On the Performance of Diffuse Ceiling Ventilation in Classrooms: A Pre-Occupancy Study at a School in Southern Sweden

Abstract

The implementation and application of diffused ceiling ventilation (DCV) is gradually gaining momentum, especially in Denmark, Finland, and the Netherlands. In countries such as Sweden, the application is limited despite the favorable conditions for implementation. The current study investigates the performance of DCV and mixing ventilation in a pre-occupancy field study for newly renovated classrooms in Southern Sweden. Two classrooms at the school were installed with diffuse ceiling ventilation while the rest had mixing ventilation. The objective of the study was to compare and evaluate the ventilation performance in terms of indoor environmental quality parameters such as thermal comfort, air quality indexes, airflow, and temperature distribution. Pre-occupancy measurements were performed in two classrooms with similar room characteristics, with one room running under mixing ventilation and the other under DCV. Constant temperature anemometers, thermocouples, and INNOVA thermal comfort were used to measure the indoor air speeds, temperature, and thermal comfort, respectively. Tracer gas measurements, with SF₆, were performed to assess air quality. Additionally acoustic measurements were conducted to assess the acoustic benefits of DCV on reducing ventilation noise. The results demonstrate that DCV offers similar indoor environmental conditions to mixing ventilation but has better acoustic performance especially on reducing the ventilation noise. Indoor environmental conditions were very homogeneous under DCV with mixing ventilation showing tendencies for short circuit ventilation. This study demonstrates that DCV has a potential for implementation in Swedish schools with minimal system modification on existing ventilation and air distribution systems.

1. Introduction

Buildings are estimated to use about 40% of global energy and contribute to about 33% of global greenhouse emissions, which has a significant impact on the climate and natural environment [1,2]. One of the building systems or services responsible for a large amount of building energy use is the ventilation system [3]. Ventilation is the largest energy using building service, accounting for about 50% of the total building energy use [4], as it is primarily one of the most important approaches to controlling the indoor environment [5,6,7]. Ventilation facilitates controlling and conditioning of the supply of indoor air and its distribution in the buildings/rooms. At the same time, ventilation works to remove indoor contaminants, e.g., heat, CO₂ (Carbon dioxide), VOCs (volatile organic compounds), and other particulate matter among other things [8,9]. The design of a ventilation system and its air distribution strategy are important for energy efficiency, simplicity and operation of the system, and for providing good indoor environmental quality (IEQ) [5,10]. Furthermore, since well-being is an important part of the sustainable development goals, the innovative ventilation system should not only reduce energy but should as well improve indoor occupants’ well-being and promote productivity [11]. Providing and sustaining healthy indoor environmental quality in an energy efficient way is one of the challenges in modern buildings today. Thus, the above criteria are often conflicting requirements and several ventilation strategies are explored in order to resolve these conflicts. Otherwise, trade-offs/compromises are made, especially in rooms with high occupancy density such as classrooms [12].

Classrooms are characterized by high ventilation demands: High ventilating airflow rates and large cooling demands due to high occupancy and increased internal heat sources [13,14]. The functional nature of classrooms makes ventilation important not just for the comfort of the students but also because health and effective learning depend on it [15,16,17]. Many ventilation and air distribution strategies have been developed and the commonly used systems, not only in classrooms, are based on homogeneous and non-homogeneous indoor environments, typical examples being mixing and displacement ventilation, respectively [18,19]. Mixing ventilation (MV) is an air distribution strategy in which supplied fresh air (or conditioned outdoor air) is mixed with indoor or contaminated enclosed air to dilute and reduce the contaminants to acceptable or less harmful levels. This air distribution strategy is characterized by high velocity and turbulence of air supply into a room to increase mixing and dilution with the room air. Cooling and heating of spaces using MV system are frequently accomplished using the same diffusers and the properties of air distribution are affected by proper diffuser selection and installation [20]. The objective is to provide good mixing and dilution of indoor contaminants in occupied space and the system is intended to cover the total volume of the enclosed space or room [3,18]. System disadvantages include high energy use to satisfy good indoor environmental criteria (good dilution), increased risk of draft (due to high air speeds and turbulence), short circuiting ventilation and indoor airflow stagnation regions (i.e., good mixing is rarely achieved) [21,22,23]. The high momentum supply of air is used to reduce ventilation short-circuiting flow and to increase mixing with the intention of creating uniformity in indoor air quality and thermal climate.

