The importance of balanced ventilation

by sadia_badhon | December 5, 2019 8:46 am

By Hans Schleibinger, PhD, Boualem Ouazia, PhD, Doyun Won, PhD, and Daniel Aubin, PhD

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Adequate ventilation is necessary to ensure acceptable indoor air quality (IAQ) and remove pollutants originating from building materials, consumer products, and human activities. When air enters a structure, it is usually a mixture of ventilation supply air and infiltrating air entering through cracks in the building envelope, such as outdoor walls. The amount of infiltrating air depends on the leakiness of the building, and is driven by what is often a negative pressure in homes relative to the atmospheric pressure.

Exhaust flows, along with the airtightness of buildings, can often generate small levels of house depressurization, resulting in a negative pressure difference between a building and the outside (read “Review of residential ventilation technologies” by M. Russel). These small pressure differences, typically up to 5 Pa (0.10 psf) cannot be felt by humans, but are enough to generate airflow and draw outdoor air into the building. Air, on its way into a facility, may pick up contaminants from unoccupied zones like attached garages or attics. In addition to the negative pressure difference between a house and the outside, Canadian homes are often at a lower or negative pressure compared to the attached garages and attics (refer to “Characterizing the cold start exhaust and hot soak evaporative emission for the test vehicle for the attached garage study” by L. Graham, ERMD Report #99-26768-1, Environment). This means contaminants from these spaces are being carried into occupied areas.

The negative pressure differential may also be due in part to the use of unbalanced mechanical ventilation systems as well as the so-called ‘stack effect,’ produced by the often large differences between indoor and outdoor temperatures. The stack effect is strongest in winter (refer to “Characterizing the cold start exhaust and hot soak evaporative emission for the test vehicle for the attached garage study” by L. Graham, ERMD Report #99-26768-1, Environment). Much like in a chimney, the stack effect causes air to be drawn up and outward, therefore, air is drawn into the home from other zones. As the purpose of ventilation is to remove contaminants from indoor spaces by displacing the infected, stale air with fresher outdoor air, the infiltration of contaminants driven by negative pressures needs to be minimized.

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Benefits of balanced ventilation

Balancing ventilation is considered the ideal way to reduce infiltration of potentially harmful gases and particles into a building (consult “Heat recovery ventilators prevent respiratory disorders in Inuit children” by T. Kovesi, C. Zaloum, C. Stocco, D. Fugler, R. Dales, A. Ni, N. Barrowman, N.L. Gilbert, and J.D. Miller). Balancing is achieved by providing the same rate of supply and exhaust air to prevent the pressurization or depressurization of zones in the home, compared to outdoor atmospheric pressure (the effect of the wind on the outside walls will also affect the pressurization between the zones depending on the direction of wind and location of zones). Balanced ventilation (i.e. a close-to-zero pressure differential between building zones) can be achieved through two adjustable fans in the system.

The benefit of balanced ventilation in improving IAQ becomes obvious when comparing it directly with exhaust-only systems. In order to ensure a fair assessment, and to compensate for weather and other environmental effects, the National Research Council Canada (NRC) Canadian Centre for Housing Technology[3] (CCHT) performed comparison tests in two full-scale research houses built side-by-side at the NRC’s Ottawa campus.

Balanced ventilation was superior when it came to removing potentially harmful gases being emitted in indoor environments. With this approach, the team found a significant reduction of gaseous contaminants, up to 70 per cent for formaldehyde, 30 per cent for the volatile compound a-pinene, and up to 28 per cent for toluene, when compared to an exhaust-only ventilation approach. This effect can be explained by the fact the supply air comes directly and only from the outside when using the balanced ventilation approach, whereas in the exhaust-only approach, part of the air enters through other (non-occupied) spaces, from where the air could potentially  pick up contaminants.

To minimize pressure-driven air infiltration, the pressure indoors should always be equal to the atmospheric pressure around the home, but not lower. As the pressure differences outdoor versus indoor fluctuate over time, the project team chose a slightly positive pressure—approximately 2 Pa (0.04 psf) higher inside—to ensure the differences were not slipping into a negative zone for too much, and too long. As can be seen in Figure 1, the research house operating with balanced ventilation could stay mostly in the positive pressure zone with this approach, with only a few short spikes into the negative zone (black line). The research house operated with an exhaust-only system showed typical negative pressure differences, usually from –1 to –3 Pa ( –0.02 to –0.06 psf), with the observation of larger negative pressure spikes (red line).

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The NRC study also showed the overall air exchange rate in the house operated with balanced ventilation was up to 28 per cent greater than in the space with exhaust-only system. This improvement was realized regardless of whether the mode of the central air distribution system in the house was set to ‘off’ (no mixing) or ‘partial’ or ‘continuous’ mixing. With respect to the internal air distribution within the house, the ‘inter-zonal airflow’ was more in the house operated with balanced ventilation system than the one with an exhaust-only assembly, meaning there was a more even air exchange between the different rooms of the living space. This is another advantage of balanced ventilation.

