	5.16	0.98	14.97	3.20	4.05	0.40	10.59	2.46
PM10 (µg/m3)	5.27	1.05	15.03	3.18	4.20	0.43	13.40	2.55

included data collected during a seven-day continuous measurement for each of the monitored heating system. The outdoor environment data is presented in

Table 4. Outdoor environment data [32,33].

	RHS				MHS
	Mean	Min	Max	SD	Mean	Min	Max	SD
Air temperature (°C)	3.00	1.30	5.00	1.10	4.20	0.80	6.80	1.90
Relative humidity (%)	79.80	72.60	87.40	5.10	70.70	53.80	81.50	8.20
Wind speed (m/s)	6.10	3.70	9.50	1.90	8.40	4.20	13.00	3.40
PM2.5 (µg/m3)	51.10	27.00	76.00	21.00	54.30	23.00	77.00	19.10
PM10 (µg/m3)	21.00	9.00	35.00	8.70	20.00	9.00	35.00	9.20

for each of the monitored heating system.

3.1. Thermal-Humidity Microclimate

Figure 6 and Figure 7 show that in the case of both heating systems, the values of the measured indoor temperatures fluctuated considerably, with a daily tendency to increase in the morning and gradually decrease in the afternoon and evening. In the case of the mechanical heating system, an increase in indoor temperatures of approximately 5.6% was recorded compared to that with the radiant ceiling heating system. The minimum measured value of the indoor air temperature during radiant heating reached the value of 22.33 °C and the maximum value of 23.51 °C. On the other hand, the lowest measured air temperature with a mechanical heating system was 23.09 °C, and the highest was 25.24 °C. The relative humidity values of the two heating systems did not fluctuate significantly and were mostly similar, ranging from 21.06% to 27.27% for the system with active radiant heating and from 22.61% to 28.8% for the system with mechanical heating. However, none of the heating systems met the legislative requirement according to which the values should be in the range of 30–70%. With regard to the values of the air flow velocity, the permissible legislative limit of 0.2 m/s was not exceeded during the entire monitoring period. A higher air velocity was recorded with the mechanical heating system. Figure 8 and Figure 9 show the outdoor temperature and humidity for each of the monitored heating system [32].

The results of the surface temperatures measurements are summarized in

Table 5. Results of all monitored surface temperature measurements of the meeting room for RHS and MHS.

Surface Temperature (°C)	RHS			MHS
	Mean	Min	Max	Mean	Min	Max
External wall	22.51	22.40	22.62	23.80	23.61	23.99
Internal wall adjacent to the office	23.11	23.02	23.20	24.24	24.02	24.47
Internal wall separating the office from the corridor	23.08	22.99	23.16	24.27	24.03	24.51
Ceiling	23.60	23.52	23.68	24.55	24.30	24.81
Floor	23.65	23.58	23.72	24.34	24.24	24.45
External wall close to the fan coil	22.95	22.46	23.45	24.14	23.83	24.46
Operative temperature	23.10			24.28

. The total average temperature of all monitored surfaces for the radiant ceiling heating system was determined to be 23.15 °C, with the highest average temperature of 23.65 °C being measured on the floor and the lowest of 22.51 °C being recorded on the external wall surface. In contrast, with fan coil heating, the highest average surface temperature of 24.55 °C was measured on the ceiling surface. Although the lowest average surface temperature was measured on the same surface as in the ceiling radiant heating, in the case of the fan coil it was higher by 0.75 °C. The total average temperature of all surfaces reached 24.22 °C. The values of the operative temperature in the case of both systems corresponded to the optimal (20–24 °C) and permissible (18–26 °C) legislative ranges. However, with a mechanical heating system, the operative temperature value was slightly higher compared to with a radiant heating system.

