Heating, Cooling, Ventilation, and Water Heating Systems
Sizing and distribution
Equipment is sized by a load calculation, ducts by a duct design method, and equipment selected to match the load. Oversizing is the most common defect: short cycling, poor latent (humidity) control, noisy ducts, and shorter equipment life. Airflow rule of thumb: about 400 CFM per ton of cooling, so a 3-ton system needs roughly 1,200 CFM.
Efficiency metrics
Furnaces and boilers use AFUE; a 90% AFUE unit with 100,000 BTU/hr input delivers about 90,000 BTU/hr. Heat pumps use HSPF2 for heating and SEER2/EER2 for cooling, with COP as the instantaneous ratio of delivered heat to energy input. Heat pump capacity and COP fall as outdoor temperature drops, so cold-climate sizing needs a balance point and deliberate backup heat. Duct location matters: ducts in an unconditioned attic can lose 20-30% of delivered capacity.
Ventilation and water heating
Whole-house ventilation rates come from ASHRAE 62.2; exhaust-only, supply, balanced (HRV/ERV), and intermittent strategies each carry different pressure consequences. Exhaust-only ventilation depressurizes the house and can backdraft natural-draft appliances. Water heating choices include storage tank, tankless, and heat pump water heaters, compared on EF/UEF and standby loss. Storage temperature is a trade-off: about 120 F limits scald risk at the tap, while Legionella control favors storing near 140 F and tempering down with a mixing valve.
Traps
Confusing efficiency (SEER2, AFUE) with capacity. Believing a bigger system heats or cools faster. Ignoring duct leakage and return-path restrictions. Forgetting that bath and kitchen exhaust change house pressure.
Worked example
A 3-ton condenser needs about 1,200 CFM. If measurement shows 900 CFM, the system is running 300 CFM/ton: the coil runs cold, suction pressure drops, and the evaporator may ice. The correction is duct and return work, not added refrigerant.