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Fix a heat pump that will not heat

We'll confirm heat mode, rule out defrost and filter, then isolate the cause—reversing valve, refrigerant, or backup heat—or tell you when to call a pro.

Troubleshooting · Home appliances
15–30 min
What you'll need
  • Replacement air filter (if dirty)
Step 1 of 10

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Steps

Goal: Confirm heat mode, rule out defrost and filter, then isolate the fault.

  • Check the thermostat. It must be set to “Heat” and the temperature above room temperature.
  • Good: Thermostat in Heat mode. Proceed to Check thermostat and defrost.
  • Bad: Thermostat in Cool or Auto—set to Heat and retest.

Check thermostat and defrost

Goal: Rule out mode setting and allow for normal defrost before assuming a fault.

  • In cold weather, the defrost cycle may blow cool air for 2–10 minutes. Wait and see if warm air returns.
  • Check the circuit breaker and disconnect. Replace the air filter if dirty.
  • Good: Warm air returns after defrost, or warm from the start. Proceed to Outdoor unit not running or No heat based on symptoms.
  • Bad: Cool air continues beyond 10 minutes—defrost control may have failed. Call a technician.

Outdoor unit not running

Goal: Get the outdoor unit to run when heat is requested.

  • If the outdoor unit does not run, the contactor, capacitor, or thermostat wiring may have failed. Use backup heat (Emergency Heat) if available.
  • Check for heavy ice on the outdoor coil—if iced over and not defrosting, call a technician.
  • Good: Unit runs after power fix, or backup heat works.
  • Bad: Unit still does not run—call a technician.

No heat

Goal: Isolate reversing valve or refrigerant issues when the unit runs but does not heat.

  • If the unit runs but blows cool or room-temperature air (and not in defrost), the reversing valve may be stuck in cool mode, or refrigerant may be low.
  • Do not attempt to add refrigerant yourself. Refrigerant work requires EPA certification.
  • Good: Technician diagnoses and repairs.
  • Bad: Continuing to run with low refrigerant can damage the compressor—call a pro.

When to get help

Call an HVAC technician if:

  • The thermostat is set correctly and the filter is clean but heating is weak or absent.
  • The outdoor unit is iced over and not defrosting.
  • The outdoor unit does not run and power checks pass.
  • You suspect refrigerant leak, reversing valve, or backup heat failure.

Refrigerant work must be done by a licensed professional.

Verification

  • Warm air comes from the vents when the thermostat calls for heat.
  • The outdoor unit runs and the fan spins (when not in defrost).
  • Filter is clean and outdoor coil is free of heavy ice.
  • No unusual noises or rapid cycling.

Escalation ladder

Work from the device outward. Stop when the problem is fixed.

  1. Confirm heat mode Check thermostat is set to Heat and above room temperature.
  2. Allow defrost Wait for defrost cycle to finish; cool air for a few minutes is normal.
  3. Power and filter Check circuit breaker, disconnect, and air filter.
  4. Outdoor coil Check for ice or blockage; heavy ice may mean defrost failure.
  5. Call a pro Reversing valve, refrigerant, defrost control, or backup heat—licensed pro.

What to capture if you need help

Before calling support or posting for help, have these ready. It speeds everything up.

  • Thermostat setting and mode
  • Whether outdoor unit runs
  • Air temperature at vents
  • Whether in defrost (cool air briefly)
  • Filter condition
  • Steps already tried

Is the thermostat set to Heat and above room temperature?

Heat pumps must be in heat mode. Cool or Auto may run in cool mode.

Check the thermostat. Set to "Heat." Set temperature above room temperature. Wait a few minutes. Yes: proceed to defrost check. No: set correctly and retest.

Yes No

You can change your answer later.

Set thermostat to Heat and retest

Set thermostat to Heat. Set temperature above room temperature. Wait 5 minutes. Check if warm air comes from vents. If yes, problem solved. If no, proceed to power and filter.

Done

Is the unit in defrost? (cool air for 2–10 min is normal)

Defrost briefly reverses the cycle. Wait for it to finish.

If you feel cool air, wait up to 10 minutes. Defrost cycle is normal. Warm air returns: heating works. Cool air continues: defrost may have failed—call a pro. Warm from start: proceed to power and filter.

Yes (cool air) No

You can change your answer later.

Wait for defrost to finish

Wait up to 10 minutes. Warm air should return. If it does, heating works. If cool air continues, defrost control may have failed—call a technician.

Done

Is power on and the filter clean?

Breaker, disconnect, and filter affect operation.

Check circuit breaker and disconnect switch. Replace dirty air filter. Good: power on, filter clean. Bad: fix and retest.

Yes No

You can change your answer later.

Fix power and filter

Reset breaker. Turn on disconnect. Replace filter. Retest. If heating returns, done. If not, proceed to outdoor unit check.

Done

Does the outdoor unit run when heat is requested?

The compressor and fan should run when the thermostat calls for heat.

When thermostat calls for heat, the outdoor unit should run. Yes: unit runs—check if air is warm. No: unit does not run—check contactor, capacitor, or use backup heat and call a pro.

Yes No

You can change your answer later.

Is warm air coming from the vents?

If unit runs but air is cool, reversing valve or refrigerant may be the issue.

Feel the air at the vents. Warm: heating works. Cool/room temp: reversing valve may be stuck, or refrigerant low. Technician must check—no DIY. Use backup heat if available.

Yes No

You can change your answer later.

Heating restored

Heating is working. Check that the outdoor coil is clear of ice and debris.

Done

Call a technician

Call an HVAC technician for: outdoor unit not running, iced-over coil not defrosting, weak or no heat (reversing valve, refrigerant, backup heat). Refrigerant work requires a licensed professional.

Done

Reviewed by Blackbox Atlas

Frequently asked questions

Why would a heat pump run but not heat?
Common causes: thermostat set to cool instead of heat, unit in defrost cycle (brief cool air), dirty filter or outdoor coil, stuck reversing valve, low refrigerant, or failed backup heat. Check thermostat and filter first.
Why does my heat pump blow cold air when heating?
During defrost, the heat pump briefly reverses to melt ice on the outdoor coil—you may feel cool air for a few minutes. If it lasts longer than 10 minutes or happens repeatedly, the defrost control may have failed—call a technician.
When should I call a technician for a heat pump that will not heat?
Call if the thermostat is set correctly and the filter is clean but heating is weak or absent. Call for outdoor unit iced over and not defrosting, refrigerant issues, reversing valve problems, or backup heat failure. Refrigerant work requires a licensed professional.

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