3.2. CO₂ Concentrations

As can be seen on Figure 10 and Figure 11, the increase in CO₂ concentrations occurred, just when the employees were in the monitored room. In the absence of people in the room, CO₂ concentrations for both heating systems ranged from approximately equal levels, ranging from about 400 to 500 ppm. During both measurement periods, the CO₂ concentration values were kept below the Pettenkofer recommended limit value of 1000 ppm [34]. A sharp increase of CO₂ concentration to 1002 ppm was recorded only during one day, when the room was heated by the fan coil, due to the presence of people in the room for a longer period of time. The subsequent decrease in CO₂ concentrations back to the original level indicated the treatment of air by air conditioning.

3.3. PM Concentrations

According to Decree No. 259/2008 and 210/2016 [35], the maximum permissible limit value for PM₁₀ concentrations, for 24 h exposure, is set at 50 µg/m³. This limit was not exceeded on any of the monitored days. Figure 12 and Figure 13 show the course of PM concentrations of two representative fractions (PM_2.5 and PM₁₀) during radiant and mechanical heating. The average PM_2.5 concentrations achieved values of 4.70 µg/m³ for RHS and 3.62 µg/m³ for MHS, which represented almost 30% decreasing of the concentrations. The average PM₁₀ concentrations achieved values of 5.27 µg/m³ for RHS and 4.20 µg/m³ for MHS, which represented a 20% drop in concentrations. In the study of Vilcekova et al. [36], the monitoring of eight offices showed that PM₁₀ mass concentrations exceeded the maximum allowable value (50 µg/m³) in 37.5% of the monitored offices. The average concentration of PM_2.5 was determined to be 10.4 μg/m³, which is higher by 54.8% and 65.2% compared to the results of this study. Figure 14 and Figure 15 show the course of outdoor PM concentration for each of the heating system [33]. Figure 16 and Figure 17 show the course of outdoor versus indoor PM_2.5 and PM₁₀ concentrations for RHS [33]. Figure 18 and Figure 19 show the course of outdoor versus indoor PM_2.5 and PM₁₀ concentrations for MHS [33].

It should be pointed out that the recorded course of concentrations was fluctuating for both systems. Visible increases in PM_2.5 and PM₁₀ concentrations were observed in the case of radiant ceiling heating, especially on the fourth day of measurement, when PM concentrations reached the maximum. The highest concentration in the case of mechanical heating was measured at the beginning of the measurement, but with this mode of heating the measured values did not tend to increase as with radiant heating. The total average concentrations of PM_2.5 and PM₁₀ of both heating systems achieved comparable results.

Table 6. I/O ratios.

	Indoor	Outdoor		Indoor	Outdoor
	PM2.5	PM2.5	I/O Ratio	PM10	PM10	I/O Ratio
RHS	1.61	76	0.02	2.29	9	0.25
	2.13	27	0.08	2.83	14	0.20
	4.11	29	0.14	4.77	14	0.34
	8.35	30	0.28	8.87	20	0.44
	6.46	50	0.13	6.76	29	0.23
MHS	6.38	77	0.08	7.47	35	0.21
	4.36	77	0.06	5.15	24	0.22
	3.45	51	0.07	4.13	9	0.46
	3.64	23	0.16	3.92	29	0.14
	1.78	68	0.03	1.98	20	0.10

