Room Heating and Cooling Load: Method

Component Load Method

The design cooling load of a room is the sum of the heat that enters it at the peak hour: conducted through the envelope, radiated in as sunshine through glass, generated inside by people, lights and equipment, and carried in by the outdoor air. Each is a simple product, and the method's accuracy lies in the inputs.

Conduction through opaque walls and roofs uses the cooling-load temperature difference (CLTD), a tabulated value for the orientation, color, mass and hour that stands in for the sol-air temperature and the wall's thermal lag; 25 to 40 °F for sunlit walls and 40 to 70 °F for roofs at the afternoon peak are typical of the ASHRAE tables. Glass conducts at the plain temperature difference. Solar gain through glass is area × SHGC × the orientation's peak irradiance (about 100 Btu/h·ft² south, 200 to 230 west, 30 north in midsummer at mid-latitudes). Internal gains: a seated person gives about 250 Btu/h sensible and 200 latent; lights and equipment their electrical input (times a usage factor the user applies). Outdoor air brought in for ventilation and leaking in as infiltration adds ṁ c_p Δt sensible and ṁ h_fg ΔW latent.

The sensible load sets the supply airflow for a chosen supply temperature difference; the latent load and the sensible heat ratio set the coil's leaving condition. The heating load is the winter conduction and outdoor-air load with no solar or internal credit.

This is the simplified component method, suitable for sizing a room's equipment and checking a detailed calculation; it does not model the time lag of radiant gains (the radiant time series or heat-balance methods do) and so tends to be conservative. Temperatures, U-values, CLTDs and irradiances are entered in the units of the selected system.