Examining the Impact of Natural Ventilation versus Heat Recovery Ventilation Systems on Indoor Air Quality: A Tiny House Case Study

: Adverse health effects can arise from indoor air pollutants, resulting in allergies, asthma, and other respiratory problems among occupants. Concurrently, the energy consumption of residential buildings, particularly concerning heating, ventilation, and air conditioning (HVAC) systems, significantly contributes to global energy usage. To address these intertwined challenges, heat recovery ventilation (HRV) has emerged as a viable solution to reduce heating and cooling demands while providing fresh ventilation rates. This study aims to investigate the indoor air quality (IAQ) of an experimental tiny house building equipped with an HRV unit by simulating real-life scenarios contributing to IAQ. The research evaluates the effectiveness of HRV compared to natural ventilation in managing particle matter (PM), total volatile organic compounds (TVOC), formaldehyde (CH 2 O), carbon monoxide (CO), and carbon dioxide (CO 2 ) levels. This research significantly contributes to the understanding of the different ventilation strategies’ impact on IAQ in tiny houses and offers valuable insights for improving living conditions in a unique building typology that is underrepresented in the research literature.


Introduction
Ambient and indoor air pollution is a leading threat to human health on a global scale.World Health Organization (WHO) data show that 2.4 billion people are exposed to dangerous levels of indoor air pollution, while the combined effects of ambient and indoor air pollution are associated with 7 million premature deaths annually [1].Furthermore, according to the American Lung Association, over 135 million people in the U.S. live in places with inadequate or unhealthy levels of air pollution [2].
In the investigation of indoor air quality (IAQ) within residential settings, many studies scrutinize various factors unique to the building, in addition to the commonly assessed IAQ parameters.For instance, there is a significant focus on examining IAQ in renovated and retrofitted residential buildings [3][4][5][6], while other studies delve into the impact of the age of construction on IAQ [7][8][9][10].The construction type itself has also been a subject of studies [11,12] concerning its influence on IAQ, with recent attention directed toward exploring the IAQ levels in high-performance and green buildings [13][14][15][16][17].
Some studies, specifically those on high-performance houses, have gained attention in recent years due to their energy efficiency and potential impact on indoor air quality.
Plavenieks et al. [18] presented a case study on indoor air quality and energy efficiency in a passive house in Latvia, emphasizing the importance of construction elements and design in achieving optimal indoor air quality.Similarly, Wang et al. [19] discussed the interactions between energy performance and indoor environment quality in passive house buildings, emphasizing the cost-efficient nature of passive house standards in reducing energy consumption.
Furthermore, the use of computational fluid dynamics (CFD) in assessing air quality has been a growing area of research in recent years.Some studies have been conducted to investigate the impact of various factors on indoor air quality, particularly focusing on carbon dioxide levels and the use of CFD simulations.Sekhar et al. [20] explored the effectiveness of a purging system in an office building in Singapore to improve indoor air quality by monitoring pollutant concentration levels.Another notable study employed numerical simulations to analyze IAQ in a studio apartment equipped with a manually controlled mechanical exhaust system.The focus was on carbon dioxide levels as an indicator of air quality.The study found that CO 2 concentrations frequently exceeded recommended levels.The numerical simulations demonstrated a strong correlation with actual measurements, validating their accuracy in predicting indoor air quality [21].
In a particular study conducted by Doll et al. [22], the impact of weatherization on IAQ in low-income, single-family residential homes was investigated.The study involved testing indoor air before and after implementing weatherization measures.The results indicated a reduction in radon levels during the heating season, decreased relative humidity in homes without pets during the heating season, and decreased PM 1 and PM 2.5 levels in homes without pets.Nevertheless, the study also revealed elevated levels of CH 2 O during the cooling season in homes without pets, along with increased concentrations of particles >1.0 mm and PM 10 during the heating season in homes with pets.
In a separate study, investigators examined indoor air quality before and after energy upgrade retrofits in social housing.The findings revealed a correlation between elevated CO 2 , VOCs, and PM 2.5 concentrations and reduced building air exchange rates [12].In a similar study, researchers examined indoor air quality following the installation of a second heating system.The authors compared an old, uncertified wood furnace with a new gas furnace in a dual-heating setup.Interestingly, the study concluded that although the wood furnace did not pose an immediate health threat to occupants due to adequate ventilation, it presented significant risks of damage from overheating and potential environmental impacts [23].
In addition to retrofitted houses, studies have also been conducted on newly built homes.In the publication by researchers Derbez et al. [24], newly constructed energyefficient houses in France were examined, featuring mechanical ventilation with heat recovery (MVHR).Compared to standard French houses, these energy-efficient homes exhibited low concentrations of benzene, ethylbenzene, m-and p-xylenes, PM 2.5 , and radon, while CO 2 and CH 2 O levels showed no significant differences.Additionally, the MVHR system facilitated air changes per hour (ACH) levels of 0.5 h-1 or higher, contributing to less humid air.However, it is noteworthy that this study did not address or explore the potential additional effects of MVHR on the overall IAQ of the buildings.In another research investigation in Japan, VOCs in 5000 homes were scrutinized, revealing the prevalence of toluene, CH 2 O, and acetaldehyde as the primary indoor VOC [25].Various factors, such as outdoor concentrations, indoor and outdoor temperature, relative humidity, month of the year, duration of open windows, usage patterns of kerosene heaters, operation times, and the age of the building influenced the concentrations of indoor VOCs and inorganic gaseous pollutants.While the study acknowledged the impact of kerosene heaters on indoor VOC concentrations, it omitted examining how mechanical ventilation might influence these concentrations.A comparative study conducted in mainland France delved into examining fourteen distinct VOCs and PM, specifically PM 10 and PM 2.5 , across 567 residences.The examined indoor concentrations were then evaluated for correlations with building characteristics, including building materials and the year of construction.
Additionally, this study explored the relationship between ventilation type and IAQ test results.Notably, findings revealed that concentrations in a specific VOC, CH 2 O, were elevated in buildings equipped with mechanical ventilation systems [9].
Conversely, a different study examining ventilation systems in school buildings in Sweden yielded contrasting results.The indoor air pollution index integrating concentrations of multiple pollutants (CO 2 , CH 2 O, PM 10 , and PM 2.5 ) was significantly higher in buildings using natural or exhaust ventilation compared to the buildings using balanced mechanical ventilation systems with either constant air volume (CAV) or variable air volume (VAV) [26].These divergent findings underscore the significance of considering information about building use and type, ventilation methods, and the area's climate when conducting IAQ research.
In the realm of mechanical ventilation studies, notable attention has been directed toward energy recovery ventilators (ERVs) and HRVs in recent studies.In a recent study by Cho et al. [27], it was discovered that an ERV system featuring an integrated cooling coil lessened the building load and enhanced the air conditioner's performance.A different interesting investigation simulated the performance of a building equipped with a ventilation system, with and without an HRV, across fifteen different Canadian cities.The findings revealed that HRV usage consistently reduced annual heating energy consumption, although energy consumption might rise during cooling seasons [28].While these studies delve into the energy performance benefits of ERVs and HRVs, they do not address the impact of these systems on IAQ.Finally, a study conducted in Chicago examined the IAQ in 40 diverse homes before and after installing new mechanical ventilation systems.The mechanical systems assessed included a continuous balanced system with an ERV.The study revealed that the homes equipped with continuous balance with ERV demonstrated the most substantial mean differences in contaminant concentrations for various pollutants [29].This suggests the potential advantages of employing an ERV in ensuring satisfactory IAQ compared to regular mechanical ventilation systems.Bao et al. [30] conducted a study focusing on the effectiveness of HRVs in reducing indoor PM.The researchers specifically installed an extra filter within the HRV system to adequately lower PM 2.5 concentrations, aiming to meet the Chinese national standard threshold of 75 mg/m 3 [30].While the additional filter improved IAQ, it concurrently led to increased energy consumption.
An alternative approach to assessing the impact of mechanical ventilation on the IAQ of a building involves introducing a pollutant or engaging in an activity known to influence IAQ within the indoor environment during the investigation.A study assessing the influence of cooking on the IAQ of Colorado passive houses and tightly constructed residences observed that PM 2.5 concentrations were typically low indoors.However, cooking substantially elevated these concentrations.Moreover, the considerable increases in PM 2.5 persisted for a prolonged period, and no significant differences were detected when operating the ventilators in various modes [31].The study also revealed heightened levels of CH 2 O in all tightly constructed buildings.
The diverse studies discussed above underscore the multifaceted nature of IAQ research within residential settings.The investigations have delved into factors such as building construction, retrofitting, ventilation systems, and specific activities like cooking, shedding light on the intricate interplay between these elements and IAQ outcomes.While some studies reveal the positive influence of energy-efficient measures and mechanical ventilation systems [32], others highlight potential challenges, emphasizing the need for a nuanced understanding of the contextual factors.As we navigate through the subsequent sections of this paper, we aim to contribute to this evolving body of knowledge, examining the individual components and their synergistic effects on IAQ in the context of the specific typology of a tiny house building equipped with an HRV system.This research study focused on assessing the indoor air quality (IAQ) of a tiny house in San Antonio, Texas, under different ventilation scenarios and experimental activities.The tiny house, located in a semi-arid climate, was equipped with a mechanical ventilation system (HRV) and a mini-split air conditioning system.Using strategically placed sensors, the study monitored various pollutants, including CO 2 , CO, TVOCs, PM, and CH 2 O, over ten days.This experiment evaluated five ventilation scenarios as follows: no ventilation, three HRV modes, and natural ventilation.Two experimental activities, first a cooking activity and then an aerosol spray activity, were conducted to introduce pollutants into the indoor environment.This study aimed to understand the efficacy of ventilation methods in managing indoor air pollutant concentrations associated with everyday activities.Ten unique scenarios were executed, each combining a ventilation method with an experimental activity.Baseline concentrations were sampled before each experiment, and standardized procedures were followed for cooking and aerosol spray activities to assess their impact on IAQ.This study stands out for its comprehensive examination of various real-world ventilation strategies and pollutant sources within the context of a tiny house, offering valuable insights for enhancing indoor air quality (IAQ) in compact living environments.Additionally, this study's novelty lies in the unique typology of the building, which is underrepresented in the existing research, and the distinctive ventilation system installed in this structure.

