Design temperature
outdoor design temperature · 99% heating design temperature
The heating design temperature is the outdoor temperature a location stays above for 99% of the hours in a year; heating systems are sized to cover the home's heat loss at that temperature.
The design temperature is a statistic, not a record low. The 99% heating value leaves about 88 hours a year colder than design. The values come from ASHRAE climate data and are tabulated in ACCA’s Manual J for load calculations.
It depends only on local weather, not on the house. A load calculation uses it to find the home’s heat loss on a design day, and the heating system is chosen so its capacity at that temperature covers the loss. For a heat pump, that means reading capacity at the design temperature from the manufacturer’s extended performance data, not the nameplate rating.
DOE’s Building America guidance adds that when the Manual J process is followed, additional upsizing or rounding up is not needed. A system sized for a colder temperature than design spends most of the season far below its capacity, or cycling on and off.
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