| Equipment | Qty | Installed W | Design W | Radiant BTU/hr | Convective BTU/hr | Total BTU/hr |
|---|
Equipment heat gain in HVAC calculations
Equipment nameplate watts are the worst-case power draw, not the typical operating load. A workstation rated 300W nameplate might draw 80-120W during normal office use. ASHRAE recommends using measured data or published usage factors rather than nameplate watts alone. Using nameplate values inflates cooling loads by 2-3x and leads to oversized equipment.
Usage factor
The usage factor (0-1) represents the fraction of nameplate watts that actually becomes heat in the space during peak load conditions. ASHRAE Fundamentals Table 18.5 provides values: desktop PCs 0.50-0.65, shared printers 0.03-0.10, copiers 0.05-0.10, servers 1.0. For kitchen appliances without ventilated hoods, use 1.0. Combine this with the lighting heat gain calculator and occupant heat gain for a complete internal gains picture.
Radiant fraction
The radiant fraction determines how much of the heat gain is delayed (radiant, absorbed by building mass) vs. immediate (convective). For cooling load calculations using the ASHRAE CLTD/CLF or RTS methods, the split matters for peak load timing. For simplified residential load calculations, the total heat gain is used directly. See the cooling load calculator for full peak load analysis.
Frequently Asked Questions
ASHRAE Handbook of Fundamentals recommends usage factors of 0.50-0.65 for desktop computers during peak occupancy hours. This accounts for the fact that not all computers run at full load simultaneously and that many users are away from their desks. For a densely occupied trading floor or call centre, use 0.70-0.80. For a typical office, 0.55 is a good default. Laptops have lower nameplate watts (45-65W) and similar usage factors. Always use measured data when available rather than estimates.
Commercial kitchen equipment heat gain uses both a usage factor and a radiation factor. The usage factor accounts for the fraction of rated power that runs during the meal service peak. The radiation factor determines how much heat enters the space versus being captured by exhaust hoods. Equipment under a Type 1 hood with good capture velocity contributes 15-25% of its heat gain to the space. Unhooded equipment contributes 50-100%. ASHRAE Fundamentals Chapter 18 Table 5 gives detailed usage and radiation factors for specific commercial kitchen equipment.