A heat pump that is not keeping the house warm in winter is not always broken. A thermostat in the wrong mode, an ordinary defrost cycle or the normal feel of heat pump air can explain it. When something has failed, it is usually the reversing valve, the defrost controls, the refrigerant charge, the compressor or the backup heat. A few checks tell the two situations apart.
Is it actually broken?
- The air feels lukewarm. A heat pump moves a large volume of warm air rather than the hot blast of a gas furnace. If the house is slowly reaching the setting, the system is working.
- It blew cool air for a few minutes, then went back to normal. That was most likely a defrost cycle; the page on a frozen heat pump shows what a normal one looks like.
- It takes hours to recover after a deep night setback. Heat pumps work at a steady pace and do not catch up quickly, and a big setback can bring on the costlier backup heat. Smaller setbacks usually work better.
Thermostat settings that cause “no heat”
- Mode. It should read Heat, not Cool, Off or Fan. In Auto, the switch between heating and cooling depends on two set points, and a narrow gap between them can make the system flip modes.
- Aux and Em. Aux heat on the display means the backup is helping because the heat pump cannot keep up alone: normal for a while, not for hours on end. Em heat means the heat pump is switched off and the house is running on backup only.
- After a thermostat replacement. Heat pump thermostats have a setting for the reversing valve, often labeled O or B, that must match the equipment. Set wrong, the system cools when you ask for heat. If the trouble began the day a new thermostat went in, check this first.
- Schedules and power cuts. An outage can reset a program and leave the system on a schedule you did not choose.
Symptom, likely cause, what it takes
| What you notice | Likely cause | What it usually takes |
|---|---|---|
| Cool air in heat mode, outdoor unit running steadily | Reversing valve stuck in cooling, or the thermostat’s O/B setting is wrong | Thermostat setup check, then valve and solenoid diagnosis |
| Outdoor unit encased in ice | Defrost control, sensor, fan motor or low charge | Defrost and fan diagnosis |
| Aux heat on constantly, bills climbing | Heat pump losing capacity: low charge, dirty coil, weak compressor | Charge and performance testing |
| Outdoor unit not running at all | Tripped breaker, failed contactor or capacitor, control fault | Electrical diagnosis |
| Outdoor unit hums, then the breaker trips | Compressor failing to start | Start components and compressor testing |
| No warm air even on Em heat | Air handler power, backup heat elements, a limit switch or the control board | Air handler and backup heat repair |
| Warm air, but weak at every vent | Clogged filter, dirty indoor coil, blower problem | Filter, coil cleaning, blower repair |
Checks you can make safely
- Set the thermostat to Heat, two or three degrees above room temperature, and wait ten minutes.
- Replace the filter if it is dirty.
- Look at the outdoor unit: is the fan turning, and is there heavy ice? A light, even frost is fine.
- Check the breakers. Many systems have one for the outdoor unit and a separate one for the indoor air handler, which also feeds the backup heat.
- Try Emergency Heat for a few minutes. If warm air comes out, the indoor side and the backup work, and the fault is on the heat pump side. If nothing warms on Em heat either, the indoor unit or its power is the place to look. Switch back afterwards, because backup heat costs more to run.
What needs a technician
Call when the outdoor unit is fully iced or never defrosts, when you suspect the reversing valve, when you hear hissing or see oily residue on the lines, when a breaker trips twice, or when the backup heat does not work either. Valves, defrost boards, refrigerant and compressors need gauges, electrical meters and EPA-certified technicians for refrigerant handling. On a heat pump repair visit you hear what was found and why, with a written estimate before work starts. If the thermostat itself is the problem, a correctly configured thermostat installation fixes it.
The coastal angle
Heat pumps suit this climate. Winters here are mild and damp rather than cold, so a heat pump spends most of its hours in the outdoor temperatures where it works most efficiently. That also makes a fault easier to spot: a heat pump that cannot hold a normal setting on a 50-degree day in Daly City is not up against the weather. Because heating is needed in most months here, a weakening heat pump leans on backup heat for much of the year, and it often shows up as high energy bills before the house feels cold. Ductless heat pumps in additions and ADUs follow the same logic; see mini-split repair.