In contrast, non-homogeneous systems provide stratified ventilation characterized by the principle to replace air, not mix the room air, primarily focusing on the removal of contaminants in the occupied region in the room [9,24]. This underpins the effectiveness of the stratified ventilation system in minimizing cross contamination, since there is no mixing in the room space. The primary challenge of this ventilation system is its complex airflow patterns which varies when there are subsequent changes in positioning of the heat sources within the room [25]. This ventilation system functions with the help of the room surfaces, e.g., floor, when transporting airflow within the room. The presence of obstacles such as people, equipment and furniture etc., in the room can result in undesirable effects on airflow distribution [25]. A typical example is displacement ventilation, where air is supplied at low velocity near or at the floor level and room airflow distribution is governed and mostly disturbed by heat sources in the room through buoyance [19,25,26]. Thus, indoor environmental conditions are characterized by a stratification profile that drives indoor contaminants from the occupied zone and accumulates it near the ceiling for eventual evacuation [8,25]. The risk of ankle draft and air distribution is strongly dependent on room setup and size, i.e., the airflow and its distribution in the classroom is affected by the occupancy density. For example, the airflow supply is strong near the diffuser and distribution distance diminishes as it penetrates the occupied region. Therefore, occupants seated near the diffusers are exposed to high air speeds (driven by the temperature difference between the occupancy thermal load and the supply temperature) while occupants furthest from the diffusers (or in the middle of the occupied zone) are exposed to little supply airflow [27]. Overall, both mixing and stratified ventilation are disadvantaged by the systems strong dependence on the air inlet supply conditions [28].

In handling high cooling and ventilation loads, the inefficiency of mixing and stratified ventilation calls for more energy-efficient ventilation systems and strategies which can ensure acceptable indoor air quality and thermal comfort. The idea of tackling these limitations and the demand for energy efficiency and improved indoor environmental quality necessitate innovative or better air distribution systems (e.g., the COVID-19 pandemic has re-enforced the central role of ventilation in reducing the risk of indoor airborne transmission). Diffused ceiling ventilation (DCV) has gained attention for use in rooms with a high heat load such as classrooms and open-plan offices [3,18,24]. The air distribution concept is based on utilizing the plenum above the suspended ceiling as a diffusion unit whereby fresh air is supplied and distributed through diffusion over the entire area of the false ceiling into the occupied zone [29]. Thus, the three main components for air distribution are the plenum, suspended ceiling, and ventilated room [3]. Zhang et al. [24] discussed three different types of ceiling diffusion based on the air path, related products, and mixing ability within the room air. The air is uniformly mixed and spread in the occupied region with aid of buoyancy flows from the room heat sources [3,24].

Previous researchers [28,30,31] have discussed the advantages of DCV over mixing and displacement ventilation. For example, Jacob and Knoll [28] found a 50% reduction in energy use when diffused ceiling ventilation was used in a classroom due to smaller fans and fewer duct losses. Nielsen and Jakubowska [31] found that DCV has a high local heat removal effectiveness attributed to the large supply diffusion area. They concluded that DVC has a high potential to manage the high internal heat load conditions in an energy-efficient manner when compared to the conventional MV air distribution system. Apart from reduced risk of draft and low fan energy use, Zhang et al. [32] explained that DCV is economical due to the low cost of ductwork and requirement for no room diffusers. An additional benefit of DCV is the use of suspended ceiling tiles made of panels with sound-absorbing properties. Due to the characteristics of the panels, they can be adapted as noise dampers as well as airflow devices without any design or physical changes [33]. Therefore, acoustic ceiling tiles can be used which increases the performance in office environments and learning in classrooms as the system dumps/limits the low frequency noise within the plenum as opposed to traditional ventilation systems (normally silencers are installed to reduce the noise, which is an extra cost) [24]. Three types of DCV panels are discussed in literature and these differ on specifications of pressure drop across the false ceiling and consequently the airflow characteristics in the ventilated room, see [3,24]. Experimental and numerical studies have explored DCV [32,33,34,35], while other studies have explored integration of other building technologies and strategies to enhance building ventilation performance [36,37,38,39]. Overall, DCV has several benefits over conventional duct-based ventilation. A common thread in literature is that DCV systems are typically smaller, use less energy, and create more uniform indoor environmental conditions [3,40].

