Troubleshooting
Heat Pump Not Heating? Causes and Fixes
Quick answer
A heat pump that will not heat is often doing something normal: supply air at 90 to 100 degrees feels cool to the hand but is heating the home, and steam during defrost looks alarming but is routine. Real faults include a stuck reversing valve, failed defrost board, refrigerant leaks, and auxiliary heat problems.
First, rule out normal heat pump behavior
Heat pumps confuse owners raised on furnaces. Furnace air leaves vents at 120 to 140 degrees; heat pump air leaves at 90 to 105 degrees, warmer than your skin but cool to the touch, while still heating the house steadily. If room temperature holds at setpoint, nothing is wrong.
Likewise, an outdoor unit that periodically stops, steams, and even makes whooshing sounds is running its defrost cycle, melting frost off the coil. Frost that fully covers the unit and never clears, however, is a genuine fault.
Real failure modes and what they cost
When the house genuinely will not hold temperature, these are the usual suspects.
| Symptom | Likely cause | Typical cost |
|---|---|---|
| Cool air in heat mode, cold outside | Low refrigerant or valve fault | $225 to $1,600 |
| Blows cold only in heat mode, cools fine | Stuck reversing valve | $450 to $1,500 |
| Outdoor unit solid ice, never defrosts | Defrost board or sensor failure | $300 to $700 |
| Huge bills, heat works | Stuck in auxiliary electric heat | $150 to $600 |
| No heat below 35 degrees | Undersized or aux heat failure | $300 to $1,200 |
| Outdoor unit will not run at all | Capacitor, contactor, or board | $150 to $600 |
The auxiliary heat trap
Most heat pump systems include electric resistance auxiliary heat that assists below roughly 30 to 35 degrees and during defrost. Two expensive failure modes hide here. If auxiliary heat fails, the home goes cold on the coldest nights even though the heat pump runs. If auxiliary heat sticks on, the home stays warm while consuming two to three times normal electricity, announced only by a shocking bill. If your thermostat shows aux or emergency heat constantly in mild weather, call for service.
Frequently asked questions
Should my heat pump run constantly in winter?
Long run times in cold weather are normal and efficient; heat pumps are designed to cruise rather than blast. Constant running while the indoor temperature falls below setpoint, however, signals lost capacity from a refrigerant leak, icing, or a component failure and deserves a service visit.
What temperature do heat pumps stop working?
Standard heat pumps lose meaningful capacity below 25 to 30 degrees and lean on auxiliary heat. Cold climate models hold full output to around minus 5 and operate to minus 13 or lower. No properly configured system simply stops; it shifts more load to backup heat.
Why is my heat pump covered in ice?
Light frost is normal and defrost cycles clear it. Ice encasing the whole unit means the defrost system failed, airflow is blocked, or refrigerant is low. Shut it to Emergency Heat and call for service; running an ice bound unit damages the fan and compressor.
Related guides
- Thermostat Not Working? Fixes That Take 5 MinutesHow to revive a dead or unresponsive thermostat, from batteries and breakers to the float switch trick, and when the problem is not the thermostat at all.
- Winter HVAC ChecklistMid winter heating care, cold snap protocols, frozen pipe prevention, and heat pump winter behavior explained.
- What Does HVAC Repair Cost?Complete 2026 HVAC repair pricing: national averages by repair type, what drives the price up or down, and how to know a quote is fair.
- HVAC Troubleshooting GuideThe master homeowner troubleshooting flowchart for any HVAC problem: no power, no heat, no cooling, weak airflow, strange noises, water leaks, and bad smells.