Air-Conditioning Design, Load Calculation, and Ductwork
This section covers air-conditioning design, load calculation, and ductwork: heat gain and loss, sensible and latent loads, equipment selection, air distribution, static pressure, and duct sizing. The exam expects you to separate building load from equipment capacity and to prove that the duct system can deliver the required airflow.
Load and selection
Use an ACCA Manual J or equivalent room-by-room load for residential work; Manual S for equipment selection; Manual D for duct design; Manual T for diffuser selection. Include orientation, insulation, windows, infiltration, ventilation, internal gains, and duct losses. Do not size by square footage or by the old unit. Match sensible and latent capacity to the load, not just nominal tons. A 3-ton unit may not deliver 3 tons at design conditions.
Airflow and ducts
Sensible airflow: CFM = sensible Btu/h / (1.08 x temperature difference). Total airflow is often estimated at 400 CFM per ton, but verify with the equipment table. Size ducts by friction rate or velocity, not by guess. Keep return paths short, seal all joints, support flex duct, and avoid sharp bends. Check external static pressure against the blower rating.
Worked example
A room has a sensible load of 24,000 Btu/h. Design indoor air is 75°F and supply air is 55°F, so the difference is 20°F. CFM = 24,000 / (1.08 x 20) = 1,111 CFM. If the equipment can deliver only 1,000 CFM at the available static, the room will run warm even though the nominal capacity looks adequate. The fix may be a duct redesign or a different equipment selection, not a lower thermostat setting.
Traps
Using nominal tonnage as delivered capacity; ignoring latent load and ventilation; sizing ducts by CFM alone without friction or velocity; burying flex duct in insulation; and forgetting return air and filter pressure drop.