Building Description
The tiny house investigated in this research (Figure 1) is situated in San Antonio, TX, USA, a city distinguished by a semi-arid climate typified by elevated summer temperatures and temperate winter conditions.The tiny house is located in Bexar County, which falls in ASHRAE climate zone 2A [33].The summer months consistently witness high temperatures surpassing 32 • C, while intermittent freezing conditions occur during the winter season, with low temperatures of 4 • C. The building stands out for its rectangular design, with measurements of 6 m for both front and rear sides and 3.6 m along the lateral dimensions, resulting in a total area of 22.3 m 2 .This compact residence's key features include dedicated living and cooking spaces, complemented by essential amenities like a bathroom and a modest storage compartment.The tiny house features a typical 5 × 10 cm wood stud construction wall with R-19 batt insulation, while the roof boasts batt R-38 insulation, and the floor incorporates R-19 rigid expanded polystyrene (EPS) insulation.The windows are of the standard vinyl variety, boasting a U-0.30 value and a solar heat gain coefficient (SHGC) of 0.25.The building features a mechanical ventilation system, incorporating an integrated HRV core.The technical specifications of the HRV are shown in Table 1.This HRV unit, configured as a through-wall fan and employing a ductless design, incorporates a ceramic regenerative heat exchanger.This ventilation system achieves heat recovery by coordinating the airflow direction and speed through the ceramic cores, periodically reversing the airflow allowing the ceramic cores to capture and release heat and optimizing interior climate conditioning with balanced ventilation [34].The tiny house also features a mini-split air conditioning (AC) system, providing a compact and efficient solution for climate control.It is composed of an outdoor condenser unit and an indoor evaporator unit integrated into the wall of the building, eliminating the need for additional ductwork.Complementing this setup, the tiny house is also equipped with a ceiling-placed fan, which is used synergistically with natural ventilation during weather conditions that do not require mechanical cooling.