In Sweden, the implementation of DCV is limited, and to our knowledge the ventilation strategy has not been adopted in schools or buildings in general. Other countries have adopted or implemented the systems in commercial buildings with research and case studies in Denmark, Finland, and the Netherlands [24,28]. The advantages for the use of diffuse ceiling ventilation in Sweden may include low cost of installation (reduced ductwork), low risk of draft (airflow diffuses through the ceiling into the occupied zone) and ability to use low supply plenum temperatures (low outdoor temperatures in Sweden can be used) [3,18,24,28,29,30,31,32,34,41]. These stipulated benefits underpin the need to investigate DCV application in Sweden.

The current study investigates the performance of DCV in a pre-occupancy field study for newly renovated classrooms at a school in south Sweden. The aim is to assess system performance of DCV which will be benchmarked against mixing ventilation for a given supply of conditions. The generated indoor environmental conditions for the actual system operation used for the current mixing ventilation are compared against DCV with considerations on indoor air speeds, temperature distribution, air quality indexes, and thermal comfort measurements. This contributes to the knowledge and understanding of implementing DVC in existing buildings and structural ventilation system without the need of replacing the whole ventilation system or changing the current practice and setpoints.

2. Materials and Methods

A grade school in the southern part of Sweden was recently renovated and the school is operated with a balanced mechanical ventilation system under mixing air distribution strategy. To explore the potential of DCV and assess its performance and suitability (not commonly adopted in Sweden), two classrooms were equipped with DCV ventilation. In this study, measurements of physical indoor environmental quality parameters were performed in one classroom with DCV and another with mixing ventilation.

2.1. Classrooms and System Description

Two similar classrooms with a floor area of about 67.3 m² and a ceiling height of 3 m were investigated and compared. It is worth mentioning that although the classrooms were very similar in design and size, the reference classroom was slightly smaller in floor area (about 4 m²). One classroom had a mixing ventilation system installed (herein referred to as the reference classroom) and the other had DCV. Figure 1 shows the rooms and respective ventilation systems. The ventilation system servicing the classrooms had centralized air handling units operating with dedicated outdoor air supply (DOS). In Sweden it is common that there is no air condition performed on the supply air except heating it to about 18 °C to avoid condensation in the ducts and to operate ventilation systems with no humidity control and no cooling of supply air during summer. None of the classrooms had active cooling systems installed. Although the ventilation system is operated under variable air volume (VAV), during measurements the system was operated as constant air volume (CAV) with the air flowrate of 280 l/s in each classroom (measured across the dedicated air supply control unit for each classroom).

Both classrooms had ceilings with “Ecophon Master E T15” sound absorbing tiles (0.6 m × 0.6 m × 0.04 m). The panels are used in all classrooms at the school for the benefit of increasing acoustic quality in classrooms. The classroom with DCV had a plenum with an overall depth of 0.8 m and the air was supplied into the plenum and diffused into the classroom through gaps between ceiling tiles and the supporting grid profiles. No structural beams were present, only exhaust ventilation service ducts, as seen in in Figure 1B, and wiring cables for electrical installations (sensors and ceiling lights). These had no influence on airflow distribution in the classroom [41].

