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Heat pump won't shut off

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Long run times in extreme weather can be normal for a heat pump. A system that never shuts off even in mild conditions, or that keeps running well past the setpoint, is a different situation.

Rule out the thermostat setting first

Confirm the setpoint is actually reasonable for the season and has not been left somewhere extreme by accident. Then check whether the thermostat is reading the room accurately. One mounted somewhere unusually warm or cold relative to the rest of the house can call for heating or cooling far longer than the rest of the home actually needs.

A thermostat that has lost calibration or connection

Some systems use a separate outdoor temperature sensor, or a smart thermostat that relies on a network connection. If the sensor reports incorrect data or the connection is lost, the system can keep calling for conditioning well past when it should have been satisfied. This is more common on newer, connected thermostats than on simple mechanical ones.

A refrigerant charge that is low

A heat pump running low on refrigerant has to work longer to move the same amount of heat, because each cycle is moving less of it. This shows up as noticeably longer run times without necessarily an outright failure. Unlike the outdoor unit's fan or a capacitor, a low charge is not something you can confirm by looking. It needs proper gauges and a technician certified under EPA Section 608 to diagnose and correct, since handling refrigerant is federally regulated.

A system that is undersized for the house

If a heat pump is simply smaller than the heating or cooling load actually requires, it can run nearly continuously in extreme weather without ever quite catching up. That looks identical to a fault from inside the house. It is an installation-sizing issue rather than something that develops suddenly. Mention it if the unit has always run long rather than starting recently.

What not to do

  • Do not keep lowering (or raising) the setpoint further to try to force the system to satisfy: if it has not caught up already, a more extreme setting usually just means more running for the same result.
  • Do not attempt to check or add refrigerant yourself. It requires certified equipment and training, and doing it incorrectly can damage the compressor.
  • Do not ignore a system that runs constantly for days on end even in ordinary weather. That pattern wastes energy and points at a fault worth diagnosing rather than living with.

When to call a licensed technician

If the thermostat setting and placement check out and the system still runs far longer than the outdoor conditions seem to justify, call a licensed HVAC technician. They can check the refrigerant charge and the outdoor sensor, and whether the system was sized correctly for the house in the first place.

Frequently asked questions

Is it normal for a heat pump to run all day in very cold or very hot weather?

Yes, longer run times in extreme conditions are normal and expected: a heat pump has to work harder as the gap between indoor and outdoor temperature grows. The concern is running constantly in mild, ordinary weather without the house ever reaching setpoint.

Could a dirty outdoor coil cause this?

Yes: debris, leaves, or a season's worth of dust on the outdoor coil reduces how efficiently the system can transfer heat, which can extend run times similarly to a low refrigerant charge.

Sources

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