Environmental Monitoring and Sensor Placement
This experiment was conducted between 26 November and 5 December 2023.Meteorological and indoor environmental conditions, including temperature, relative humidity, carbon dioxide (CO2), carbon monoxide (CO), total volatile organic compounds (TVOCs), formaldehyde (CH2O), and particulate matter (PM0.3,PM0.5, PM1, PM2.5, PM5, and PM10), were continuously monitored over those ten days, with data collected at 5 min intervals.Five different types of data sensors were used (Table 2) and strategically placed in the building (Figures 2 and 3).All devices were factory-calibrated according to the manufacturer's specifications.Additionally, the TVOC, CO, and CH2O sensors were manually user-calibrated right before the beginning of the experiment.Given the similar accuracy and resolution parameters of indoor and outdoor air temperature and relative humidity sensors, any errors or discrepancies among these sensors were deemed negligible.The outdoor temperature and relative humidity sensors were placed in positions, protected from direct solar radiation and rain.The PM, TVOC, CO, and CH2O monitoring devices were also positioned at an elevation of 0.5 m, following the method demonstrated by Pei et al. [36], and at a minimum distance of 1.5 m from all walls to minimize the particle dispersion effects on the device's readings [37].

Environmental Monitoring and Sensor Placement
This experiment was conducted between 26 November and 5 December 2023.Meteorological and indoor environmental conditions, including temperature, relative humidity, carbon dioxide (CO 2 ), carbon monoxide (CO), total volatile organic compounds (TVOCs), formaldehyde (CH 2 O), and particulate matter (PM 0.3 , PM 0.5 , PM 1 , PM 2.5 , PM 5 , and PM 10 ), were continuously monitored over those ten days, with data collected at 5 min intervals.Five different types of data sensors were used (Table 2) and strategically placed in the building (Figures 2 and 3).All devices were factory-calibrated according to the manufacturer's specifications.Additionally, the TVOC, CO, and CH 2 O sensors were manually user-calibrated right before the beginning of the experiment.Given the similar accuracy and resolution parameters of indoor and outdoor air temperature and relative humidity sensors, any errors or discrepancies among these sensors were deemed negligible.The outdoor temperature and relative humidity sensors were placed in positions, protected from direct solar radiation and rain.The PM, TVOC, CO, and CH 2 O monitoring devices were also positioned at an elevation of 0.5 m, following the method demonstrated by Pei et al. [36], and at a minimum distance of 1.5 m from all walls to minimize the particle dispersion effects on the device's readings [37].

Assessment of Current Regulations
The pollutants selected for the experiment include CO 2 , PM in multiple sizes, CO, TVOC, and CH 2 O. Exposure limits for all these pollutants are shown in Table 3.The choice of these pollutants was predominantly influenced by the criteria delineated in the indoor air quality guidelines of the World Health Organization [38,39].It is important to initially reference the available regulations and standards about these pollutants.This approach ensures a comprehensive understanding of indoor air quality levels.It facilitates an evaluation of the efficacy of the ventilation methods employed in the experiment to mitigate exposure to said pollutants.
Regarding CO 2 specifically, the most frequently encountered is 1000 ppm, representing a comfort-oriented standard articulated in ASHRAE standard 62 [40].Interestingly, CO 2 levels of 1000 ppm are not directly harmful but are generally considered an indication of poor ventilation [41].The 24 h time-weighted average (TWA) is the primary standard for assessing PM concentrations.Specifically, the National Ambient Air Quality Standards (NAAQS) define TWA limits of 150 µg/m 3 for PM 10 and 35 µg/m 3 for PM 2.5 [42].However, the NAAQS guideline addresses ambient PM concentrations, not indoor PM concentrations.At the time of this study, there are no U.S. national regulations for indoor PM concentrations in residential settings.However, a second standard for PM is particulates not otherwise regulated (PNOR).The Occupational Safety and Health Administration (OSHA) has established a 10 mg/m 3 8 h TWA and 15 mg/m 3 permissible exposure limit (PEL) for total dust [43].Regarding CO, the primary standard is the 8 h TWA limit of 35 ppm established by the National Institute of Occupational Safety and Health (NIOSH) [44].However, the OSHA has also established 25 ppm 8 h TWA as a limit for CO levels.
Concerning CH 2 O, the California OSHA has an established 0.75 ppm 8 h TWA and 2 ppm short-term exposure limit (STEL).Furthermore, for CH 2 O, California's NIOSH has also established a 0.016 ppm 10 h average limit and a 0.1 ppm ceiling (15 min) concentration [43].
Federal U.S. standards for VOCs are currently restricted to specific pollutants like CH 2 O and benzene.The U.S. Environmental Protection Agency (EPA) stated that "No federally enforceable standards have been set for VOCs in non-industrial settings" [45,46].In the absence of IAQ guideline values for TVOCs found in indoor air, the principle of as low as reasonably achievable (ALARA) provides a sensible procedure [47,48].

Description of Experimental Scenarios
Indoor air quality assessments were conducted employing the following five ventilation experimental scenarios: (i) no ventilation, (ii) HRV on mode 1, (iii) HRV on mode 2, (iv) HRV on mode 3, and (v) natural ventilation only.These ventilation modes represent different settings in which air exchange occurred during the indoor testing.Further details about each ventilation scenario are as follows: • Ventilation method (i): There was no intentional exchange of indoor and outdoor air, and the doors and windows remained shut.The HRV system was turned off; For each of these five ventilation methods, two experimental activities took place, whose purpose was to introduce pollutants into the indoor environment of the tiny house.These activities were a cooking activity and an aerosol spray activity.During these activities, an air quality monitoring investigation was conducted on five prevalent pollutants that impact IAQ, as follows: TVOC, PM, CO, CH 2 O, and CO 2 .Continuous measurements of these pollutants were taken at 5 min intervals.The selection of these pollutants was primarily guided by the parameters outlined in the World Health Organization's air quality guidelines [38,39].Recognizing the prevalence of indoor particle emissions from activities such as cooking or candle burning [49], these activities were employed to introduce particle emissions into the house environment.VOCs such as CH 2 O, benzene, polycyclic aromatic hydrocarbons, PM 2.5 , and ultrafine particles are the typical combustion gases related to these activities [50].Additionally, considering the established role of aerosol sprays as common sources of TVOC particle emissions [45], a standard air freshener aerosol spray was utilized to introduce VOC emissions into the house environment.Therefore, ten unique scenarios were executed (Table 4) within the confines of the tiny house structure, encompassing all five ventilation modes and the two experimental activities.Each experimental scenario spanned 12 h, from 2 p.m. to 2 a.m. the next day.The objective of these scenarios is to evaluate the efficacy of the various ventilation methods in managing the rise of indoor air pollutant concentrations typically associated with everyday activities like cooking and the use of common aerosol air fresheners.Before the aforementioned pollutants were introduced, for each of the ten experimental scenarios, the tiny house was aired by natural ventilation (window and door open) for 60 min to reset the indoor air environment so previous conditions did not impact the different experiments.Furthermore, baseline concentrations of the exterior pollutants were also sampled before each experiment.During the experiments, the occupancy of the tiny house was consistently limited to one person, the researcher conducting the experiment.
A standardized cooking procedure was executed to evaluate the performance of the residence and its ventilation system.Two pieces of ham, approximately 50 g in total, were fried in a 30-cm induction-ready stainless-steel skillet for 8 min (4 min on each side).Preheating for one minute preceded the cooking process on an induction hot plate (portable induction cooktop) set to 400 watts (W).Following the 8 min of cooking, the ham pieces were removed and covered with a paper towel.The induction surface and the pan were wiped down after a brief cooling period to remove the excess grease.This method was deliberately conducted to mimic a typical cooking scenario, allowing for the assessment of its impact on the building's IAQ.Regarding the second pollutant, air fresheners have been identified as contributors to elevated levels of various indoor air pollutants, particularly TVOCs [51].In the context of this experiment, an aerosol air freshener was employed.Specifically, five sprays were administered within two minutes, releasing the air freshener directly into the room's ambient air.This method was employed to simulate and assess the impact of aerosol air fresheners on IAQ, particularly focusing on the ensuing increase in TVOC concentrations.