2.2. Measurements

2.2.1. Room Air Speed and Temperature

The measurements were conducted in August 2022. During this time, the classrooms were unoccupied and unfurnished (schools had closed for summer break). Thus, to simulate occupant’s heat load, 60 W incandescent lamps were distributed in the classroom and a 100 W was used for the teacher (See Figure 2). The choice on the rating of the incandescent lamps was in accordance with recommendation from ASHRAE fundamentals [42]. These lamps were used as heat sources to represent sensible heat generated by students and teachers, respectively.

Figure 2 shows the measurement setup and measurement positions in the classrooms. Room air speeds and temperature were measured with in-house constant temperature anemometer system (CTA88), points shown in Figure 2. The CTA probes were calibrated for an air speed measurement range of 0.05–3 m/s and had an air speed accuracy of 0.05 m/s and temperature range of 10–40 °C with a sensor accuracy of 0.2 °C. The sampling interval for all measurements was set to 60 s, with the response time of 0.2 s to 90% of a step change. Additionally, thermocouples were used to measure the surface temperatures and to complement the room air temperatures. T-type thermocouples (class 1) with a tip diameter of 1.5 mm and a factory accuracy of ±0.3 °C were used in surface temperature measurements in both the plenum and the classroom (shielded with ventilation tape to reduce the influence of air temperature). One central measurement point was chosen on each surface for the measurements. The thermocouples were calibrated in the range of 10–30 °C with 2 Hz frequency operation mode with an uncertainty of ±0.1 °C. A measurement resolution at heights of 0.1, 0.6, 1.1, 1.7, and 2.6 m was performed for all air speed and temperature measurements. The importance of the temperature measurements was to help create a picture of room conditions with thermal distribution and heat removal efficiency of the air distribution systems (DCV and MV) in the room.

2.2.2. Thermal Comfort Assessment

The measurements to estimate thermal comfort were performed with the thermal comfort data logger INNOVA 1221 [43]. The logger was managed on a computer through a software-INNOVA 7701 running on a thermal comfort code based on ISO 7730 [44]. The software enables coordination of the inbuilt modules and connected transducers to derive information and estimate PMV (Predicted Mean Vote) and PPD (Predicted Percentage of Dissatisfied), in the current study on PMV results are presented. The equipment has four transducers that measure the shielded room air temperature, air speed, humidity and dry heat loss. The temperature transducer measures with an accuracy ±0.2 °C and the velocity transducers with an accuracy of ±5%. Thermal comfort measurements were performed as stipulated in the standards [44,45]: at the height of 0.6 m from the floor for a sitting person and with a measurement interval of 3 min. 1.2 met and 0.75 clo. were used as personal input values for metabolic rate and clothing insulation, respectively.

2.2.3. Ventilation Efficiency Measurement

The tracer gas decay method was used to analyze ventilation performance and representative air quality indexes in the classrooms. SF₆ (sulfur hexafluoride) was used as a tracer gas. A total of 8 measuring points in the breathing zone were studied. These positions were kept the same and didn’t change in the room with mixing ventilation. During analysis, the indexes that were considered for air quality performance were:

• Local air change index which is the measure of how fast supplied-air reaches a specified point and replaces the old air in the room [27]. This is expressed as

$${\epsilon }_{p }^a=\frac{{\tau }_n}{{\tau }_p}$$

(1)

where ${\tau }_n$ is the nominal room time constant (h) and ${\tau }_p$ is the local mean age of air (h).

The local air change index evaluates air distribution and the quality of air at specific points in the room. Perfectly mixed cases occur when ${\tau }_{n }={\tau }_p$, ${\epsilon }_{p }^a=1$. When the local air change index is greater than 1, then the locational air quality is good, and the ventilation system is effective in contaminant removal. When the local air change index is less than one, it shows poor air quality or relatively old air retention. This implies the smaller the mean age of air, the longer the old air retention or the poorer the air quality. Thus, the local air change index can be used to ascertain the effectiveness of contaminant removal by the ventilation system.

b.

