| Heat Gain Source | Sensible BTU/hr | Latent BTU/hr | Total BTU/hr | % of Total |
|---|
Understanding internal heat gains in HVAC load calculations
Internal heat gains are a critical component of any accurate cooling load calculation. In residential buildings they typically account for 10-20% of peak cooling load. In commercial offices they often represent 40-60%. A server room or commercial kitchen can have internal gains so large they require cooling even in winter; for a commercial kitchen specifically, the kitchen exhaust calculator shows how much of that heat a properly designed hood actually captures before it reaches the space.
Sensible vs. latent heat gains
Sensible heat raises dry-bulb temperature. Latent heat adds moisture (raises humidity). Both must be handled by the cooling system. An AC sized only for sensible load will struggle in high-occupancy spaces with significant latent gain. Check the SHR calculator to confirm equipment can handle your space's latent load. The psychrometric chart calculator shows the combined effect on space conditions.
Lighting: the biggest controllable internal gain
Switching from T8 fluorescent to LED fixtures typically reduces lighting power density from 1.2-1.5 W/ft² to 0.6-0.9 W/ft², well below the roughly 0.9-1.0 W/ft² that ASHRAE 90.1's lighting power density table allows for a typical office. In a 10,000 ft² office that's a reduction of 6,000-9,000 W, or 20,000-30,000 BTU/hr of cooling load. It's one of the most cost-effective cooling load reduction strategies available. Use the lighting heat gain calculator to quantify the impact for your specific fixture inventory.
Equipment loads: harder to estimate accurately
Equipment nameplate watts are often 2-3x higher than actual operating watts. A workstation rated at 300W might draw 80-120W in typical use. ASHRAE recommends using measured data where possible. Servers are a notable exception to that pattern: Lawrence Berkeley National Laboratory's data center research shows they run at a high, fairly constant load around the clock, which is why server room cooling is typically sized closer to nameplate rather than derated with a diversity factor. For office equipment, ASHRAE Handbook Fundamentals Chapter 18 provides typical heat gain values per unit area for common building types. See the equipment heat gain calculator for device-by-device analysis.
Frequently Asked Questions
Yes and no. Internal gains reduce heating load by offsetting envelope heat loss. In a heating load calculation, internal gains are typically not credited against peak heating load because they may not be present during the coldest night when equipment is off. However, for annual energy calculations and VAV system design, internal gains from lights and equipment can significantly offset heating requirements, particularly in interior commercial zones. The heat load calculator handles peak heating load using standard load calculation methodology.
Modern desktop workstations (PC + monitor) typically draw 100-150W in active use. Laptops draw 30-60W. Nameplate watts are worst-case maximums. For a typical office with 20 workstations, actual simultaneous peak draw is roughly 20 × 120W × 0.70 diversity = 1,680W = 5,730 BTU/hr. Use the equipment heat gain calculator for device-level estimates with ASHRAE usage factors built in.
Internal gains feed directly into the cooling load calculator as a separate input alongside envelope loads (walls, roof, windows, solar gain) and ventilation loads. For commercial buildings, the commercial load calculator has internal gains built in as a section, and you can use these detailed results to override the defaults.