Air Infiltration Assessment
In order to measure the air infiltration of the tiny house building, a blower door test was conducted.This infiltration test is, among others [52], the most reliable assessment of air infiltration in buildings.The blower door test quantitatively assesses the tiny house's airtightness by measuring the air exchanged per hour, expressed as air changes per hour (ACH).Conducting the blower door test (Figure 4) for the tiny house construction involved a methodical procedure to evaluate its air tightness, which is described as follows: Initially, all windows and doors were shut.Then, a specialized fan was installed at the exterior doorway, generating a pressure differential between the interior and exterior of the structure.This pressure contrast facilitates the measurement of air infiltration and exfiltration through any unintended leaks or openings in the building envelope.In particular, the fan operated at a standardized pressure of 50 Pascal (Pa), and a manometer was employed to gauge the air pressure inside and outside the house.The airflow rate necessary to sustain this pressure difference served as an indicator of the overall air tightness of the building.Two tests were conducted, one involving pressurizing the house and the other de-pressurizing it.Finally, the average of these measurements was utilized to determine the air infiltration for the building envelope at 5.4 ACH.For residential buildings in Texas, the IECC code [53] states that the minimum ACH should be 5 ACH tested at 50 Pa.Therefore, this shows that the building of this case study was not built at the building code standards in terms of air tightness.

Particulate Matter Readings during the Experiment
The results of the air monitoring experiment provide crucial insights into PM concentrations across various scenarios analyzed in this study, as depicted in Figure 5.The measured PM concentrations, presented in μg/m 3 for a 12 h duration in each of the 10 scenarios summarized in Table 4, facilitate a direct comparison with the OSHA permissible exposure limit (PEL) of 10,000 μg/m 3 for total dust [43].

Particulate Matter Readings during the Experiment
The results of the air monitoring experiment provide crucial insights into PM concentrations across various scenarios analyzed in this study, as depicted in Figure 5.The measured PM concentrations, presented in µg/m 3 for a 12 h duration in each of the 10 scenarios summarized in Table 4, facilitate a direct comparison with the OSHA permissible exposure limit (PEL) of 10,000 µg/m 3 for total dust [43].

