EcoGuide · Heat Pumps · Published 2026-09-06 · Last Updated 2026-09-06 · 1,100 words
Heat Pump Iced Up? How to Tell Normal Defrost From a Real Problem
Some Ice Is Normal. Here Is the Line.
A heat pump extracts heat from outdoor air, which makes the outdoor coil colder than the air around it. Below about 40 °F with any humidity present, moisture condenses on that coil and freezes. This is expected physics, not a fault, and every heat pump is built to deal with it.
The system handles it by running a defrost cycle: it briefly reverses itself into cooling mode, sending warm refrigerant out to the coil to melt the frost off. According to Lennox, the system checks for frost every 30 to 90 minutes during heating, and a defrost cycle typically runs 5 to 15 minutes.
The line is simple: a light coat of frost that clears within about 15 minutes is the system working. Solid ice that encases the coil, builds up over days, or never clears is a problem.
What a Normal Defrost Looks Like
People call for service over a normal defrost more often than over a real fault, because it looks alarming if you have not seen it before. During a defrost you should expect:
- Steam rising from the outdoor unit. That is water vapour coming off a melting coil, not smoke. It is the single most common reason people think their heat pump is on fire.
- The outdoor fan stopping while the compressor keeps running. Counterintuitive, but correct — the fan would blow the heat away from the coil it is trying to warm.
- A whoosh or clunk as the reversing valve changes over. Normal.
- Slightly cooler air indoors for a few minutes, because heating has paused. Many systems bring in auxiliary heat during defrost specifically to mask this.
- Water and localised ice on the pad below. Melt water has to go somewhere.
If the air indoors feels cool at other times too, that is a separate and usually normal thing — see why heat pump air feels cold.
When the Ice Is a Real Problem
Go and look at the unit. What you see tells you which problem you have.
| What the unit looks like | What it means | What to do |
|---|---|---|
| Light frost on the coil, gone within 15 minutes | Normal defrost | Nothing. |
| Solid ice encasing the whole coil or the fan | Defrost is not working, or not keeping up | Service call. Do not chip it off. |
| Ice on the top of the unit or under the fan grille | Water dripping from above, or a blocked drain below | Check gutters, downpipes and roof runoff. |
| The unit sits in a pool or in packed snow | Melt water cannot drain away and refreezes | Clear the base; the unit should sit above grade. |
| Defrosting very often, or for far longer than 15 minutes | Dirty coil, faulty sensor, or low refrigerant charge | Service call. |
| Iced up in mild weather, above 40 °F | Not a weather problem — usually airflow or charge | Service call. |
The last row is the most diagnostic. Ice at 20 °F is the system doing its job; ice at 50 °F means something is wrong regardless of the weather.
What You Can Safely Do Yourself
- Clear space around the unit. Manufacturers generally want clearance on all sides and above. Snow drifts, leaves, a fence panel or a bush grown in close all restrict the airflow the coil needs.
- Clear snow away from the base, gently and by hand or with a soft brush. The unit needs to shed melt water; sitting in packed snow guarantees it refreezes.
- Look up. A leaking gutter or a roof valley dumping water onto the unit will ice it up in a way no defrost cycle can fix. This is a surprisingly common cause and a cheap fix.
- Check your filter. A blocked indoor filter restricts airflow across the indoor coil, which can push the system towards icing. It is the cheapest thing on this list.
- Pour lukewarm water over a fully iced coil, only if you need heat urgently and only with the system switched off. It is a stopgap, not a repair.
Never do these: chip or scrape ice off the coil — the fins and refrigerant tubing are thin aluminium and copper, and puncturing them turns a service call into a replacement. Do not use hot water, which can crack components, and do not point a heat gun or torch at it. Do not run the system with the ice in place hoping it clears.
Why Excessive Icing Costs You Money
Every defrost cycle is a period where the system is cooling the outdoors instead of heating your house, and most systems fire auxiliary electric heat strips during it to stop you noticing. Those strips are resistance heat — the expensive way to warm a house, and the reason a heat pump bill spikes in cold weather.
So a system defrosting far more often than it should is not only failing to heat efficiently, it is actively running your most expensive heat source repeatedly. That is usually what people are seeing when a winter bill jumps without an obvious cause.
If your bill has climbed and you are not sure whether icing, aux heat or the thermostat is responsible, work through why aux heat spikes a heat pump bill and why a heat pump runs constantly in winter.
Frequently Asked Questions
Q: Is it normal for a heat pump to ice up in winter?
A: A light coat of frost on the outdoor coil is normal below about 40 °F, and the system clears it automatically. Lennox puts the defrost check at every 30 to 90 minutes with a cycle lasting 5 to 15 minutes. Solid ice that encases the coil, or frost that never clears, is not normal.
Q: Why is steam coming off my heat pump?
A: That is water vapour from a defrost cycle, not smoke. The system has reversed briefly into cooling mode to warm the outdoor coil and melt the frost off it. It is the most common reason people call for service on a perfectly healthy heat pump.
Q: Why has the outdoor fan stopped while the unit is still running?
A: During defrost the fan stops deliberately. Running it would blow away the heat being used to melt the coil. It restarts when the cycle finishes.
Q: Should I chip the ice off my heat pump?
A: No. The coil fins and refrigerant tubing are thin aluminium and copper, and puncturing a line turns a repair into a replacement. If you need heat urgently, switch the system off and pour lukewarm — not hot — water over the coil.
Q: My heat pump is icing up in mild weather. Why?
A: Icing above roughly 40 °F is not weather-driven, so it points at the system rather than the conditions: restricted airflow from a dirty coil or blocked filter, a faulty defrost sensor, or low refrigerant charge. That last one means a leak, which needs a technician.
Q: Does icing make my electricity bill go up?
A: Yes, indirectly. Most systems run auxiliary electric heat strips during defrost so you do not feel the pause, and those strips are the most expensive heat in the house. A system defrosting excessively is running them far more often than it should.
Related Guides
- Heat pump blowing cold air? — why that is usually normal
- Heat pump runs constantly in winter — when long run times are correct
- Emergency heat vs auxiliary heat — what the difference actually costs
- Why aux heat spikes your bill — the expensive half of a cold snap