Heat pump blowing cold air or covered in ice?
Light frost on a heat pump's outdoor unit is normal; it melts off in short defrost cycles. Thick ice that doesn't clear within a day blocks airflow and usually means a defrost failure. Use Emergency Heat only if the heat pump isn't working or can't keep up, and call for service.
Why is my heat pump covered in frost or ice in winter?
A heat pump's outdoor unit pulls heat out of cold outside air, so its coil runs colder than the air around it. In cold, humid weather that coil collects frost, the same way a car windshield frosts over overnight.
Light frost on the coil is normal and expected. Your heat pump is built to deal with it on its own.
Unlike a central AC compressor that sits idle all winter, a heat pump's outdoor unit keeps running through the cold months. That is why heat pump owners see frost and ice that straight air-conditioning owners never do.
Why is there steam coming off my heat pump?
Steam rising off the outdoor unit usually means it is in a defrost cycle, melting off frost it just collected. Several times a day, the system automatically reverses briefly to clear the coil.
During those few minutes you may hear the unit run differently, too. That is the defrost cycle doing its job, not a malfunction.
The system's backup electric heat may also switch on automatically during defrost, which is why an Aux Heat light can flicker on at the same time.
How do I tell normal frost from a real ice problem?
The line between normal and a problem is simple: does it clear on its own? Light frost that melts off within hours is fine. Thick, solid ice that stays put between defrost cycles is not.
Look at three places on the outdoor unit:
- The coil (the finned sides of the unit)
- The fan blades on top
- The base of the unit
Solid ice building up on any of these blocks airflow. A heat pump that can't move air loses heating capacity fast, which is often why a house stops warming up even though the system is running.
If ice doesn't clear across a full day, especially if it builds up even in mild weather, that points to a stuck reversing valve or a failed defrost control board. Neither is normal winter operation, and both are jobs for a technician.
What's the difference between Aux Heat and Emergency Heat?
Aux Heat runs itself; Emergency Heat is a setting you turn on by hand and must remember to turn off. Mixing them up is the single most expensive mistake a heat pump owner makes in winter.
| Aux Heat | Emergency Heat | |
|---|---|---|
| How it turns on | Automatically | Manually, at the thermostat |
| What it does | Adds backup electric heat for a few minutes at a time | Stops using the heat pump and runs on backup electric heat alone |
| When you'll see it | During defrost, or when it's too cold for the heat pump to keep up alone | Only when the heat pump has failed |
| Is it a problem? | No, normal behavior | It means the heat pump needs service |
An Aux Heat light that comes on in cold weather or during defrost is normal. If it stays on constantly, have a technician check the system. You don't need to do anything about it.
When should I use Emergency Heat?
Use Emergency Heat only if the heat pump isn't working or can't keep up, and you need heat until a technician can get there. Call for service. It is a bridge, not a setting to leave on.
Emergency Heat is the most expensive way your system can produce heat. Never leave it on just because it happens to feel warmer.
If you do switch to it:
- Call for service right away. Needing Emergency Heat is itself a sign the heat pump needs a technician.
- Set a reminder to switch back to normal Heat once the real fix is done.
- Watch your electric bill. A week left on the wrong setting can add up to a substantial difference.
If your system runs but the house isn't warming up, check the thermostat first. Make sure it isn't stuck on Emergency Heat by mistake before assuming a bigger failure.
What can I check myself before calling a technician?
A quick look takes about 5 minutes and costs nothing. Go through these:
- Check the outdoor unit for ice. Is it light frost that clears, or thick ice that stays?
- Confirm the thermostat is set to normal Heat, not Emergency Heat.
- Remember that an Aux Heat light in cold weather is normal, but one that stays on all the time is worth a technician check.
- Keep the outdoor unit clear of snow drifts and debris so air can move through it.
- Save your technician's number now, in case you ever need Emergency Heat.
If the outdoor fan isn't spinning while the system is calling for heat, and it isn't in a defrost cycle, stop there. That is a safety issue as well as a repair one, and it needs a professional.
Doing it yourself
$0 — understanding the settings yourself is free; the cost is in getting them wrong.
Calling a pro
$75–200 service call; repairs average $150–650 (national average around $400). A defrost control board, a common winter failure, runs $200–500.
