Heat Pump Running Constantly: When It Is Normal

Find the balance point for your house

The house loses more heat the colder it gets. Double the difference between inside and out, and the heat leaking through the walls roughly doubles.

The heat pump makes less the colder it gets, because it is pulling heat out of colder air.

Somewhere those two meet. That outdoor temperature is the balance point. It is different for every house, and on many ordinary systems it sits somewhere around freezing.

Above it, the heat pump has spare capacity and cycles on and off. Below it, it runs flat out all the time, and the thermostat brings in the backup electric heat to make up the difference.

So nonstop running on a cold day is a sign it is sized sensibly, not badly. A heat pump big enough never to run constantly would short cycle and do a poor job of drying the air for the rest of the year.

Put it against the weather

Cold outside, running nonstop, house holding temperature. Normal. Nothing to do.

Cold outside, running nonstop, house slipping a degree or two in the coldest part of the night. Also usually normal, and it is what the backup heat is for.

Mild outside, forty degrees or above, running nonstop and falling behind. Not normal. A heat pump should have plenty of capacity at that temperature. Something is holding it back.

Air from the vents feels cool. Heat pump air is warm rather than hot, around 85 to 100 degrees, Carrier says. Measure it before judging it by hand: what heat pump air should measure.

In summer, nonstop running on the hottest days is the same story as any air conditioner: why a system runs all day and holds 78.

The causes, cheapest first

A clogged filter. Starved of air, a heat pump moves less heat and runs longer. First thing to check, every time: how often a filter really needs changing.

A dirty outdoor coil. In winter the outdoor coil is where the heat comes from, and a matted coil gathers less of it.

A deep overnight setback. Dropping the thermostat six degrees at night means a long recovery every morning, often on the expensive backup heat. Small setbacks only on a heat pump.

Low refrigerant. A heat pump that is short of charge runs longer and heats less, and it gets worse every year: why low refrigerant always means a leak.

A defrost problem. A coil that never fully clears spends the whole day half blocked.

The fan setting. If it is the indoor fan that never stops rather than the heat pump, check the thermostat fan is on Auto, not On. Many variable speed systems also run the fan slowly on purpose between cycles.