| Load Component | Heating BTU/hr | Cooling BTU/hr | % of Total Cooling |
|---|
Commercial vs. residential load calculations
The biggest difference between commercial and residential load calculations is the dominance of internal gains. A well-insulated office building can require cooling on a cold winter day because the lighting, computers, and people generate more heat than escapes through the envelope. That never happens in a residential building.
Internal gain density drives commercial design
A typical open-plan office has 1.0 W/ft² of lighting, 1.5 W/ft² of equipment, and 1 occupant per 150 ft² (roughly 250 BTU/hr sensible each). Combined, that's about 5-7 BTU/hr·ft² of internal gain. The envelope load in a modern insulated building might add another 3-5 BTU/hr·ft². The result: peak cooling loads of 400-600 ft² per ton, compared to 600-900 ft² per ton in residential. And many interior zones need cooling year-round.
Diversity factors
Not all zones peak at the same time. East-facing zones peak in the morning, west-facing in the afternoon. Internal zones peak at midday. The central plant can be sized to 70-85% of the sum of zone peaks using a diversity factor. See the VAV diversity calculator for this analysis. This tool calculates individual zone peaks — apply diversity at the plant sizing stage.
Frequently Asked Questions
Commercial buildings have significantly higher internal heat gains from lighting, equipment, and occupants per square foot. Office lighting alone can add 1-3 W/ft2 of heat gain. Commercial buildings also have more complex zoning needs, VAV systems, and diversity factors. ASHRAE Block Load calculations use diversity factors acknowledging that not every zone peaks at the same time, reducing total plant size versus summing individual zone peaks. Residential room-by-room load calculations assume worst-case simultaneous load for all rooms.
Use ASHRAE 0.4% design conditions for commercial buildings: the outdoor temperature exceeded only 0.4% of annual hours (about 35 hours per year). This is more conservative than the 1% condition used in some residential calculations. ASHRAE Fundamentals Table 14-3 gives design temperatures for hundreds of Canadian cities. For Vancouver: 29C DB/21C WB summer, -7C winter. Toronto: 33C/24C summer, -18C winter. Edmonton: 29C/18C summer, -29C winter.