Time to check: 5 minutes to check the outdoor unit.
When to stop and call a pro
- Ice on the outdoor unit doesn't clear across a full day, especially if it builds up even in mild weather (likely a stuck reversing valve or failed defrost control board).
- The outdoor fan isn't spinning while the system is calling for heat outside a defrost cycle — a safety issue as well as a repair one.
- You've had to switch to Emergency Heat — the heat pump itself needs service.
- Ice on the unit comes with unusual noise or short-cycling (turning on and off rapidly), which points to a mechanical or electrical fault.
Want the full step-by-step?
The full $6 guide adds the complete troubleshooting table matching each winter symptom to its most likely cause and fix, plus the full 'When to stop and call a pro' list and the honest cost comparison. It also includes a printable heat pump winter checklist, down to setting a reminder to switch Emergency Heat back off once the real fix is done.
Get “Heat Pump in Winter: Ice, Defrost & Emergency Heat” — $6 →Or buy the guide inside the Heat Kit (5 guides, $14) or The Winter Home (all 20 winter guides, $39). Instant PDF, no subscription.
Frequently asked questions
Is it normal for my heat pump's Aux Heat light to come on?
Yes. Aux Heat is automatic backup electric heat that kicks in on its own during a defrost cycle, or when it's cold enough outside that the heat pump alone can't quite keep up. Seeing the light flicker on in cold weather is normal, expected behavior and not a sign of trouble.
What happens if I leave my heat pump on Emergency Heat?
Your system will run on backup electric heat alone, around the clock, at several times the normal cost. Emergency Heat is the most expensive way a heat pump system can make heat. Leave it on only until a technician fixes the heat pump, then switch back to normal Heat.
How much does it cost to fix a heat pump that keeps icing up?
A heat pump service call runs $75–200, and repairs average $150–650, with a national average around $400. Thick ice that won't clear across a full day often traces to a failed defrost control board, a common winter-specific failure that costs $200–500 to replace.
What happens if I ignore ice on my heat pump?
A heat pump stuck in a bad defrost cycle can freeze up and lose its heating capacity entirely. Ice on the coil, fan blades or base blocks airflow, and a heat pump that can't move air loses heat fast. If ice doesn't clear across a full day, call a pro.
Why is my heat pump turning on and off rapidly in winter?
Rapid on-and-off running is called short-cycling. When it happens alongside ice on the outdoor unit or unusual noise, it points to a mechanical or electrical fault. That combination is not normal winter operation and is worth a professional diagnosis rather than a DIY fix.
Sources
Safety guidance and key figures were checked against these outside sources on October 22, 2026. Dollar figures are typical ranges, not quotes.
- What is a Heat Pump and How Does it Work? (You Installed an ASHP, Now What?) (U.S. DOE Building Science Education Solution Center / CEE). Defrost cycle (5-15 min, reverses to melt ice, aux heat may run), frost normal below 32F, constant ice or stuck defrost means call a pro, keep unit clear of snow and uncovered, aux heat costly, signs to call a technician.
- What to Confirm After the Install (air-source heat pump fact sheet) (NYSERDA (New York State Energy Research and Development Authority)). Defrost triggers when coil is below 32F, lasts about 5-15 minutes; frost on outdoor unit is normal and cleared by defrost; do not cover the unit.
- Emergency Heat (EM Heat) explainer (North Georgia EMC (electric cooperative)). Aux and emergency heat use the same backup coils; aux comes on automatically for defrost, emergency heat is a manual setting that keeps coils running; uses lots of power and shows on the bill; if the heat pump isn't keeping the home warm, have an HVAC tech check it.
- Emergency Heat for heat pump systems (Google Nest Help (thermostat manufacturer)). Emergency heat is turned on manually when primary heating isn't working or isn't enough; usually expensive and inefficient; Aux heat can be automatic; frequent need means have a technician check.
- Electric Heat Lock Out on Heat Pumps (Washington State University Extension Energy Program). Electric strip (backup) heat is expensive; avoid running it when the heat pump can do the job; strip heat can kick on from fast thermostat recovery.
More winter fixes
For general homeowner reference only. When a repair involves gas, major electrical, or structural work, call a licensed professional.