Heat-Pump Cold-Temperature Capacity and Auxiliary Heat

a heat pump's capacity falls as it gets colder, so interpolating the AHRI 47 F and 17 F rating points down to the design temperature gives the delivered capacity, and the shortfall below the Manual J design load is the auxiliary strip heat (kW) the installer must add. A unit sized on its 47 F nameplate is badly undersized at 5 F - the classic cold-climate conversion failure. Prefer a published low-temperature data point over extrapolation when a conversion hinges on it. A sizing check, not a performance guarantee.

slope = (cap_47_btuh - cap_17_btuh) / (47 - 17); cap_design = max(0, cap_17_btuh + slope x (design_temp_f - 17)); shortfall = max(0, design_load_btuh - cap_design); aux_kw = shortfall / 3412.

The AHRI 210/240 low-temperature rating points (the 47 F and 17 F integrated heating capacities) and a linear capacity-versus-temperature interpolation, by name; 3,412 Btu/h per kW is a named constant.

The AHRI rating points and the linear interpolation are public; manufacturer expanded-performance tables are published in submittal data.

Estimate. AHJ and licensed professional govern.

Field names used by the API: cap_47_btuh, cap_17_btuh, design_temp_f, design_load_btuh, slope, cap_design, shortfall, aux_kw, covers

• Rating points capacity interpolated linearly between the AHRI 47 F and 17 F integrated heating capacitiesAHRI 210/240
• Auxiliary heat the shortfall below the design load is the backup heat, at 3,412 Btu/h per kWfirst principles
• Design load the design load and design temperature come from a Manual J; defrost and cycling derate the field capacityACCA Manual J