Heat pumps: efficient until the strip heat kicks in

Typical connected load

In cooling mode a heat pump behaves like an air conditioner: roughly 1 kW per ton, so 3 to 5 kW for a typical residential system.

In heating mode the compressor draw is similar, 2 to 4 kW, and the efficiency is excellent. The problem is the auxiliary heat. Nearly every air-source heat pump includes resistive backup elements sized to heat the house without any compressor contribution at all. Those banks commonly run 10 to 20 kW, staged in 5 kW increments.

So the same piece of equipment can be a 3 kW load or a 20 kW load depending on the outdoor temperature and what the thermostat has decided to do. Always check the auxiliary heat breaker size or the element model numbers in the air handler access panel. The nameplate on the outdoor unit tells you nothing about the strip heat inside.

Why it spikes demand

Two things trigger auxiliary heat, and both of them create demand spikes at the worst possible times.

The first is low outdoor temperature. Below the balance point, the compressor cannot keep up and the thermostat brings in strip heat to make up the difference. Cold mornings are exactly when the utility's winter on-peak window falls.

The second, and the one that catches people, is recovery from setback. When a thermostat sees a large gap between room temperature and setpoint, most controls interpret that as an emergency and energize auxiliary heat immediately rather than letting the compressor work at it. Program a nighttime setback and your heat pump will bring on 15 kW of resistance heat at 6 a.m. every winter morning.

Defrost cycles do the same thing on a shorter timescale. When the outdoor coil ices up, the unit reverses into cooling mode to melt it, and strip heat comes on so the house does not get blasted with cold air.

Thermal storage and deferrability

The compressor stage has the same building thermal mass to coast on that an air conditioner does. Moderate deferrability, measured in minutes.

The auxiliary stages are a different case. They are pure resistance, no storage of their own, but they are frequently unnecessary. A great deal of strip heat operation happens because the thermostat's recovery logic called for it, not because the house was actually going to get cold. Holding back one or two stages while the compressor works often produces a slightly slower recovery and no comfort complaint at all.

What an interruption feels like

Compressor stage held: the same slight drift you would get from any cycling.

Auxiliary stages held: the house takes longer to come up to temperature after a setback. If you set back to 62 overnight and expect 70 by 6:30 a.m., you might get there at 7:00 instead. The most common fix has nothing to do with the controller. Stop setting back the thermostat deeply in a heat pump home. Setback saves money in a gas-heated house and costs money in a heat pump house.

Where it sits in the priority order

Split. The compressor stage sits with the air conditioners, priority 2 or thereabouts, with minimum on and off times protecting it exactly as they protect an AC compressor. The auxiliary stages sit lower, below cooling and above the water heater, and they get no minimum on or off restriction because there is no compressor to protect.

On a residential Energy Sentry installation the heat pump's compressor is usually controlled through a low-voltage relay that breaks the 24V thermostat loop at the Y wire, or through a line-voltage relay on the condenser circuit. It is important that the relay does not break the R wire, because cutting thermostat power causes the thermostat to reboot on every control cycle.

The honest caveat

Heat pumps are the load type where installation detail matters most. Getting the staging right requires knowing how many auxiliary stages exist, how they are wired, whether the thermostat has an adjustable balance point or lockout, and how cold the climate actually gets. In a cold-climate home there are hours where strip heat is genuinely needed and holding it back will make the house cold. A well-configured system knows the difference. A poorly configured one does not.

This is a load type we want eyes on before we quote. If you have a heat pump with electric backup, expect us to ask for photos of the air handler access panel and the breaker panel.

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