Ventilation and Air Distribution
Ventilation and distribution questions test whether you understand how air actually moves through a building and why it must be replaced. Cover the three ventilation strategies - exhaust-only, supply-only, and balanced - and know that balanced systems (HRV/ERV) are preferred in tight, cold-climate homes because they control both pressurization and heat recovery. Understand ASHRAE 62.2 as the residential ventilation standard and the concept of required continuous ventilation rate based on occupancy and floor area.
Air distribution is about pressure and balance: supply and return paths, static pressure, duct velocity, and the fan laws. Know that total external static pressure is the sum of supply and return losses, and that a fan moving more air needs more horsepower roughly with the cube of speed. Understand registers, grilles, diffusers, dampers, and the difference between face velocity and throw. Balanced air means return air roughly equals supply air in each zone, preventing pressurization that drives moisture into walls.
Traps: assuming an ERV and HRV do the same thing (ERV also transfers moisture); forgetting that exhaust-only ventilation depressurizes a house and can backdraft combustion appliances; and confusing cfm with velocity (fpm). In Alaska, cold outdoor air and ice formation on HRV cores are real design concerns.
Worked example: a 2,000 ft² home with 3 bedrooms. A common rule of thumb gives roughly 0.35 air changes per hour minimum; if the home volume is 16,000 ft³, that is about 5,600 ft³/h, or roughly 93 cfm continuous - check the actual standard formula rather than relying on the shortcut.