Air Quality
Air Quality tests how installation affects filtration, duct pressure boundaries, ventilation, combustion air, and moisture control. The exam does not treat IAQ as an accessory; it treats it as part of a safe, balanced system.
Filtration and pressure drop
Select a filter with adequate MERV for the application and verify its pressure drop. MERV 8–13 is common for residential retrofit, but a high-MERV filter in a small slot can exceed the blower's external static allowance. Use a manometer; do not judge by filter thickness. Seal the filter rack to prevent bypass.
Pressure boundaries and ventilation
Return air must not be drawn from garages, appliance closets, or bathrooms. Seal ducts in unconditioned space. Provide combustion air for fuel-burning appliances and verify CO safety. Where required, provide mechanical ventilation per ASHRAE 62.2 or local code. Keep indoor humidity in a range that discourages mold, typically below 60% RH.
Worked example
A 3-ton blower is rated 0.50 in. w.c. external static. Measurements are filter 0.35, return grille 0.10, supply 0.08, and coil 0.15, totaling 0.68 in. w.c. Airflow falls below design. Replace the restrictive 1-inch filter with a larger media filter or lower MERV, then re-measure.
Traps
- Assuming higher MERV is always better.
- Blocking combustion air openings.
- Returning air from a garage.
- Filter bypass around the frame.