presents I/O ratios for PM_2.5 and PM₁₀ concentrations during radiant and mechanical heating. The ratio values between indoor and outdoor (I/O) concentrations have been widely used to study the relationship between indoor and outdoor concentrations of pollutants [37]. The I/O ratio was less than 1 during all measurement days, which meant that the amounts of PM_2.5 and PM₁₀ concentrations in the outdoor environment were higher, so they can be a source of PM in the indoor environment of buildings. The I/O ratio for PM_2.5 ranged from 0.02–0.2 and that for PM₁₀ ranged from 0.2–0.44 for RHS. The I/O ratios of PM_2.5 and PM₁₀ ranged from 0.03 to 0.16 and from 0.01 to 0.46 for MHS, respectively. The results showed that the room ventilated by a central air handling unit using the filter technology with high-efficiency (more than 99%) achieved low I/O ratios. Therefore, outdoor PM concentrations should not affect the level of PM occurrence in the indoor environment. In the study of Burdova et al. [38], aimed at the monitoring of an air-conditioned university library also in Kosice, Slovakia showed that the I/O ratios of PM₁₀ are within the range of 7.1–31.43 which are higher by 97.2–98.6% for RHS and 98.5–99.8% for MHS compared to the results of PM₁₀ in this study, and the authors proposed to design high-efficiency HEPA filters with an efficiency of up to 99.97%. This present study also confirmed the need of using filters with the highest level of separation. It should also be noted that regular care and replacement of filters must be taken care of, so that the filters do not become the source of pollutants. This study also points to the necessity of using fresh air units with a stream of fresh air. Combining this measure with a suitable heating system can significantly contribute to reducing PM concentrations. Radiant heating is the method with the lowest level of air and dust swirling. Mechanical heating can be characterized as a heating with a higher risk of air swirling and dust spreading but is often used due to the possibility of bringing fresh air into a ventilated room with occupants.

3.4. VOC Concentrations

The results of the determination of VOC in the indoor environment under different heating conditions are given in

Table 7. Results of VOC measurements in indoor air with different heating systems.

	RHS	MHS
Toluene (µg/m3)	<9.96	<9.96
Xylenes (µg/m3)	<21.16	<21.16
Styrene (µg/m3)	<7.04	<7.04
Tetrachloroethene (µg/m3)	<15.19	<15.19

. Since the air was exchanged by air conditioning during the measurements, the concentrations of selected volatile organic compounds (toluene, sum of xylenes, styrene, and tetrachloroethene) specified in the Decree of the Ministry of Health of the Slovak Republic No. 259/2008 [31] were below the quantification limits and therefore below permissible limits.

4. Conclusions

Subsequent conclusions have been drawn from the above results of thermal-humidity microclimate and of indoor air quality monitoring under different heating conditions. In terms of thermal comfort, a system with radiant ceiling heating provides more favorable results. Low relative humidity was recorded for both heating systems, which could be due to insufficient air conditioning settings. The results of measuring CO₂ concentrations were almost identical for both systems and did not exceed the recommended limit value of 1000 ppm. The increase in CO₂ concentrations was mainly related to the presence of employees in the monitored room. On none of the monitoring days, whether in the case of a mechanical heating system or a radiant ceiling heating system, the average 24 h concentration of PM₁₀ did not exceed the legally permissible limit of 50 µg/m³. At the beginning of the measurements, we assumed that PM concentrations would be higher in the case of a mechanical heating system, which caused higher air flow and created higher vertical stratification. This assumption was not confirmed. The presence of selected volatile organic compounds in the room has not been demonstrated due to effective ventilation by air conditioning. In conclusion, it can be stated that both systems meet the requirements set out in the decree of the Ministry of Health of the Slovak Republic, but it is not clear from the results obtained which of the systems provides a better quality of the indoor environment. The results of the evaluation were comparable, and smaller fluctuations in values can be attributed to other factors such as the presence of people in the monitoring room or the overall HVAC system.

The company that occupies and owns the building deals with ventilation and air-conditioning, and therefore, this company cares about regular maintenance and replacement of filters, which was reflected in the low values of the monitored parameters. It was confirmed that the replacement of filters and regular maintenance and cleaning of the air handling system is necessary to maintain a high quality of the indoor environment.

Ensuring the right indoor environment in buildings is key to the performance and well-being of occupants. Our future work will be focused on measuring the IEQ in various types of buildings as well as with different HVAC systems. While existing ventilation systems in most buildings do not supply enough clean outdoor air to dilute the virus concentration as is emphasized also in a study [39], our future work will be aimed at reducing the risk of airborne infection as well as minimizing energy consumption. The findings of another study [40] revealed that the highest priority for improvement should be temperature and air quality and humidity, as well as cleanliness and visibility.