These positive effects can translate into lower concentrations of the volatile organic indoor contaminants mentioned above.

The study also confirmed the increased energy efficiency of balanced ventilation when using an energy recovery ventilator. In the heating season[5] of 2016-2017, the team recorded four to eight per cent less whole-house space heating energy consumption[6], which supports previous findings.

MODES OF CENTRAL AIR DISTRIBUTION SYSTEMS

Two full-scale research houses were built side-by-side at the National Research Council Canada’s (NRC’s) Ottawa campus. One house was operated with exhaust-only ventilation, drawing air from the master bathroom, and the other operated with an energy recovery ventilator exhausting indoor air from the kitchen and bathrooms. The tests included the following central air distribution system scenarios. 1) Activation of central system (furnace) fan only when there is a need for heating or cooling. This means the central air distribution system would usually be off, resulting in no mixing during this period, the typical mode in homes. 2) Central air distribution system with continuous mixing, with the central system fan always on at low speed. The fan would switch to high speed if there is a need for heating or cooling. 3) Central air distribution system intermittently mixing supply air 20 per cent of the time. 4) Central furnace fan continuously ‘off.’ This mode was used to define a baseline, which was required for research purposes.

Reducing contaminant transfer from attached garages into occupied spaces

Canadians appreciate having attached garages, and use them in many ways. According to Natural Resources Canada[7] (NRCan), 61 per cent of Canadian dwellings have one. Unfortunately, homes with attached garages have up to three times higher indoor concentrations of the carcinogenic volatile compound benzene, when compared to homes without. This was found in many Canadian studies (for more information, read “Predictors of indoor air concentrations in smoking and non‐smoking residences” by M. Heroux, N. Clark, K.V. Ryswyk, et al; “Housing characteristics and indoor concentrations of selected volatile organic compounds (VOCs) in Québec City, Canada” by M. Héroux, D. Gauvin, N.L. Gilbert, et al; and “Predicting personal exposure of Windsor, Ontario residents to volatile organic compounds using indoor measurements and survey data” by C. Stocco, M. MacNeill, D. Wang D, et al). The origins of benzene are tanks of vehicles and other gas-powered equipment and gas storage cans. Due to this potential health hazard, Health Canada, the World Health Organization (WHO), and the European Commission recommend the reduction of residential benzene to as low levels as possible (consult “Predictors of indoor BTEX concentrations in Canadian residences” by A.J. Wheeler, S.L. Wong, C. Khoury, and J. Zhu J; “The Index Project: Critical Appraisal of the Setting and Implementation of Indoor Exposure Limits in the EU” by the Institute for Health and Consumer Protection, 2005; and Residential Indoor Air Quality Guideline, Science Assessment Document: Benzene by Health Canada).

Benzene finds its way from attached garages into living spaces through leaks in the shared walls and doors. This contaminant transfer is mainly driven by the pressure difference, if the pressure in the home is lower relative to the garage.

Two mitigation approaches have been found to be effective by NRC researchers. The first possibility is the installation of an exhaust fan in the attached garage, ensuring a negative pressure of 5 Pa relative to the living space. This means pollutants emitted in the attached garage are largely drawn outdoors, thus limiting the ingress of benzene into the occupied space. In a field study with occupied homes, it was demonstrated benzene intrusion[8] into the studied homes could be reduced by 60 per cent. The second possibility is perfectly sealing the garage-home interface, which will also reduce benzene ingress into the home with the same efficiency (results will be published shortly by the NRC) (consult “Improved Sealing of Attached Garages Reduces Infiltration of Polluted Air into Adjoining Dwelling Spaces” by Daniel Aubin, Gary Mallach, Melissa St-Jean, Tim Shin, Keith Van Ryswyk, Ryan Kulka, Hongyou You, Don Fugler, Eric Lavigne, and Amanda Wheeler).

Measuring airflows

To support understanding of how air circulates through buildings, and in order to provide and refine science-based recommendations for ventilation, NRC researchers developed a Canadian analytical capacity called ‘multi-tracer gas method’ to help understand how pollutants move from adjacent zones like attached garages, attics, basements, or sub-slabs into living spaces. This analytical method is now available to exactly describe and quantify the amount and direction of airflow between these zones. Knowing the airflows will directly tell building professionals how the pollutants are moving into a facility. Pollutants mainly follow the airflow within a building—this translates into knowledge of how to design, improve, and adjust ventilation systems for a Canadian context.

The multi-tracer gas method (read “Tracer Gas Method,”), also called the perfluorocarbon tracer gases (PFT) method, uses PFTs—harmless volatile tracer gases—to mimic the movement of potentially harmful pollutants. These gases do not pose a threat to human health or the environment at the quantities emitted during the tests. This provides the opportunity to measure airflows in occupied buildings.