Air change efficiency: this is the measure of how efficiently the supply air is distributed and the room and it is defined by the expression:

$${\epsilon }^a=\frac{{\tau }_n}{2〈\tau 〉} \left(100\right) \left[\%\right]$$

(2)

where ${\tau }_n$ is the nominal room time constant (h) and $\tau$ is the room mean age of air (h).

The air change efficiency is important in understanding the mixing airflow patterns that exist in an occupied region. According to Kabanshi et al. [46], stratified indoor environments have air change efficiency greater than 50%; perfect mixing is equal to 50% and in poorly mixed environments it is less than 50%. Qualitative information about contaminant exposure can be provided using the air change efficiency. Lower contaminant exposure occurs when the air change efficiency is greater than 50% and higher contaminant exposure exists in the occupied space when the air change efficiency is less than 50%. Therefore, a ventilation system with a high level of efficiency can also efficiently remove contaminants [8].

2.2.4. Acoustic Measurements

In addition to “Ecophon Master E T15” sound absorbing ceiling tiles (0.6 m × 0.6 m × 0.04 m), “Ecophon Acousto Wall A” sound-absorbent wall panels were installed in the classrooms, covering 6.1 m² and 4.9 m² of the wall area for MV and DCV, respectively. The ceiling with acoustically active tiles provided the largest possible area of sound absorption in the classrooms and had the most substantial effects on the acoustic parameters in the room, lowering reverberation time and SPL in the rooms. Since DCV has no supply ventilation components in the classroom as opposed to mixing which has exposed supply diffusers into the classroom, it was predicted that the SPL/noise level due to mechanical noise from the ventilation system would be lower in the room with DCV than in the room with mixing ventilation.

The measurement equipment included an omnidirectional loudspeaker (B&K Type 4292), an omnidirectional microphone (4188-A-021), an amplifier (B&K), a microphone amplifier (B&K TYPE 1704-A-002), and measurement software (Dirac version 6). All measurements were made according to room acoustic standards SS-EN ISO 3382-2:2008 and SS-EN ISO 16032:2004.

3. Results and Discussions

3.1. Temperature Distribution

Table 1. Mean temperatures measured in the classrooms.

Measurement Points	DCV		MV
	Mean (°C)	SD	Mean (°C)	SD
Supply	18.1	0.03	18.3	0.03
Exhaust 1	23.1	0.05	22.0	0.09
Exhaust 2	22.6	0.06	21.9	0.06
Room temperature	23.0	0.08	22.2	0.07
Surface—Corridor wall	23.1	0.23	22.8	0.03
Surface—Board wall	23.6	0.01	22.7	0.04
Surface—Window wall	24.3	0.03	22.9	0.05
Surface—Back wall	24.0	0.02	22.8	0.03
Surface—ceiling	24.5	0.04	23.2	0.05
Surface—floor	24.0	0.03	22.7	0.04
Plenum—Corridor wall	20.9	0.02	-	-
Plenum—Window wall	20.0	0.02	-	-
Plenum—Board wall	20.1	0.03	-	-
Plenum—Back wall	19.2	0.02	-	-

shows the mean indoor temperature distribution. As shown, for similar supply conditions, the mean temperature in the room does not differ much although MV had lower temperatures than DCV, slightly cooler by 1 °C. This is consistent with the analysis of mean temperature measured at different locations and height in the room, shown in Figure 3. A closely stacked temperature profile illustrates and strengthens the uniform and homogenous temperature distribution in both classrooms, although this is more apparent under DCV with MV exhibiting a slightly more scattered profile. The results are in agreement with what was reported in earlier studies [3,24].

In DCV, buoyancy forces through the aid of heat sources are driving the airflow in the room, thus stratification should be expected. However, the measurement does not show a temperature gradient. This confirms that the temperature field under DCV does not differ to that under MV. Thus, both systems are performing as total volume dilution systems.

