Air Distribution, Ventilation, and Indoor Air Quality
Moving air
Airflow is measured in CFM and relates to heat transport by the sensible heat equation: Btu/hr = 1.08 × CFM × ΔT. The familiar 400 CFM per ton is a nominal rating, not a design rule - humid climates and tight envelopes often call for 350 CFM/ton or lower to improve latent removal, and high-sensible applications may use more. Duct design follows Manual D: start from the available static pressure (blower rating minus filter, coil, grilles, and register losses), convert to a friction rate, and size each run by equivalent length including fittings.
Diagnostics
Total external static pressure (TESP) is the single best airflow check: measure with a digital manometer across the blower in both the return and supply plenums. High TESP means a restrictive filter, dirty coil, or undersized duct - and low airflow means low capacity, coil icing, and high superheat. Fan laws tell you that CFM varies with speed, static pressure with the square of speed, and horsepower with the cube.
Ventilation and IAQ
ASHRAE 62.2 sets whole-house ventilation rates; local exhaust (bath ~50 CFM intermittent, kitchen 100 CFM intermittent) removes moisture and pollutants at the source. Balanced ventilation with an ERV or HRV recovers energy while bringing in outdoor air. Keep indoor relative humidity roughly 30–50%; use MERV-rated filtration appropriate to the blower's static budget; control CO, radon, and combustion byproducts.
Worked example
A 3-ton system moving 1,200 CFM with a 20°F temperature rise delivers 1.08 × 1,200 × 20 = 25,920 Btu/hr. If measured TESP is 0.90 in. wc against a 0.50 in. wc blower rating, airflow is below design - find the restriction rather than guessing at charge.