A heat pump that blows cool air in winter is often working normally. The two usual reasons are a defrost cycle, which lasts a few minutes, and the fact that heat pump air is simply less hot than furnace air. If the air stays cold for a long time, check the thermostat settings, the filter and the outdoor unit. If those are fine, the cause is usually a refrigerant, valve or control problem that needs a technician.
Work through the checks in this order. The first two are normal behavior; the rest are things you can look at before calling anyone.
1. It’s in defrost mode (normal)
In cold, damp weather, frost builds up on the outdoor coil. To melt it, the heat pump briefly switches into cooling mode. The outdoor fan stops, you may see steam rising from the unit, and if the backup heat doesn’t come on, the indoor air can feel cool for a few minutes. Then it switches back. If the cool air happens now and then and doesn’t last, this is almost certainly what you’re seeing. Our guide to how a heat pump works explains the cycle.
2. The air is warm, just not hot (normal)
A heat pump delivers a steady stream of moderately warm air rather than the hot blast of a gas furnace. Because that air is cooler than your skin, it can feel cool when you hold your hand right at the vent, even while the house is warming. If the rooms are holding the temperature you set, the system is doing its job.
3. A thermostat setting
- Mode set to Cool or Auto. Make sure it’s on Heat, and the set temperature is above the room temperature.
- Fan set to On. With the fan on “On” instead of “Auto,” the blower keeps running between heating cycles and pushes room-temperature air through the vents, which feels cool. Switch it to Auto.
- Dead batteries or a blank screen. Replace the batteries if your thermostat uses them.
4. A dirty air filter
A clogged filter cuts airflow across the indoor coil, which reduces how much heat reaches the rooms. The Department of Energy suggests cleaning or changing filters once a month or as needed. It’s the cheapest fix to try.
5. The outdoor unit is iced over or blocked
A light coat of frost that comes and goes is normal. A unit encased in thick ice that doesn’t clear after a defrost cycle is not, and it means the heat pump can’t collect heat. Snow drifts, leaves or a dripping gutter overhead can also smother the unit. Clear snow and debris from around it by hand or with a soft brush, but leave the ice on the coil itself alone. If it stays frozen for hours, the defrost system probably isn’t working.
6. The outdoor unit isn’t running
If the indoor blower runs but the outdoor fan and compressor are silent for a long time, the heat pump isn’t heating at all. Check whether the breaker for the outdoor unit has tripped and that the outdoor disconnect switch is on. If you reset a tripped breaker and it trips again, stop and call a technician.
7. Problems that need a technician
- Low refrigerant. A leak means less refrigerant to carry heat, so output falls and the coil may ice up. Refrigerant must be handled by a certified technician, who also needs to find and fix the leak, not just top it up.
- A stuck reversing valve. If the valve that switches between heating and cooling is stuck in cooling, the system will steadily blow cold air in heat mode.
- A failed defrost control or sensor. The unit may stay frozen, or get stuck in defrost.
- Backup heat not working. In very cold weather a heat pump relies on its auxiliary heat. If those heating strips or their breaker fail, the house gets cool air when the heat pump falls behind.
- Compressor or outdoor fan failure.
If the outdoor unit is broken and the house is getting cold, switching the thermostat to Emergency Heat runs the backup heat on its own until help arrives. It costs more to run, so use it only as a stopgap.
What you can check yourself vs. when to call a pro
| Safe for most homeowners | Call an HVAC technician |
|---|---|
| Wait out a defrost cycle | Cold air that lasts more than about 30 minutes |
| Set the thermostat to Heat and the fan to Auto | Thick ice that doesn’t clear |
| Replace the air filter | A breaker that trips again after a reset |
| Clear snow and debris from around the outdoor unit | Hissing, oily spots on the lines, or iced refrigerant pipes |
| Reset a tripped breaker once | Grinding, buzzing or a silent outdoor unit |
Safety first. Never chip, pry or pour hot water on ice on the outdoor coil; it can damage the fins and tubing. Turn off power at the disconnect before clearing debris near the fan, and don’t open electrical panels on the unit.
Frequently asked questions
How long should a heat pump defrost cycle last?
Usually just a few minutes. Length varies by model and weather, but if the unit seems stuck in defrost or the cool air goes on far longer, call a technician.
Why does my heat pump blow cold air only when it’s very cold outside?
Heat pump output drops as the outdoor temperature falls, and defrost cycles become more frequent. If your backup heat isn’t kicking in, the gap shows up as cool air. A technician can check the auxiliary heat.
Can I add refrigerant myself?
No. Refrigerant handling is restricted to certified technicians, and low refrigerant means there’s a leak that needs to be found and repaired.
Sources
- U.S. Department of Energy, Air-Source Heat Pumps
- U.S. Environmental Protection Agency, Section 608 Technician Certification