3.2. Air Speed Distribution

Figure 4 shows the measured local mean air speeds at different locations and heights in the room. The room air speed at the majority of all the measuring heights under DCV is below 0.2 m/s which follows ISO 7730 on recommended indoor air speeds in total volume dilution ventilation systems. Position E, which is in the center of the room, had varying air speeds at different heights while positions I and H both had high air speeds at a height of 0.1 m. It is not clear why there was a variation, but this can be attributed to the influence of convective heat flows from the lamps or measuring equipment.

Figure 5 illustrates the contour plots of the air speed distribution in the classrooms. As can be seen, the air speed distribution between the two ventilation systems shows a slightly different trend, especially at 0.1 m and 2.6 m height. Relatively high velocities were found at ankle level (0.1 m) under DCV and this could be a result of the heat source generating momentum flow due to buoyancy forces (this is expected to be higher close to heat loads). At a height of 1.1 m in the classroom with DCV (see also Figure 4), the center of the room had slightly high air speeds. In general, the DCV exhibits a more homogeneous distribution than mixing ventilation. Under MV, the air speeds are predominantly higher near the ceiling due to the inlet jets from the diffusers, the airflow exhibits ceiling attachment characteristics (Coanda effect) and seem to merge along the classroom asymmetry. The airflow then attaches to the front wall and flows to impinge on the floor. This is registered in the front of the classroom (near the teacher or board), and the air speeds begin to diminish as it flows inwards in the classroom. Thus, mixing ventilation has different air speeds at different heights from the floor in contrast to DCV which has more homogeneous conditions [3,24].

3.3. Local Heat Removal Efficiency (LHRE)

Figure 6 shows the local heat removal efficiency as a result of ventilation, defined as the ratio (T_E − T_S)/(T_L − T_S) where subscripts denote the following: E is exhaust, L is local point, and S is supply. The results show DCV as having a more uniform distribution when compared to mixing ventilation. This is because diffusion from the ceiling and buoyance forces in the classroom are interacting creating an evenly distributed dilution effect [29]. This shows that the DCV has cooling potential even at the lowest level (0.1 m) which is evenly distributed in the occupied zone.

The local heat removal efficiency and dilution capacity have a direct relationship with temperature and velocity distribution in the room. DCV having a more uniform temperature distribution when compared to mixing ventilation (see Figure 3) indicates better local heat removal or dilution capacity in the classroom. Mixing ventilation performance shows tendency to divert away from the room mean value at different room heights. The system design (type and location of supply diffusers) and high flowrates used could have affected the system performance of MV on heat removal and dilution capacity. This also means that performance of MV depends on the design and placement (balancing) of diffusers in the room. Thus, there is no room for error. This is not the case with DCV since all design constraints are accounted for in plenum as a unit supply diffuser. In the current mixing ventilation setup, perhaps the system can perform better under spiral cone diffusers and a VAV system. Ceiling attachment of airflow would be reduced, and airflow demands would depend on occupancy. In the current installation at the school, the MV system is disadvantaged on energy performance and will likely be the same on indoor air quality. Studies from Irwin et al. [47] explain that heat removal effectiveness and contaminants removal usually follows the same trend. This implies that from the result in Figure 6, DCV will have a high cooling capacity and contaminant removal effectiveness in an energy-efficient manner when compared to the conventional MV air distribution system. Overall, DCV demonstrates better homogeneous and complete dilution conditions.

3.4. Thermal Comfort Prediction

Figure 7 below shows the thermal comfort measurements of both MV and DCV. As shown, the boxplots of PMV measurements fall in Category I (shaded region), as stipulated in ISO 7730, under DCV as opposed to MV. The conditions in the room with DCV are predicted to be slightly warm while those under MV are slightly cool. This implies using DCV will likely have a lower percentage of people dissatisfied [29,32] while MV has a higher risk of draft perception especially on the teacher’s position which has high velocities. This agrees with the conclusion by Zhang and Heiselberg [29] who state that DCV has the properties of exhibiting a high thermal comfort level. It should be mentioned that both DCV and MV were in compliance with thermal comfort requirements as stipulated in the standards ISO 7730 and ASHRAE 55.