Particulate Matter Readings during the Experiment
The results of the air monitoring experiment provide crucial insights into PM concentrations across various scenarios analyzed in this study, as depicted in Figure 5.The measured PM concentrations, presented in μg/m 3 for a 12 h duration in each of the 10 scenarios summarized in Table 4, facilitate a direct comparison with the OSHA permissible exposure limit (PEL) of 10,000 μg/m 3 for total dust [43].The aerosol experiment scenarios consistently yielded significantly higher peak concentrations than the cooking scenarios.For instance, in the absence of ventilation, the peak concentration during the aerosol experiment exceeded 2000 μg/m 3 , surpassing the cooking experiment peak of 900 μg/m 3 .Similar trends persisted across different ventilation modes, with HRV mode 1 registering peaks over 2000 μg/m 3 for the aerosol experiment, notably higher than the cooking experiment's corresponding value of approximately 550 μg/m 3 .Finally, the highest peak PM concentrations were observed during HRV mode 2 in the aerosol experiment, exceeding 6000 μg/m 3 .Despite these spikes, none of the concentrations breached the OSHA 10 mg/m 3 threshold.
During the experiments, which each spanned 12 h from 2 p.m. to 2 a.m. the next day, the time variable notably influenced the dynamics of PM concentrations within the tiny house environment.For particulate matter, spikes and subsequent drops were observed within the initial 2 h of the experiment, with elevated levels persisting above baseline for approximately 6 h after that.This pattern underscores the relatively rapid response of PM The aerosol experiment scenarios consistently yielded significantly higher peak concentrations than the cooking scenarios.For instance, in the absence of ventilation, the peak concentration during the aerosol experiment exceeded 2000 µg/m 3 , surpassing the cooking experiment peak of 900 µg/m 3 .Similar trends persisted across different ventilation modes, with HRV mode 1 registering peaks over 2000 µg/m 3 for the aerosol experiment, notably higher than the cooking experiment's corresponding value of approximately 550 µg/m 3 .Finally, the highest peak PM concentrations were observed during HRV mode 2 in the aerosol experiment, exceeding 6000 µg/m 3 .Despite these spikes, none of the concentrations breached the OSHA 10 mg/m 3 threshold.
During the experiments, which each spanned 12 h from 2 p.m. to 2 a.m. the next day, the time variable notably influenced the dynamics of PM concentrations within the tiny house environment.For particulate matter, spikes and subsequent drops were observed within the initial 2 h of the experiment, with elevated levels persisting above baseline for approximately 6 h after that.This pattern underscores the relatively rapid response of PM concentrations to environmental changes, suggesting a prompt but prolonged impact on indoor air quality following pollutant introduction.The observed duration of elevated PM levels highlights the need for effective ventilation strategies to expedite the reduction of PM concentrations and maintain optimal indoor air quality over an extended period.
The air quality monitoring results are also presented in terms of an 8 h time-weighted average (TWA) in Figure 6, to be directly comparable to the OSHA PNOR limit of 15 mg/m 3 (8-h PEL) [43].PM is presented in micrograms per cubic meter (µg/m 3 ) for different particle size fractions (0.5, 1, 2.5, 5, 10 µm) as well as in total particulate matter (TPM).The graph is split into two parts, depicting the cooking experiment on the left and the aerosol experiment on the right.The results for all the conducted experiments under different ventilation conditions, namely no ventilation, HRV mode 1, HRV mode 2, HRV mode 3, and natural ventilation, are presented as follows.
Upon reviewing the outcomes of the cooking experiment, it is evident that, as anticipated, the data indicate the highest total particulate matter (TPM) levels in the absence of ventilation (refer to Figure 6), peaking at 167 µg/m 3 .Moreover, that trend continues, with the PM values across all particle sizes for the cooking experiment with no ventilation, whereas using mechanical ventilation (HRV modes 1, 2, and 3) showed generally lower TPM values.Surprisingly, HRV mode 1, which provides the least ventilation among the HRV modes, namely 10 CFM, showed the lowest TPM concentrations at 55 µg/m 3 .Finally, the data show that natural ventilation provided lower TPM concentrations than no ventilation (142 µg/m 3 ) but was not as effective as any of the active HRV systems for the cooking experiment.
mg/m 3 (8-h PEL) [43].PM is presented in micrograms per cubic meter (μg/m 3 ) for different particle size fractions (0.5, 1, 2.5, 5, 10 μm) as well as in total particulate matter (TPM).The graph is split into two parts, depicting the cooking experiment on the left and the aerosol experiment on the right.The results for all the conducted experiments under different ventilation conditions, namely no ventilation, HRV mode 1, HRV mode 2, HRV mode 3, and natural ventilation, are presented as follows.Upon reviewing the outcomes of the cooking experiment, it is evident that, as anticipated, the data indicate the highest total particulate matter (TPM) levels in the absence of ventilation (refer to Figure 6), peaking at 167 μg/m 3 .Moreover, that trend continues, with the PM values across all particle sizes for the cooking experiment with no ventilation, whereas using mechanical ventilation (HRV modes 1, 2, and 3) showed generally lower TPM values.Surprisingly, HRV mode 1, which provides the least ventilation among the HRV modes, namely 10 CFM, showed the lowest TPM concentrations at 55 μg/m 3 .Finally, the data show that natural ventilation provided lower TPM concentrations than no ventilation (142 μg/m 3 ) but was not as effective as any of the active HRV systems for the cooking experiment.
Overall, higher PM concentrations were observed for the aerosol experiment results than in the cooking experiment.Among the aerosol scenarios, the highest PM concentrations were monitored using HRV mode 2, reaching a TPM of 475 μg/m 3 .Furthermore, HRV mode 1 and 3 concentrations were close at 265 μg/m 3 and 259 μg/m 3 TPM, respectively.Surprisingly, the scenario that eliminated ventilation was found to be more effective at mitigating PM concentrations (192 μg/m 3 ) than the scenarios utilizing HRV in the case of the aerosol experiment.That could result from the HRV airflow leading to particle resuspension inside the environment of the tiny house.Last but not least, natural ventilation was more effective at lowering PM concentrations than no ventilation (186 μg/m 3 ).These surprising findings demonstrate that implementing HRV systems may not consistently enhance IAQ in all building types and scenarios.Factors such as building design, air tightness, pollutant sources, and HRV unit placement may influence the effectiveness of Overall, higher PM concentrations were observed for the aerosol experiment results than in the cooking experiment.Among the aerosol scenarios, the highest PM concentrations were monitored using HRV mode 2, reaching a TPM of 475 µg/m 3 .Furthermore, HRV mode 1 and 3 concentrations were close at 265 µg/m 3 and 259 µg/m 3 TPM, respectively.Surprisingly, the scenario that eliminated ventilation was found to be more effective at mitigating PM concentrations (192 µg/m 3 ) than the scenarios utilizing HRV in the case of the aerosol experiment.That could result from the HRV airflow leading to particle resuspension inside the environment of the tiny house.Last but not least, natural ventilation was more effective at lowering PM concentrations than no ventilation (186 µg/m 3 ).These surprising findings demonstrate that implementing HRV systems may not consistently enhance IAQ in all building types and scenarios.Factors such as building design, air tightness, pollutant sources, and HRV unit placement may influence the effectiveness of mechanical ventilation in reducing air pollutant levels.This underscores the need for further research to understand IAQ's multifaceted nature and how it relates to HRV systems.
To summarize the aerosol experiment results, HRV modes 1, 2, and 3 all significantly showed increased PM levels, with HRV mode 2 exhibiting the highest TPM concentrations at 475 µg/m 3 .Admittedly, natural ventilation outperformed all the other ventilation methods in the aerosol experiment, showing generally lower PM values of all sizes compared to no ventilation.
Overall, looking at all the PM results presented in the 8 h TWA, some repeating patterns can be noticed as follows: During the cooking experiment, the results indicate that the ventilation modes, especially HRV mode 1, demonstrate effectiveness in reducing PM levels.However, modes 2 and 3, which offer more ventilation (15 and 20 CFM), were not more effective at lowering PM concentrations.This pattern can be seen for both the cooking and the aerosol experiment, which showcases that a higher mechanical ventilation rate did not result in lower PM concentrations.Another consistent trend observed across the experiments is that natural ventilation scenarios yield some improvement compared to no ventilation, albeit not significantly.For instance, during the cooking experiment, natural ventilation resulted in a 15% reduction in TPM compared to no ventilation.Similarly, during the aerosol experiment, natural ventilation led to a mere 3% reduction in TPM compared to no ventilation.This marginal improvement might be attributed to the building's inherent leakiness (with an air change rate of 5.4 ACH), which allows for some air exchange through its envelope.
It is also important to note the results of the airtightness of the tiny house building in relation to the monitoring results of these experiments.The 5.4 ACH shows that the building could be considered relatively "leaky", meaning that many uncontrolled air changes happen without purposefully using natural or mechanical ventilation.Due to the nature of the leaky building, the HRV system may not have consistently enhanced the indoor air quality (IAQ) of the building during the everyday activities tested.Other researchers, such as Militelo et al. [31], have found parallel results.The authors of [31] demonstrated that the considerable increases in PM 2.5 showed no significant differences when operating mechanical ventilation in various modes.More studies are needed to compare the results of this study to a more airtight tiny house building equipped with an HRV system.Finally, none of these scenarios exceeded OSHA's limit of 15 mg/m 3 (8-h PEL) [43] for total dust.The researchers acknowledge that this regulation is not meant for residential scenarios but is referenced as the closest relative regulation for PM in indoor environments currently in the U.S.

