Load Calculations and System Sizing
Candidates must understand that sizing begins with a room-by-room load calculation, not rules of thumb. Know the components of a Manual J-style load: conduction through envelope assemblies, solar gain through glazing, internal gains from occupants and appliances, and latent load from moisture. Heating load is generally driven by transmission and infiltration; cooling load is driven by solar, internal, and latent gains and peaks in late afternoon for west-facing glass.
Understand design conditions: winter outdoor design temperature and summer outdoor/indoor design temperatures. A system sized to a mild day will be oversized on the design day, causing short cycling, poor dehumidification, and premature wear. Know the difference between sensible heat ratio and total capacity, and why latent capacity matters in humid climates.
Equipment selection traps: matching nominal tonnage to load without checking actual capacity at design conditions; ignoring duct losses; and confusing block load with room-by-room load. Also be alert to questions about oversizing penalties: higher first cost, humidity problems, and temperature swings.
Rule of thumb: One nominal ton (12,000 Btu/h) per 400–600 sq ft is a crude field check only - never a substitute for a calculation. Worked example: a 2,000 sq ft home with a calculated 30,000 Btu/h sensible and 6,000 Btu/h latent load has a total of 36,000 Btu/h (3 tons) and an SHR of 0.83; the selected equipment must deliver at least that sensible capacity at design conditions, not just nominal tonnage.