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