Heating and Cooling Load Basics

A load calculation estimates how much heat a building loses in winter and gains in summer under design conditions. The result, in BTU per hour, drives equipment selection and duct design.

• Envelope losses and gains through walls, ceilings, floors and windows.
• Infiltration, the uncontrolled air leaking in and out.
• Solar gain, which depends on window orientation, area and shading.
• Internal gains from people, lighting, cooking and appliances.
• Ventilation air brought in deliberately.
• Duct losses when ducts run through unconditioned space.

Oversized equipment short cycles. In cooling that means poor dehumidification and a clammy house. In heating it means temperature swings, noise and more wear on ignition components. Undersized equipment cannot hold setpoint on the worst days. Matching the load is also what makes duct sizing and airflow targets meaningful.

1. 1Establish indoor and outdoor design conditions for the location.
2. 2Measure the building and record construction details for each surface.
3. 3Calculate heat flow through each surface at the design temperature difference.
4. 4Add infiltration and ventilation loads.
5. 5Add solar and internal gains for the cooling calculation.
6. 6Separate the cooling result into sensible and latent portions.
7. 7Select equipment against the manufacturer expanded performance data at the design conditions, not the nominal rating.

Heat pumps add a balance point analysis, the outdoor temperature at which the heat pump capacity equals the building heat loss, which determines how much supplemental heat is needed.

• Sizing to match the equipment being replaced, which repeats an old mistake.
• Using nominal tonnage instead of actual capacity at design conditions.
• Ignoring duct losses in an unconditioned attic, which can add a large fraction to the real load.
• Adding a safety factor on top of a calculation that already includes design margins.

New installations, change outs, additions, and any job where comfort complaints persist after the equipment checks out mechanically.