Learning Center
Heat Pump Problems Explained
Heat pumps heat and cool using the same refrigerant circuit — which means their problems look different than a traditional AC. Here's how to read the symptoms.
Common Heat Pump Problems
Heat pump blowing cool air in winter (heating mode)
Investigate SoonPossible causes:
- •Thermostat set incorrectly (verify it's in HEAT mode, not COOL or EM HEAT)
- •Outdoor temps below the heat pump's efficient operating range (typically below 30–35°F) — the backup auxiliary heat should be running
- •Low refrigerant charge (heat pumps need refrigerant to heat, same as to cool)
- •Reversing valve stuck or failing (the reversing valve switches the system between heating and cooling modes)
- •Defrost cycle in progress (normal, lasts 5–15 minutes)
What to do:
Check thermostat mode first. If set correctly and outdoor temps are above 35°F, call for service — reversing valve or refrigerant issues require a technician.
AUX HEAT light on constantly (even in mild weather)
Schedule ServicePossible causes:
- •Heat pump not generating enough heat — system relying on backup electric strips
- •Low refrigerant (reduced heating capacity)
- •Failing reversing valve (not switching to heating mode properly)
- •Dirty outdoor coil (reducing heat-absorption efficiency in winter)
- •Defrost cycle issue (not defrosting, keeping outdoor coil inefficient)
What to do:
AUX HEAT running occasionally (during very cold weather) is normal. AUX HEAT running continuously on mild days (above 40°F) is not — the heat pump should be handling most of the load. Schedule service.
Ice or frost on the outdoor unit that doesn't clear
Call SoonPossible causes:
- •Defrost board or sensor failure (system not initiating defrost cycles)
- •Reversing valve not functioning (can't switch to defrost mode)
- •Low refrigerant (reduces defrost effectiveness)
- •Outdoor fan motor issue
What to do:
Light frost on the outdoor coil is normal — the heat pump's defrost cycle clears it periodically. A solid block of ice that doesn't clear in 1–2 hours indicates a defrost system failure. Do not attempt to chip or remove ice manually — you can damage the fins and coil.
System won't switch from heating to cooling (or vice versa)
Schedule ServicePossible causes:
- •Reversing valve stuck or failed (the valve that switches refrigerant flow direction)
- •Control board issue
- •Thermostat wiring issue
What to do:
Allow 5 minutes for the system to fully stop before switching modes. If it still won't switch after waiting, call for service. Reversing valve replacement is a professional repair.
Heat pump running but no cooling in summer
Call SoonPossible causes:
- •Low refrigerant (most common cooling-mode failure)
- •Dirty outdoor coil (preventing heat rejection)
- •Compressor failure
- •Reversing valve stuck in heating position
What to do:
See our full AC troubleshooting guide. Low refrigerant is the most common cause — if the system is running but not cooling, and the refrigerant lines have ice on them, shut the system off and call.
Heat pump making unusual noises
Depends on noise typePossible causes:
- •Loud banging or clanking: loose or broken component — shut off and call
- •Grinding: bearing failure in outdoor fan motor or compressor
- •Hissing at outdoor unit: refrigerant leak
- •Chattering relay: normal startup click (occasional) vs. chattering = relay failing
- •Gurgling or bubbling sounds: refrigerant flow sound (normal during defrost)
- •Loud swoosh during defrost: normal — refrigerant reversing direction
What to do:
Loud, new, persistent mechanical noises (banging, grinding) warrant shutting the system off. Soft, consistent new sounds (mild hum change, occasional gurgling) can be monitored and reported at next service visit.
Frequently Asked Questions
Heat Pump Not Performing Right?
Heat pump diagnostics require a technician with the right instruments — refrigerant gauges, electrical meters, and heat pump-specific knowledge. NC License #27258 · Authorized Trane Dealer.