If your heat pump seems to be blowing cold air in heat mode, first check whether the air is actually cold or just cooler than you expect. Heat pumps deliver air around 90 to 100°F, which can feel cool on your hand compared with a gas furnace (120°F or more). The air also turns cool for a few minutes during normal defrost cycles. If the air stays at room temperature or below, the most common causes are thermostat settings, a defrost problem, low refrigerant, a stuck reversing valve, or backup heat that isn’t working.
Quick test: hold a kitchen thermometer in a supply vent for a few minutes. If it reads 85 to 105°F, the heat pump is working normally. If it reads close to room temperature, keep reading.
1. Thermostat set wrong
- Mode should be HEAT (or AUTO), not COOL. Fan on AUTO, not ON; with the fan on ON, the blower runs between heating cycles and moves room-temperature air.
- On thermostats configured for the wrong system type, the reversing valve may be wired backward (O vs. B setting), so “heat” actually cools. This usually shows up right after a new thermostat installation. See thermostat wiring colors explained.
2. Normal defrost mode
In cold, damp weather, frost builds on the outdoor coil. The heat pump periodically switches into defrost, briefly running in cooling mode to melt the ice. During that time, the indoor air may feel cool unless backup heat kicks in. Defrost usually lasts 5 to 15 minutes. See heat pump defrost mode.
3. Outdoor unit iced over
If the defrost cycle isn’t working, the coil stays frozen and the heat pump can’t absorb heat. See heat pump covered in ice.
4. Dirty filter or blocked outdoor unit
Low airflow indoors or outdoors cuts heating output. Replace the filter and clear snow, leaves or debris from around the outdoor unit.
5. Low refrigerant
Low refrigerant reduces heat output in winter just as it reduces cooling in summer, and always means a leak. Call a technician.
6. Stuck reversing valve
The reversing valve switches the heat pump between heating and cooling. If it sticks in cooling position, the heat pump blows cool air in heat mode. A technician can test the valve and its solenoid coil.
7. Backup or auxiliary heat not working
On very cold days, the heat pump relies on backup heat strips or a furnace. If those aren’t working (tripped breaker for the strips, failed sequencer), the house may get cool air. Check the air handler’s breakers. See heat pump emergency heat.
8. Outdoor unit not running
If the indoor fan runs but the outdoor unit is silent, check the breaker and disconnect. A humming outdoor unit with a stopped fan usually means a failed capacitor. See bad AC capacitor symptoms; heat pumps use the same parts.
Using an ecobee? Backup heat that runs too often has its own fix: ecobee aux heat running too much.
If the thermostat says it’s heating but the house won’t warm up, see thermostat says heat on but no heat.
If it heats in short bursts and shuts off, see heat pump short cycling.
Frequently asked questions
Why does my heat pump feel cold even though it’s running?
Heat pump air is warm but cooler than furnace air, and air moving across your skin feels cooler. Measure it with a thermometer before assuming a problem.
Why does my heat pump blow cold air for a few minutes?
It’s probably in defrost mode. That’s normal in cold, humid weather.
Can I use emergency heat if my heat pump blows cold air?
Yes, as a temporary measure while you wait for service, but it’s expensive to run.
Disclaimer: this guide is for general information only. HVAC systems involve high voltage and refrigerant. When in doubt, contact a licensed HVAC professional.