Load Calculation and System Sizing
This domain tests the calculation chain from envelope to selected equipment: ACCA Manual J loads, Manual S equipment selection, Manual T terminal selection and Manual D ducts. Expect questions on design conditions, load components, and the consequences of oversizing.
Method and rules
- Use ASHRAE design conditions: 99% heating design dry bulb, 1% cooling design dry bulb and 1% coincident wet bulb.
- Build the load from conduction (UA x delta-T), solar gain through glazing (SHGC, area and orientation), internal gains from people, lights and appliances, infiltration (blower-door CFM50 or ACH), and outdoor ventilation air per ASHRAE 62.2.
- Select equipment on both total and sensible capacity at the design indoor and outdoor conditions, and confirm the latent capacity meets the latent load. Verify a matched AHRI combination.
- Oversizing causes short cycling, poor dehumidification and wider temperature swings; it is not a safety margin.
Worked example
A room has UA = 250 Btu/h-F with a 40F heating difference: 10,000 Btu/h. Add 3,000 Btu/h solar, 1,200 internal and 1,500 infiltration = 15,700 Btu/h sensible, or about 1.31 tons. Ventilation air at 150 CFM adds roughly 2,400 Btu/h latent, so total is 18,100 Btu/h. A nominal 1.5-ton unit matches the total on paper, but its sensible capacity at 95F outdoor may be only about 15,000 Btu/h, which is short of 15,700. The correct action is to check the expanded rating and either step up capacity or reduce glazing or infiltration, not to assume nominal tonnage equals delivered capacity.
Traps that cost marks
- Square-foot rules of thumb instead of a room-by-room calculation.
- Ignoring duct gains and losses, typically 10 to 20 percent in unconditioned attics.
- Sizing on total capacity while the space is sensible-dominated.
- Forgetting ventilation latent load when selecting.