3.5. Ventilation Efficiency

Table 2. Local Air Change Index and Air Change Efficiency.

Ventilation	Local Air Change Index								Air Change Efficiency
	${\mathit{\epsilon }}_{\mathit{p}}^{\mathit{a}}=\frac{{\mathit{\tau }}_{\mathit{n}}}{{\overline{\mathit{\tau }}}_{\mathit{p}}}$								${\mathit{\epsilon }}^{\mathit{a}}=\frac{{\mathit{\tau }}_{\mathit{n}}}{2\mathit{·}〈\overline{\mathit{\tau }}〉}\mathit{·}100 \left[\mathit{\%}\right]$
	A2	A3	A4	A5	B2	B3	B4	B5
MV	1.09	1.07	0.99	0.95	1.02	1.00	0.97	1.03	53.85
DCV	1.07	1.08	1.03	1.07	1.05	1.03	1.05	1.04	48.60

below illustrates the ventilation effectiveness of both DCV and MV at all the measuring positions. The ventilation effectiveness of these two systems was measured based on the local air change index and air change efficiency. As mentioned earlier, the Air change index represents how fast the supply air reaches a specific point in the room and air change efficiency represents how supply air is efficiently distributed in the room. The DVC in all the measuring positions (Table 2) has a local air change index greater than one, which indicates a low concentration of contaminants in the room [27]. The MV at some measuring positions as seen in Table 2 also has a local air change index greater than one with the exception of positions A4, A5, and B4. This could be attributed to the positions not being in the direction of the supply airflow or being located in airflow stagnation regions, thus indicating non-uniformity of air and short-circuiting airflow in the room. This will result in regions having a high concentration of contaminants in the room. The results indicate better performance of DCV over MV and shows how the airflow fluid mechanics differ in the two systems [29].

According to Kabanshi et al. [46], satisfactory ventilation is obtained when air change efficiency is greater than 50%. Table 2 further illustrates the air change efficiency of the two systems. It was observed that the DCV has a better air change efficiency (53.85%) when compared to the mixing ventilation (48.60%). Though critics might suggest that these numbers do not necessarily imply the room has good indoor air quality, they can be used to illustrate a well-ventilated space [27,48].

In the era of the COVID-19 pandemic, an inadequate indoor environment may increase the risk of microbial infection in an enclosed environment. The recent pandemic has prioritized ventilation systems toward contaminant removal and dilution [49]. Sandberg [8,23] and Xu et al. [50] identified that ventilation is not just a significant strategy in the removal of indoor contaminants (e.g., viruses) and decreasing the risk of exposure in the indoor environment, but it is also a process of spreading contaminants and as such dilution becomes a critical aspect. A study by Li et al. [51] and Sandberg [8,23] demonstrated that the spread of airborne infectious diseases is strongly associated with ventilation rate and airflow. However, the system capacity to dilute and create homogeneous conditions are critical especially in high occupancy spaces because even though there is exposure, the potency of the virus or contaminants to cause infection is reduced. Thus, the MV system may be disadvantaged because of short circuiting ventilation, as also discussed in this study, and this promotes contaminant flooding in certain regions of the occupied zone [3,8]. This is demonstrated by Li et al. [52] who explained the probable airborne transmission of SARS-CoV-2 in the MV system in the Guangdong Province of China in 2020.

3.6. Room Acoustics

Table 3. Results on acoustic measurements.

Ventilation	T20 (s)		C50 (dB)	G (dB)	Sound Pressure Level dB (A) (30) *
	125 Hz (0.60) *	250 to 4000 Hz (0.50) *	250 to 4000 Hz (≥6) *	250 to 4000 Hz (≤19) *
DCV	0.72	0.65	5.1	17.6	30
MV	0.63	0.59	5.4 (≥6)	18.2	33

* The recommended values according to Swedish standard SS 25268, although it should be noted that 0.1 s higher values are permitted at 125 Hz octave band.

shows a comparison between the basic acoustic parameters in the DCV and MV rooms, including reverberation time (T₂₀), speech clarity (C₅₀), sound strength (G) and the A-weighted sound pressure level (SPL). The first three parameters did not differ much, and in some cases the classroom with MV ventilation performed better than DCV. This was to be expected since the classroom with MV had a slightly larger sound absorbing panel, so the basic acoustic parameters are better.

