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Heat pump not heating: causes, fixes and what it costs
A heat pump that will not heat often keeps the house liveable on auxiliary resistance heat, so the first thing most people notice is the electricity bill rather than the cold.
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A heat pump failing to heat is usually stuck in the wrong mode, iced on the outdoor coil, low on charge, or leaning entirely on auxiliary resistance heat. Call (855) 321-3194 and we connect you with a local heating provider.
What you are seeing
Heating symptoms
- Cool or lukewarm air while the system runs
- Auxiliary or emergency heat indicator permanently lit
- The outdoor unit is encased in ice
- A loud clunk when the system changes mode
Running cost symptoms
- Electricity bills climbing sharply in winter
- The system runs continuously without satisfying
- It cools correctly in summer but will not heat
- Weak airflow from the supply registers
Do and do not
| Do | Do not |
|---|---|
| Check whether the aux or emergency heat light is on | Leave the system on emergency heat indefinitely |
| Let a normal defrost cycle finish before judging | Chip ice off the outdoor coil |
| Note whether cooling mode still works | Assume a steaming outdoor unit is a fault |
| Report the outdoor temperature when it fails | Pour hot water over the outdoor unit |
| Ask for charge to be weighed rather than estimated | Accept a top-up without a leak search |
What causes it, ranked by how often
- Stuck reversing valve. The valve that switches the refrigerant flow between heating and cooling. Stuck in the cooling position, the system runs normally while delivering cool air.
- Defrost system failure. The outdoor coil frosts in cold damp weather and the defrost cycle clears it. A failed defrost board or sensor leaves the coil iced and heating stops.
- Low refrigerant charge. Heating capacity falls away faster than cooling capacity as charge drops, so a leak shows up in winter first.
- Auxiliary heat carrying the load. The compressor side is contributing nothing and resistance strips are doing all the work. The house stays warm and the bill rises sharply.
- Outdoor unit not running. A failed capacitor, contactor or fan motor stops the outdoor section, leaving only auxiliary heat.
- Wrong thermostat. A conventional control fitted to a heat pump cannot operate the reversing valve or stage auxiliary heat properly.
How a technician diagnoses it
- Confirm which mode the system is actually in
Checking the reversing valve position separates a control fault from a mechanical one.
- Observe a defrost cycle
Whether the unit initiates and completes defrost correctly tells you about the board and the sensor.
- Measure charge in heating mode
Readings taken in heating are what matter here, since a charge adequate for cooling may not be adequate for heating.
- Check auxiliary heat staging
Whether resistance heat is engaging appropriately or simply running all the time.
When to shut the system off
- The outdoor unit is fully encased in ice and not clearing it.
- The breaker trips when the system attempts to start.
- There is a burning smell from the air handler or outdoor unit.
- The system is running on emergency heat for days with no visit booked.
Service pages
Related guides
Safety first
Call now, or call the utility first?
Call now
- No cool air during a heat advisory
- No heat with outdoor temperatures below freezing
- Burning smell from the vents
- Water pooling at the air handler
- Ice on the line set
- Breaker tripping on every cycle
Call (855) 321-3194 and give your ZIP code.
Shut it off and call the utility or 911 first
- You smell gas: leave the building, then call the gas utility or 911
- Smoke or sparking at the equipment
- A carbon monoxide alarm is sounding: leave and call 911
Reference: CPSC recalls and US Fire Administration heating fire safety.
Questions
Heat pump not heating questions
Why is my heat pump blowing cool air?
Most often the reversing valve has not completed the switch into heating mode, so the system is effectively still in cooling. It can also be a defrost cycle, which is normal and lasts only a few minutes.
Is it normal for the outdoor unit to ice up?
Some frost in cold damp weather is normal, and the defrost cycle clears it periodically. A unit that stays encased in ice is not normal and points at the defrost board, the sensor or the refrigerant charge.
What is auxiliary heat and why is it expensive?
Auxiliary heat is electric resistance heating that supplements the heat pump when it cannot meet demand. It is far less efficient than the compressor, so a system leaning on it constantly produces a much larger bill.
Why does it cool fine but not heat?
Heating uses the reversing valve and the defrost circuit, neither of which is involved in cooling. A fault in either produces a system that performs perfectly in summer and fails in winter.
Can a heat pump work in freezing weather?
Modern cold-climate units hold useful capacity well below freezing, though output falls as temperature drops and auxiliary heat covers the shortfall. Whether a given unit suits your climate is a sizing question.
What does a reversing valve cost?
Typically a figure the provider confirms on site including labor, which makes it one of the more expensive heat pump repairs. A defrost board is considerably less at a figure the provider confirms on site.
Written and fact-checked by the HVAC Emergency USA editorial team. Last updated . Editorial policy.