Heat pumps are incredibly efficient — until they’re not. If your heat pump seems to be running but your house isn’t warming up during cold weather, you’re not alone. This is one of the most common heat pump complaints, and it has a few distinct causes depending on the temperatures outside.
First: Understand How a Heat Pump Works in Winter
A heat pump doesn’t generate heat the way a furnace does — it moves heat from outside air into your home. When outdoor temps drop below about 35–40°F, there’s less heat energy in the air to extract, and the system works harder for less output. Most heat pumps have a supplemental electric resistance strip heater that kicks in during very cold weather. If that backup heat isn’t working, you’ll feel the difference.
Common Causes
1. The Outdoor Unit Is Frozen Over
Some frost on the outdoor coil is normal and the heat pump has a defrost cycle to handle it. But if the entire unit is encased in a thick layer of ice, airflow is blocked and the system can’t extract heat. This can be caused by a faulty defrost board, low refrigerant, or a stuck reversing valve. Don’t try to chip the ice off — you can damage the coil fins. Call an HVAC tech.
2. Low Refrigerant
Refrigerant doesn’t “get used up” — if levels are low, there’s a leak somewhere. Low refrigerant drastically reduces heating capacity. You’ll notice the system runs constantly but barely warms the house. This requires a licensed HVAC technician to diagnose and repair.
3. Dirty Air Filter
Same story as with furnaces — a clogged filter starves the system of airflow and can cause the unit to run inefficiently or trigger safety shutoffs. Check and replace your filter first before doing anything else.
4. Reversing Valve Stuck in Cooling Mode
The reversing valve is what switches the heat pump between heating and cooling mode. If it sticks in the cooling position, the system blows cold air in winter. This requires a pro to diagnose and usually replace.
5. Backup Heat Not Working
Check your thermostat — if it’s set to “Emergency Heat” mode and you’re still not getting warmth, the electric strip heaters may have failed. A technician can test them with a multimeter.
What You Can Do Yourself
- Replace the air filter
- Clear debris, leaves, or snow from around the outdoor unit (maintain at least 18 inches of clearance)
- Make sure all supply vents inside are open and unblocked
- Check the thermostat settings and replace batteries if needed
Everything else — refrigerant, reversing valves, defrost boards — needs a certified HVAC technician. Heat pumps are more complex than furnaces and not the place to experiment.