Heat Pump Performance

A heat pump in Miami with 95 degree F outdoor air delivers a COP of 4.0. The same unit in Minneapolis at -10 degree F delivers a COP of 1.8. Same machine, same electricity input, dramatically different performance. The physics of why is the Carnot limit applied in reverse.

The maximum COP of a heat pump follows the Carnot formula: COP_max = T_hot / (T_hot - T_cold), where temperatures are in Kelvin. As the gap between indoor and outdoor temperatures widens, the COP drops. At T_cold = -10 degrees F (249 K) and T_hot = 70 degrees F (294 K): COP_max = 294 / (294 - 249) = 6.5. Real COP: roughly 1.5-2.5. At T_cold = 50 degrees F (283 K, ground temperature): COP_max = 294 / (294 - 283) = 26.7. Real COP: 3.5-5.0.

Compare air-source in cold weather. At 0 degrees F outdoor air: COP of about 2.0. Each kWh of electricity delivers 2 kWh of heat. A resistive heater: COP = 1.0. The heat pump is still better than resistance, but barely.

Compare ground-source. Same cold day, ground at 50 degrees F: COP of about 4.0. Each kWh delivers 4 kWh of heat. Twice as efficient as the air-source unit.

At what outdoor temperature does an air-source heat pump become less efficient than a gas furnace?

The crossover point. A 95% efficient gas furnace delivers 0.95 units of heat per unit of gas energy. At a COP of 1.0, a heat pump matches that (if electricity and gas cost the same per BTU). But electricity typically costs 3-4x more per BTU than gas. So the economic crossover occurs at a COP of roughly 2.85 (0.95 x 3) to 3.8 (0.95 x 4), which corresponds to outdoor temperatures of about 25-35 degrees F for most air-source units.

This temperature sensitivity is why building electrification policy must account for climate zone, and why cold-climate heat pump technology and ground-source systems matter for northern states.

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