Total Volatile Organic Compounds Concentrations
While no national legislation exists on maximum indoor TVOC concentrations in the U.S., researchers widely agree that elevated TVOC levels indicate poor air quality [45,47].Figure 7 illustrates the results of the experiments, displaying TVOC concentrations.The cooking experiment generally exhibited no significant TVOC spikes, aligning with expectations, as cooking is known to increase PM rather than VOCs.An exception occurred in the cooking experiment with HRV 1, in which a spike to 300 ppb was observed, which was potentially attributed to external factors like researcher movement rather than the cooking itself.Conversely, the aerosol spray experiment consistently demonstrated substantial TVOC concentration increases, reaching a peak of 560 ppb in the no ventilation scenario (scenario 6).Notably, HRV mechanical ventilation, particularly in mode 3, proved effective in reducing TVOC concentrations during the aerosol experiment, outperforming the other HRV modes and reaching a ceiling of 260 ppb.During the natural ventilation scenario, peak TVOC concentrations reached 274 ppb, which is only a 3% increase over the case of the HRV at mode 3.  The data show that the effectiveness of the HRV system in reducing pollutant concentrations during the aerosol experiment varied depending on the type of pollutant.We previously saw that the PM monitoring results exhibited how the use of HRV was not as effective as natural ventilation at lowering PM levels in the environment of the tiny house.Notably, in the case of TVOCs, the HRV system was more effective in reducing concentrations, suggesting its efficacy in managing this particular type of pollutant.This discrepancy in effectiveness underscores the importance of considering the specific nature of contaminants and their sources when designing and implementing ventilation systems.It may be necessary to tailor ventilation strategies to address the particular characteristics of the pollutants in a given environment.Additionally, this observation highlights the complexity of IAQ management and the need for a nuanced approach to ventilation systems for comprehensive pollutant control.
During each experiment, which spanned 12 h from 2 p.m. to 2 a.m. the next day, the time variable notably influenced the dynamics of TVOC concentrations within the tiny house environment.Looking at the TVOC concentrations, while maximum spikes oc- The data show that the effectiveness of the HRV system in reducing pollutant concentrations during the aerosol experiment varied depending on the type of pollutant.We previously saw that the PM monitoring results exhibited how the use of HRV was not as effective as natural ventilation at lowering PM levels in the environment of the tiny house.Notably, in the case of TVOCs, the HRV system was more effective in reducing concentrations, suggesting its efficacy in managing this particular type of pollutant.This discrepancy in effectiveness underscores the importance of considering the specific nature of contaminants and their sources when designing and implementing ventilation systems.It may be necessary to tailor ventilation strategies to address the particular characteristics of the pollutants in a given environment.Additionally, this observation highlights the com-plexity of IAQ management and the need for a nuanced approach to ventilation systems for comprehensive pollutant control.
During each experiment, which spanned 12 h from 2 p.m. to 2 a.m. the next day, the time variable notably influenced the dynamics of TVOC concentrations within the tiny house environment.Looking at the TVOC concentrations, while maximum spikes occurred within the first 2 h, elevated levels endured for an extended duration.Notably, in the no ventilation scenario of the aerosol experiment, elevated TVOCs persisted for about 5 h.HRV mode 1 marginally accelerated the decrease in TVOC concentrations, while HRV mode 2 and natural ventilation required approximately 2 h for a significant drop.HRV mode 3 exhibited the most rapid decline, with concentrations decreasing considerably within 30 min.This elucidates the importance of ventilation modes in promptly mitigating pollutant levels, underscoring the efficacy of HRV mode 3 in rapidly improving IAQ conditions within the tiny house environment.

Measurements of Formaldehyde
In the air monitoring experiments, CH 2 O concentrations were found to remain consistently low, ranging from 15 to 20 ppb (Figure 8), well below the established regulations by the California OSHA and NIOSH (Table 3).The cooking experiment with HRV mode 3 exhibited two notable spikes in CH 2 O concentrations, reaching 87 and 83 ppb, respectively.However, even with these significant spikes, the concentrations did not exceed the NIOSH ceiling limit of 0.1 ppm (100 ppb).These results indicate a generally low-risk exposure to formaldehyde under monitored conditions, aligning with best indoor air quality and safety practices.The regulatory limits provided by Cal-OSHA and NIOSH serve as critical benchmarks for occupational exposure, and the consistent readings well below these limits suggest the effective control of formaldehyde emissions in the studied environments.
These spikes during the cooking experiment suggest that certain activities, such as cooking, can temporarily elevate formaldehyde levels indoors.This could be attributed to the combustion processes and formaldehyde release from cooking materials.The HRV system's mode of operation likely influenced the dispersion and ventilation of formaldehyde, allowing these spikes to occur.Despite the transient increases in formaldehyde concentration, the overall levels remained compliant with regulatory standards.This indicates that the HRV system, even in mode 3, is generally effective in maintaining safe indoor air quality.This finding underscores the importance of continuous air monitoring, particularly in environments in which formaldehyde-emitting activities are performed, to ensure that even short-term exposures remain within safe limits.