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As an added benefit, the PFT method will also exactly determine the overall air exchange rate of a building under real-life conditions. Using this method, one can determine the positive effect of ventilation solutions including window opening. The NRC enhanced this method by allowing concentrations of potentially harmful volatile organic compounds (VOCs) to be quantified at the same time. This feature allows one to characterize the air quality in every zone of a building, and understand the relationship between airflows and IAQ. Obviously, this feature reduces the analytical cost, when both ventilation and VOC determinations are required for IAQ investigations.

The PFT method is a new capacity for Canada, and was essential to perform the study presented here. It is now also available for all Canadian IAQ investigators when looking for the best ventilation options for their clients, as well as for solving issues.

Conclusion

Achieving the desired IAQ is a complex enterprise, especially in climates where natural ventilation is not an option in all seasons. Several elements need to come together to achieve this objective, including controlling emissions from building materials and consumer products, improving the quality of supply air (in case the ambient air is polluted), reducing ingress of soil gases like radon, and limiting the infiltration of breathable particles and gases through the building envelope.

Controlling the pressure differences between the zones in the building and the outside atmosphere contributes to the objectives of ventilation, such as improving IAQ, managing better the moisture in building envelopes and materials, and reducing the risk of mould growth.

Limiting undesired airflow from zone to zone, and finding optimal pressure differences supporting the objective of ventilation, which is the outcome of balanced ventilation, will support the generation of the desired healthy IAQ, while respecting project’s energy and sustainability goals.

The results presented here originate from the 2017 heating season and were presented at the 39^th Air Infiltration and Ventilation Centre (AIVC) Conference (2018) (for more information, read “Residential balanced ventilation and its impact on indoor pressure and air quality” by Boualem Ouazia, Daniel Aubin, Doyun Won, Wenping Yang, Stephanie So, Chantal Arsenault, Yunyi Li, and Jacqueline Yakobi-Hancock, presented at the 39^th Air Infiltration and Ventilation Centre (AIVC) Conference, 2018). A full report on this study was released in April 2019 (Consult “Residential balanced ventilation system effectiveness and tested indoor pressure and air quality impacts” by Boualem Ouazia, Doyun Won, Daniel Aubin, Wenping Yang, Stephanie So, and Chantal Arsenault, National Research Council Canada (NRC) report No. A1-009760, March 2019). NRC continues to work with partners from industry and the health community to improve ventilation with the objective of providing healthy indoor environments for all Canadians using energy-efficient approaches.

THE TRACER GAS METHOD

[10] Tracer gases are used to show building professionals how potential gaseous contaminants may move through buildings, and from unoccupied to occupied zones, as they behave like, or very similar to, the volatile contaminants one is interested in measuring. The experimental technique is based on the evaporation of volatile gases called perfluorocarbon tracer gases (PFT), and was originally developed at the U.S. Brookhaven National Laboratory (read the “Air filtration measurements in a home using a convenient perfluorocarbon tracer technique” article by Russell N. Dietz and Edgar A. Cote). In every zone of interest, a source of a specific tracer gas called an emitter is deployed, passively releasing a specific tracer gas at a constant and known rate. Simultaneously, tracer gases floating around from all zones of interest are passively sampled in all zones of interest. By determining in the laboratory the quantities of tracer gases sampled, one can determine how air flows within a building.

Hans Schleibinger, PhD, is an environmental engineer who has worked in the areas of air and water pollution control, prevention of mould growth, hospital hygiene, toxicology, and environmental analysis in Germany. He has been a senior research officer at the National Research Council Canada (NRC) since 2005, evaluating products and technological solutions and creating evidence-based knowledge for Canadian stakeholders. He can be reached at hans.schleibinger@nrc-cnrc.gc.ca[11].

Doyun Won, PhD, is a senior research officer at NRC, working on indoor air quality (IAQ) and ventilation. She has 20 years of experience in testing building materials and consumer products for chemical emissions, predicting their impacts on IAQ, and developing IAQ mitigation strategies. She earned her PhD in Environmental Engineering from the University of Texas at Austin. Won is a member of the International Standards Organization (ISO) Technical Committee 146-SC6 on “Indoor Air”. She can be reached at doyun.won@nrc-cnrc.gc.ca[12].

Boualem Ouazia, PhD, is a senior research officer at NRC as an expert in the areas of HVAC, ventilation and air distribution effectiveness, heat/energy recovery ventilation, thermal comfort, IAQ, and environment. A mechanical engineer, Ouazia is the community infrastructure thrust lead for NRC’s Arctic Program, leading research projects focused on improving housing for northern and remote communities and IAQ for Canadians. He can be reached at boualem.ouazia@nrc-cnrc.gc.ca[13].

Daniel Aubin, PhD, is an environmental chemist who has worked in both the areas of outdoor and indoor air chemistry in Canada. Aubin is a senior research officer at NRC and the team leader for the Indoor Air Quality Group. His recent work has focused on developing and validating methods and technologies to reduce occupant exposures to harmful pollutants. He has expertise in the areas of environmental analytical chemistry, children’s environmental health, building science, and in the conduct of intervention field studies with human participants. He can be reached via e-mail at daniel.aubin@nrc-cnrc.gc.ca[14].

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