| Fixture | Qty | Input W | Adjusted W | Heat to Space | Heat to Plenum |
|---|
Lighting heat gain in HVAC cooling load calculations
Every watt of lighting power eventually becomes heat in the building. A 2,000 W lighting system adds 6,824 BTU/hr to the cooling load — that's roughly 0.57 tons. In a 10,000 ft² office with 1.2 W/ft² of fluorescent lighting, that's 12 kW or 40,900 BTU/hr of cooling load from lighting alone. Switching to 0.7 W/ft² LED cuts that to 23,900 BTU/hr, saving 17,000 BTU/hr or about 1.4 tons from the cooling plant.
Ballast factors and driver factors
Electronic fluorescent ballasts have a ballast factor of 0.85-1.0 depending on the lamp/ballast combination. Magnetic ballasts are 1.0-1.2 — they waste more energy as heat. LED drivers are essentially 1.0. Always use the actual ballast factor from the luminaire spec sheet, not an assumed value. ASHRAE Handbook Fundamentals Chapter 18 notes that using incorrect ballast factors is a common source of error in lighting load calculations. For a detailed fixture inventory, combine this tool with the internal heat gain calculator.
Space fraction vs. plenum fraction
Recessed light fixtures in a return-air plenum ceiling transfer some heat directly to the plenum airstream and some to the space below. Typical space fractions: surface-mounted = 1.0, recessed in plenum = 0.55-0.70. The plenum heat still loads the cooling coil via return air, but with a time lag. For peak load calculations, it's conservative (and common practice) to use a space fraction of 1.0. Use the split for detailed hourly energy modeling. The results here feed directly into the commercial load calculator.
Frequently Asked Questions
LED luminaires convert 80-95% of input power to light (vs 5-10% for incandescent). However, all electrical input power eventually becomes heat in the space since even the light energy is absorbed by surfaces and converted to heat. The heat gain from lighting equals the total wattage times the space fraction (how much heat stays in the space vs. going up through the ceiling into the plenum). For LED downlights with no return air plenum, 100% of wattage becomes cooling load. For fixtures in a return-air plenum ceiling, 40-60% enters the conditioned space directly.
ASHRAE 90.1-2022 allows maximum lighting power density of 0.82 W/ft2 for open-plan office space and 0.75 W/ft2 for enclosed offices. Well-designed LED offices often achieve 0.5-0.7 W/ft2. Compare this to older fluorescent installations at 1.5-2.0 W/ft2 and incandescent at 3.0+ W/ft2. Reducing lighting power density is one of the most cost-effective ways to reduce cooling load because it reduces both the direct heat gain and the heat from the lighting energy itself.