If your heat pump isn't heating or is blowing cold air, first check whether it's in a defrost cycle: on cold, damp mornings the outdoor unit may stop its fan and give off steam for a few minutes, which is normal. If it isn't defrosting, check that the thermostat is set to HEAT with the fan on AUTO, replace a dirty filter, and look at the outdoor unit. If the air stays cold, call a technician.
Fallbrook Heating & Air installs and repairs heat pumps across North County, and we've been working on home heating and cooling here since 1972. Many "my heat pump is broken" calls turn out to be a normal defrost cycle or a thermostat setting, so start here before you worry.
Safety first. Don't open the panels on the outdoor unit or the indoor air handler. They carry high voltage, and capacitors can hold a charge after the power is off. Don't chip ice off the outdoor coil or pour hot water on it; the fins and tubing are easily damaged. If a breaker trips more than once, or you smell burning, turn the system off and call us.
Why is my heat pump blowing cold air?
The most common reason is a normal defrost cycle. Other causes are the thermostat set to COOL or the fan set to ON, a clogged filter, backup heat not working, a stuck reversing valve, or low refrigerant from a leak. Heat pump air also feels warm, not hot, which some people mistake for cold air.
A heat pump moves heat rather than burning fuel. In winter it pulls heat from the outdoor air and brings it inside, the reverse of what it does in summer. The air from your vents will feel noticeably cooler than air from a gas furnace, even when everything is working. Hold your hand at a supply vent for a minute rather than judging by the first puff.
What is a heat pump defrost cycle?
When the outdoor coil gets cold enough, moisture in the air freezes on it as frost, which blocks airflow and cuts heating. To clear it, the heat pump briefly switches into cooling mode, sending warm refrigerant to the outdoor coil. The outdoor fan stops, frost melts, steam rises, and then normal heating resumes.
Here's what a normal defrost looks and sounds like:
- The outdoor fan stops while the compressor keeps running.
- Steam or vapor rises from the outdoor unit. It's not smoke.
- You hear a whoosh or click as the system switches into and out of defrost.
- Water drips under the outdoor unit as the frost melts.
- Indoor air feels cooler for a few minutes, unless backup heat comes on.
Fallbrook's December nights average about 41°F, so frost and defrost cycles are most likely on cold, damp mornings. A thin, even layer of frost that clears on its own is normal. A coil encased in solid ice that doesn't clear is not.
| Cause | Sign | Can you fix it yourself? | Typical fix |
|---|---|---|---|
| Normal defrost cycle | Steam from the outdoor unit; fan stopped; cool air for a few minutes | Nothing to fix | Wait; heating resumes on its own |
| Thermostat mode or settings | Set to COOL, OFF or EM; fan on ON; setpoint below room temperature | Yes | Set HEAT, fan AUTO, setpoint above room temperature |
| Backup (auxiliary) heat not working | House can't catch up on cold mornings; AUX shows but air stays cool | Partly (check breaker once) | Heat strip, relay or control repair |
| Dirty filter | Weak airflow; filter gray or matted | Yes | Replace the filter |
| Stuck reversing valve | Cold air in HEAT mode, but cooling works | No | Valve solenoid or valve repair |
| Low refrigerant (leak) | Weak heat; heavy ice on the outdoor coil that won't clear | No. EPA certification required | Find and repair the leak, then charge to spec |
What should I check on the thermostat?
Make sure it's set to HEAT, not COOL, AUTO with a low heat setpoint, or EM. Set the fan to AUTO; on ON, the blower runs between heating cycles and blows room-temperature air. Set the temperature a few degrees above the room, and replace the batteries if the screen is dim or blank.
If you recently installed a new thermostat and the heat pump now cools when you ask for heat, the thermostat may be set up for the wrong reversing-valve setting. That's a configuration fix, and we can sort it out.
AUX vs EM. AUX on the screen means backup heat (usually electric heat strips, or a furnace in a dual-fuel system) is helping. That's normal on cold mornings. EM, or emergency heat, turns the heat pump off completely and runs only backup heat. Use it only if the heat pump has failed, and then call us.
Quick checks you can do safely
- Wait out defrost. If the outdoor unit is steaming with its fan stopped, give it several minutes.
- Thermostat. HEAT mode, fan AUTO, setpoint a few degrees above the room, fresh batteries.
- Filter. Hold it to a light. If light barely gets through, replace it.
- Breakers. The outdoor unit, the air handler and the heat strips may each have a breaker. Reset a tripped one once. If it trips again, leave it off and call.
- Outdoor unit. Clear leaves, weeds and anything stacked around it. Don't cover it.
- Vents. Open supply registers and keep returns clear.
What if the reversing valve is stuck?
The reversing valve is what switches a heat pump between heating and cooling. If it sticks in cooling, you get cold air in HEAT mode, even though the system cooled fine all summer. It can be an electrical problem with the valve's solenoid or a mechanical problem with the valve itself. Both need a technician.
Replacing the valve body means opening the refrigerant circuit, which requires EPA Section 608 certification. Don't tap on the valve or try to free it yourself.
Could it be low refrigerant?
Yes. A heat pump that's low on refrigerant heats weakly, and its outdoor coil may ice heavily and stay iced. Low refrigerant means a leak. A certified technician needs to find and repair the leak, then charge the system to the manufacturer's specification. Topping it up without fixing the leak doesn't last.
See our refrigerant leak repair page. If the indoor coil is icing in summer, our frozen AC guide applies to heat pumps too.
When to call a technician
Call if cold air continues well beyond a defrost cycle, if the outdoor coil stays encased in ice, if the system cools when set to HEAT, if a breaker trips again, or if you've had to switch to EM heat. Those point to a control, valve, refrigerant or electrical problem.
When you book a heat pump repair online, the problem is pre-filled for us, or call (760) 728-8716. Learn more on our heat pump page. If you have a gas furnace instead, see our furnace not working guide, and if you're weighing a switch, read our heat pump vs furnace guide. A yearly check covers defrost controls, the reversing valve and refrigerant charge.
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Frequently asked questions
Why is my heat pump blowing cold air?
Most often it's a normal defrost cycle, which can send cooler air through the vents for a few minutes. Other causes include the thermostat set to COOL or the fan set to ON, a dirty filter, a stuck reversing valve, or low refrigerant. If cold air lasts longer than a defrost cycle, call a technician.
Why is steam coming off my heat pump?
That's usually the defrost cycle. The heat pump briefly reverses to warm its outdoor coil and melt frost, and the meltwater turns to vapor in the cold air. The outdoor fan typically stops while it happens, and you may hear a whoosh when it switches back. It's normal and usually ends within minutes.
What does AUX or EM heat mean on my thermostat?
AUX means backup heat, usually electric heat strips or a furnace, is helping the heat pump, which is normal on cold mornings or after a large setpoint change. EM or emergency heat turns the heat pump off and runs only backup heat. Use it only if the heat pump has failed, and call us.
Does a heat pump work in Fallbrook winters?
Yes. Fallbrook's December nights average around 41°F, mild weather for a heat pump. You may see more defrost cycles on cold, damp mornings, and the supply air will feel warm rather than hot. If the house won't reach the setpoint on a typical winter day, have the system checked.