Load Calculation, Sizing, and System Design
Building the load
Manual J is the accepted residential method: room-by-room envelope areas with U-factors, window solar and orientation, infiltration, internal gains, and design indoor/outdoor temperatures. The basic conduction equation is Btu/hr = U × A × ΔT; infiltration and ventilation add sensible and latent load, and duct gains or losses (often 10–20% in unconditioned space) are added as a system multiplier. Manual S then selects equipment whose capacity at the actual design conditions matches both the sensible and the latent load.
Sizing discipline
Manufacturer capacity tables are stated at ARI conditions (95°F outdoor / 80°F dry-bulb, 67°F wet-bulb indoor), so a unit's nominal tonnage is not its capacity at your design temperatures. Oversizing causes short cycling, poor dehumidification, higher humidity, and shortened equipment life; undersizing loses setpoint on design days. Match the airflow (roughly 350–450 CFM/ton based on the sensible heat ratio) and verify the selected blower can deliver it against the duct system's static pressure.
Traps
Replacing equipment by matching the old nameplate tonnage instead of running a load calculation; using indoor design conditions of 75°F when the load was calculated at 70°F; forgetting duct losses and outdoor-air loads; and confusing total capacity with sensible capacity in a hot, dry climate where sensible capacity is the limiting number.
Worked example
A wall of 200 ft² with U = 0.10 faces a 70°F indoor and 10°F outdoor design condition. ΔT = 60°F. Load = 0.10 × 200 × 60 = 1,200 Btu/hr. Repeat for every envelope element, then add infiltration, internal, and duct loads for the block load.