Consistent with our hypothesis that the ventilation noise would be higher in the MV room due to the presence of mechanical noise transported through the ducts, the noise level (SPL) in the MV room is 3 dB (A) higher. Notably, 3 dB corresponds with a doubling of the sound energy in the room [53].

The results indicate that the benefits of diffuse ceiling ventilation are not only for thermal comfort. Reducing the sound energy caused by ventilation noise by up to half in the classrooms with DCV is likely to have an effect on speech perception in the classroom due to the reduced masking action of the noise signal on the basilar membrane as the SPL level decreases [54]. Ventilation noise can be defined as a steady-state energetic masker due to its spectral and temporal characteristics, and while not as harmful to speech perception as informational maskers (i.e., masking sounds in which there is audible language content), energetic maskers have a marked effect on speech perception [47]. Lower levels of energetic masking, therefore, will likely provide additional benefits for communication and speech perception over and above those provided by the already-optimized acoustics in the rooms measured [55].

As large associated bodies of literature as well as acoustic standards and recommendations tailored for special listening needs suggests (e.g., BB93, BATOD [56]), the benefits of optimized acoustics and lower sound levels are even more pronounced for those for whom listening to speech in the presence of noise is a challenge, e.g., those learning in a second language [57], children with sensory [58] or cognitive impairments, as well as young children [59]. For these children, who are most often mainstreamed into schools alongside typically developing peers, the benefits of speech perception, communication, and academic performance improve in tandem with optimization of acoustic conditions in the classroom.

Additionally, it should be noted that although SPL was higher in the DCV room, the SPL levels in the unoccupied classrooms both met the requirements for classrooms in the Swedish Standards (SS 02 52 68), which are currently classified as Class C sound environments, i.e., meeting the minimum requirements of the Swedish building code by having unoccupied noise levels of under 45 dB. However, only the room with DCV met the requirements for Class B sound environments which ensures “high” acoustic quality in a room, which is a maximum unoccupied noise level of 30 dB.

4. Conclusions

This study compared and discussed the ventilation performances of diffused ceiling ventilation and mixing ventilation installed in a classroom of a newly renovated school in the southern part of Sweden. As shown, DCV demonstrated better dilution and uniformity in the airflow, temperature, thermal comfort, and noise performance characteristics. The room air speeds in both classrooms were predominantly below 0.2 m/s the recommended indoor air speeds, in accordance with ISO 7730. DCV is characterized by a low momentum supply which enables the system to be free of draughts, thus improving thermal comfort through higher heat removal efficiency and contaminant removal when compared to mixing ventilation. This is verified in the measured PMV of the diffused ceiling ventilation at all the measuring points falling between 0.1 to 0.15 and −0.15 to −0.24 for mixing ventilation. Both systems satisfy the requirements for thermal comfort under the measurement conditions, although the classroom with MV shows an increased risk for draft perception. On the system performance on reducing ventilation noise, DCV reduced the sound energy level by up to half that of mixing ventilation, thus contributing to better indoor environments for inclusive learning. A pre-occupancy study in an unfurnished room was performed herein. It would be interesting to perform and compare system performance in a full classroom and under different weather conditions. Additionally, a human subject study would provide insights on student’s perception of classrooms with DCV and MV. Overall, the current study shows that DCV can be implemented in existing mixing ventilation systems with minimal or no changes on the operation of the ventilation system. The ventilation performance is improved in terms of uniformity and heat removal with the benefits of reduced ventilation noise. We, however, recommend that further investigations are conducted to understand the performance under different weather conditions, variable air volume control, and a varying occupancy capacity. This could be innovative towards improving system performance for better indoor environmental quality in classrooms and low energy use.