Concentration of Carbon Monoxide
As for CO, the primary standards set by NIOSH (35 ppm) and OSHA (25 ppm) were not surpassed, with concentrations remaining stable between 12 and 14 ppm throughout the majority of the experiments (Figure 9).During the cooking experiment, the absence of These results indicate a generally low-risk exposure to formaldehyde under monitored conditions, aligning with best indoor air quality and safety practices.The regulatory limits provided by Cal-OSHA and NIOSH serve as critical benchmarks for occupational exposure, and the consistent readings well below these limits suggest the effective control of formaldehyde emissions in the studied environments.
These spikes during the cooking experiment suggest that certain activities, such as cooking, can temporarily elevate formaldehyde levels indoors.This could be attributed to the combustion processes and formaldehyde release from cooking materials.The HRV system's mode of operation likely influenced the dispersion and ventilation of formaldehyde, allowing these spikes to occur.Despite the transient increases in formaldehyde concentration, the overall levels remained compliant with regulatory standards.This indicates that the HRV system, even in mode 3, is generally effective in maintaining safe indoor air quality.This finding underscores the importance of continuous air monitoring, particularly in environments in which formaldehyde-emitting activities are performed, to ensure that even short-term exposures remain within safe limits.

Concentration of Carbon Monoxide
As for CO, the primary standards set by NIOSH (35 ppm) and OSHA (25 ppm) were not surpassed, with concentrations remaining stable between 12 and 14 ppm throughout the majority of the experiments (Figure 9).During the cooking experiment, the absence of ventilation showed a unique trend, with CO levels consistently remaining at lower concentrations.This could be attributed to the lack of airflow, preventing the dispersal of CO into the surrounding environment.The absence of gas ovens in the cooking method, which utilized an induction surface instead, most likely contributed to the lack of a significant rise in CO levels.Additionally, CO concentrations did not exhibit notable responses to the various ventilation methods employed in the experiment.

Readings of Carbon Dioxide
The CO2 concentration data unveiled intriguing findings, showcased in two distinct graphs corresponding to each conducted experiment (Figure 10).CO2, a product of exhalation, typically poses no threat below the recognized limit of 1000 ppm and is commonly used as an indicator of insufficient ventilation [33].Despite the tiny house remaining unoccupied for most of the experiment, the brief presence of one researcher for approximately 1 h per scenario resulted in CO2 concentrations surpassing 1000 ppm in certain instances, with a notable peak at 1600 ppm.This threshold was momentarily exceeded in 3 out of the 10 experiments (Figure 10).The results confirmed the anticipated relationship between elevated ventilation levels and accelerated reduction in CO2 concentrations, underscoring the significance of implementing effective ventilation strategies for maintaining IAQ.

Readings of Carbon Dioxide
The CO 2 concentration data unveiled intriguing findings, showcased in two distinct graphs corresponding to each conducted experiment (Figure 10).CO 2 , a product of exhalation, typically poses no threat below the recognized limit of 1000 ppm and is commonly used as an indicator of insufficient ventilation [33].Despite the tiny house remaining unoccupied for most of the experiment, the brief presence of one researcher for approximately 1 h per scenario resulted in CO 2 concentrations surpassing 1000 ppm in certain instances, with a notable peak at 1600 ppm.This threshold was momentarily exceeded in 3 out of the 10 experiments (Figure 10).The results confirmed the anticipated relationship between elevated ventilation levels and accelerated reduction in CO 2 concentrations, underscoring the significance of implementing effective ventilation strategies for maintaining IAQ.
Both experiments demonstrate significant peaks in CO 2 concentrations shortly after 3 p.m., which correspond to the presence of a researcher or activity within the tiny house.The data clearly show that ventilation strategies (HRV 1, HRV 2, HRV 3, and natural ventilation) are effective in reducing CO 2 concentrations more quickly than the no ventilation scenario.Among the HRV modes, HRV 2 appears to be particularly effective, maintaining lower CO 2 levels throughout the monitoring period.In all ventilation scenarios, CO 2 levels decline steadily after the initial peak, highlighting continuous ventilation's importance in maintaining indoor air quality.Although HRV 1 has the highest initial peak, it shows a consistent decrease, underscoring that even though peaks can be high due to certain activities, proper ventilation can effectively manage and lower these levels over time.Natural ventilation shows effectiveness comparable to mechanical HRV systems, indicating that natural ventilation can be a viable option for maintaining air quality, although it may depend on external conditions like weather and the ability to open windows.occupied for most of the experiment, the brief presence of one researcher for approxi-mately 1 h per scenario resulted in CO2 concentrations surpassing 1000 ppm in certain instances, with a notable peak at 1600 ppm.This threshold was momentarily exceeded in 3 out of the 10 experiments (Figure 10).The results confirmed the anticipated relationship between elevated ventilation levels and accelerated reduction in CO2 concentrations, underscoring the significance of implementing effective ventilation strategies for maintaining IAQ.Both experiments demonstrate significant peaks in CO2 concentrations shortly after 3 PM, which correspond to the presence of a researcher or activity within the tiny house.The data clearly show that ventilation strategies (HRV 1, HRV 2, HRV 3, and natural ventilation) are effective in reducing CO2 concentrations more quickly than the no ventilation scenario.Among the HRV modes, HRV 2 appears to be particularly effective, maintaining lower CO2 levels throughout the monitoring period.In all ventilation scenarios, CO2 levels The findings underscore the importance of ventilation in small, confined spaces like tiny houses.Even a minimal human presence can rapidly elevate CO 2 levels to potentially uncomfortable or unhealthy levels, suggesting that these environments are susceptible to occupancy changes.While CO 2 typically poses no health threat at levels below 5000 ppm, its concentration can serve as a proxy for the presence of other pollutants and the overall effectiveness of the ventilation system.High CO 2 levels can indicate a potential buildup of different contaminants, such as volatile organic compounds (VOCs) or particulate matter, generated from activities like cooking or household products.The regular monitoring of CO 2 can be a practical method for assessing indoor air quality.By maintaining CO 2 levels below 1000 ppm, it is likely that other pollutants are also being kept in check, contributing to a healthier indoor environment.

Temperature and Relative Humidity Levels
In the air monitoring experiment, the indoor and outdoor temperature and relative humidity were closely observed.The HRV unit, designed to regulate indoor comfort by providing fresh, ventilated air and removing stale air, demonstrated its effectiveness.Despite the considerable variability in outdoor relative humidity, indoor levels remained more stable overall (Figure 11).Specifically, in all the HRV scenarios, the indoor relative humidity remained between 50 and 60%.The indoor temperature also exhibited less variability (15-20 • C) than outdoor temperature fluctuations (8-22.5 • C) throughout the day.The findings suggest that the HRV unit contributes to maintaining consistent indoor conditions.The graphs highlight the importance of ventilation systems in regulating temperature and humidity in indoor environments.HRV systems effectively stabilize indoor conditions compared to no ventilation or natural ventilation alone.The consistent decrease in relative humidity underscores the need for balanced ventilation strategies to maintain comfort and air quality, especially in more tightly sealed environments.Further studies could explore seasonal variations and longer-term impacts on indoor environmental conditions, which could provide additional insights into overall indoor humidity levels and temperature regulation.

