Load Calculations and System Sizing
Core concepts
Manual J (residential) and Manual N (commercial) load calculations determine heating and cooling loads from envelope, orientation, internal gains, infiltration, and ventilation. Manual S selects equipment based on sensible and latent capacity, not just nominal tonnage. Manual D sizes ducts; Manual T selects registers. Know the components of a load: conduction through assemblies, solar gain through glazing, internal gains from people, lights, and appliances, and infiltration/exfiltration. Understand design conditions (ASHRAE 1% cooling, 99% heating) and how altitude and outdoor design temperature affect equipment selection.
Traps in questions
- Sizing to square footage alone is a classic error; questions may present a rule-of-thumb and ask why it is inadequate.
- Sensible vs. latent load: equipment must satisfy both, and a unit can be undersized for latent even if total capacity matches.
- Oversizing causes short cycling, poor humidity control, and comfort complaints; it does not "add safety margin."
- Duct losses and infiltration are often omitted by novices; questions may test whether you included them.
- Design conditions are location-specific; do not use a national average.
Worked example / rule of thumb
A common screening rule is roughly 20–30 BTU/h per square foot of conditioned floor area for cooling in a mixed climate, but this is only a sanity check. A 2,000 ft² home at 25 BTU/h·ft² suggests ~50,000 BTU/h (~4.2 tons). If the Manual J shows 38,000 BTU/h sensible and 6,000 BTU/h latent, select equipment whose sensible and latent capacities at design conditions meet or exceed both, not merely 44,000 total.
Study guidance
Practice reading a load summary and matching it to a manufacturer's expanded performance table.