Heating and Cooling Loads
Load components
Cooling load is the sum of envelope conduction, solar gain, internal gain, and latent load, each peaking at a different hour. Conduction uses U × A × ΔT with the outdoor design dry-bulb. Sunlit opaque surfaces are handled with sol-air temperature or a CLTD/CLF or radiant time series method that accounts for thermal mass, so peak wall gain lags peak outdoor temperature by hours; roofs lag more than walls, and west glass peaks later than south. Internal gains come from people (sensible plus latent, both set by activity level), lights, and equipment, and must include fan and duct heat gain.
Ventilation versus infiltration
Ventilation is deliberate outdoor air sized by ASHRAE 62.1 with a people component and an area component; infiltration is uncontrolled leakage driven by wind and stack effect. They are not interchangeable, and double counting them is a classic error.
Heating
Heating design typically ignores solar and internal gains, uses the 99% or 99.6% outdoor design dry-bulb, and adds duct loss and warm-up allowance.
Traps
Mixing U-factor with R-value (U = 1/R_total, and R totals include inside and outside film resistances); omitting latent load from outdoor air; forgetting duct gain and fan heat; and applying one worst-case hour to every component.
Worked example
A 2,000 ft² roof with U = 0.05 Btu/h·ft²·°F, 95°F outside and 75°F inside: Q = 0.05 × 2,000 × 20 = 2,000 Btu/h.