Conclusions
This study on a tiny house building in San Antonio, Texas, sheds light on indoor air quality (IAQ) management through different ventilation scenarios and activities.Results unveil varied impacts of ventilation methods on pollutant concentrations.These findings underscore the need for tailored ventilation strategies to enhance IAQ in compact dwellings [54].In this section, we will present key conclusions drawn from this study.

Conclusions
This study on a tiny house building in San Antonio, Texas, sheds light on indoor air quality (IAQ) management through different ventilation scenarios and activities.Results unveil varied impacts of ventilation methods on pollutant concentrations.These findings underscore the need for tailored ventilation strategies to enhance IAQ in compact dwellings [54].In this section, we will present key conclusions drawn from this study.
This study highlights the critical importance of quantifying air tightness.The results indicate that the building may be relatively "leaky", with an air change rate of 5.4 ACH.For residential buildings in Texas, the IECC code mandates a minimum ACH of 5 ACH tested at 50 Pa, indicating that the building in this case study does not meet the code standards for air tightness.
Regarding particulate matter (PM) concentrations, different ventilation scenarios had varying impacts during the cooking and aerosol experiments.HRV mode 1 effectively reduced PM levels during the cooking experiment, while natural ventilation was more effective during the aerosol experiment.The aerosol scenarios consistently showed elevated PM concentrations, particularly in HRV mode 2, that exceeded 6000 µg/m 3 .
Total volatile organic compound (TVOC) concentrations also varied based on ventilation methods, especially during the aerosol experiment.The HRV system, particularly in mode 3, significantly reduced TVOC concentrations, outperforming natural ventilation.Formaldehyde (CH 2 O) concentrations remained consistently low, with notable spikes during the cooking experiment under HRV mode 3.
Carbon monoxide (CO) concentrations did not exceed NIOSH and OSHA standards, likely due to the absence of gas ovens in the cooking method, contributing to stable CO levels.However, CO 2 concentrations surpassed 1000 ppm in some instances, highlighting the need for effective ventilation.The results confirmed the expected relationship between higher ventilation levels and faster decreases in CO 2 concentrations, emphasizing the importance of ventilation strategies for maintaining indoor air quality (IAQ).
During the 12 h experiment from 2 p.m. to 2 a.m., the influence of time on various pollutant concentrations in a tiny house environment was notable.Initially, PM concentrations spiked and then gradually decreased over approximately six hours, suggesting a swift yet prolonged impact on indoor air quality.In contrast, TVOC concentrations spiked within the first two hours and remained elevated for up to five hours without ventilation.Different ventilation modes showed varying effectiveness in reducing TVOC concentrations, with HRV mode 3 demonstrating the most rapid decline within just 30 min.These findings underscore the importance of ventilation strategies in promptly improving indoor air quality in tiny house environments.
Finally, the HRV unit proved effective in regulating indoor temperature and relative humidity, contributing to more stable conditions compared to outdoor fluctuations.Long-term studies and seasonal variations should be considered to gain a comprehensive understanding of indoor humidity levels and temperature regulation, particularly with natural ventilation.
Based on the results presented in this study, several conclusions can be drawn as follows: • Overall, this study's findings suggest that while HRV systems may be effective in certain scenarios, their effectiveness can vary depending on the type of experiment and pollutant being considered; • This study indicates that different ventilation methods may have varying impacts on different types of pollutants, emphasizing the need for a nuanced approach to IAQ management; • The building's air tightness may impact the effectiveness of ventilation systems in improving IAQ, highlighting the need for further studies in different building conditions; • Mechanical ventilation strategies, especially HRV mode 1, proved effective in reducing PM during cooking activities.However, a higher rate of mechanical ventilation through the HRV was not more effective in lowering concentrations of PM; • Mechanical ventilation strategies consistently proved effective in reducing TVOC concentrations during the aerosol experiment;

•
This study calls for further research, especially in more airtight buildings equipped with HRV, to better understand the impact of ventilation on IAQ during everyday activities.
In summary, this research provides valuable insights into the effectiveness of ventilation methods in managing IAQ, highlighting the importance of considering specific pollutants and building characteristics in designing ventilation strategies.This study makes a substantial contribution to enhancing our comprehension of IAQ performance within compact dwellings featuring HRV systems.This study provides valuable insights for improving living conditions in a unique building typology that has been insufficiently explored in existing research.It stands out for its comprehensive examination of various real-world ventilation strategies and pollutant sources within the context of a tiny house, offering significant findings for enhancing indoor air quality (IAQ) in compact living environments.Additionally, this study's novelty is underscored by the distinctive typology of the building and the innovative ventilation system installed, both of which are underrepresented in the current literature.

Figure 1 .
Figure 1.Exterior and interior views of the tiny house and HRV unit.

Figure 1 .
Figure 1.Exterior and interior views of the tiny house and HRV unit.

Figure 2 .
Figure 2. Tiny house plan and sensor placement.

Figure 2 .
Figure 2. Tiny house plan and sensor placement.

Figure 2 .
Figure 2. Tiny house plan and sensor placement.

Figure 4 .
Figure 4. Blower door test of the tiny house building envelope.

Figure 4 .
Figure 4. Blower door test of the tiny house building envelope.

Figure 5 .
Figure 5. PM concentrations during the experiments.

Figure 5 .
Figure 5. PM concentrations during the experiments.

Figure 6 .
Figure 6.Eight-hour TWA of the PM concentrations during the experiments.

Figure 6 .
Figure 6.Eight-hour TWA of the PM concentrations during the experiments.

Figure 9 .
Figure 9. CO concentrations during the experiments.

Figure 11 .
Figure 11.Indoor and outdoor air temperature and relative humidity.

Figure 11 .
Figure 11.Indoor and outdoor air temperature and relative humidity.

Table 2 .
Characteristics of the monitoring devices.

Table 3 .
Tested pollutants and their regulatory limits.