Heat Pump Frozen in Winter: Normal Frost or a Fault?

By Ray Bennett · Updated 2 October 2026 · 6 min read · Checked against the sources at the end

Heat Pump Frozen in Winter: Normal Frost or a Fault?

Short answer

A coat of frost on the outdoor coil in cold damp weather is normal and the unit clears it itself every hour or so. Ice over the fan, ice filling the base, or ice that is still there hours later is a fault. Three of the four causes are outside the machine and cost nothing to put right.

A heat pump frozen in winter is usually doing what it is supposed to. The outdoor coil runs below freezing in heating weather, so moisture in the air lands on it as frost, and the unit melts it off every so often by itself.

What matters is whether the ice clears. Frost that is gone an hour later is the machine working. A solid block that is still there tonight is not.

Before you start

What to carry: a torch, a garden hose and gloves. Ten minutes to look, and two to four hours to thaw one on a day above freezing, Lennox says.

Gas or electric? A heat pump is neither. It has no flame and no flue, only a box outside that runs all winter and electric strips indoors as backup, so nothing about gas appliances applies here.

Go and look properly. Take the torch round the whole unit, including the top and underneath. Where the ice is tells you more than how much of it there is.

Two things that turn ice into a new heat pump. Never chip it off. The fins and the thin tubing behind them go through easily with a screwdriver, and a punctured coil is a refrigerant leak, which usually means a new unit rather than a repair.

Never use boiling water. The shock can crack things. Cool or lukewarm water from a hose, with the power off, is the only safe way to help it along, and most of the time just switching it off and waiting is enough.

Is this frost, or is it a fault?

A coat of frost on the coil is the weather. Ice over the fan or filling the base is the machine failing to clear itself.

Normal. A white coat of frost on the coil fins, which comes and goes. Every half hour to hour and a half the unit runs backwards for a few minutes to melt it, the outdoor fan stops, steam pours off and the indoor backup heat covers the house. Then it goes back to heating. That whole performance is a defrost cycle and it is described in more detail on the page about a heat pump that seems not to be heating.

Not normal. Ice on top of the fan grille. Ice filling the base so the fan can catch it. Ice that has been there since this morning. Ice that comes straight back within a day of thawing. Any of those is a fault, and the section below names it.

What to do right now

Check 1Free, two to four hours of waiting

Stop it making more ice, then let it go

Put the thermostat on Emergency Heat. That locks the outdoor unit out and runs the electric strips instead, so the house stays warm while nothing is adding to the ice.

Then leave it. On a mild day it will thaw on its own. If you want to help it, a hose on cool water over the coil, never anything sharp and never boiling.

While it thaws, clear the snow and the leaves away from all four sides and off the top, and look at the base to see whether meltwater can actually get away.

Emergency Heat costs two to three times as much to run, so put it back to Heat once the unit is clear.

Why it iced up in the first place

Three of these four are outside the machine, and all three are free to put right.

Check 2

Look above the unit before you look inside it

Follow the wall upwards. A gutter with a blocked downpipe, a roof valley that sheds onto the unit, or a dripping outside tap puts water onto the top of a machine that is already below freezing.

The giveaway is ice on top of the fan grille rather than on the coil, because that water never went through the coil at all.

Fixing the gutter fixes the heat pump, and it costs nothing.

Check 3

Check the base can drain, and that the unit is up off the ground

The base pan has holes in it so that defrost water runs away. Leaves, grit and a nest of twigs block them, and then every defrost cycle leaves its water behind to freeze again. That is what builds a block of ice from the bottom up.

Clear the holes once the ice is gone.

In snow country the unit is meant to sit on a stand, high enough that it is clear of the deepest snow you get. One sitting on a slab at ground level, in a hollow that fills with meltwater, will ice up every winter no matter what else is right.

Check 4

Then it is the defrost system, and that needs a technician

If the outside is clear, the base drains and it still ices up solid, the unit is not running its defrost cycle properly.

Three things do that job: a control board that decides when to defrost, a small sensor clipped to the coil that tells it how cold the coil is, and the changeover valve that actually reverses the flow. Any of the three failing gives you the same picture.

A fan that is not turning at all does the same thing for a different reason, and that is usually the capacitor, which holds a charge and is not a do it yourself part.

Low refrigerant also ices coils, in patches rather than evenly, and that means a leak. Neither of those is a homeowner job.

Stopping it happening again

Keep it clear. At least two feet on all four sides, Lennox and Trane say, and nothing on top. Cut the shrubs back in autumn rather than in January.

Never put a full cover on it in winter. Covers are for units that are shut down for the season, which means air conditioners. A heat pump runs all winter and a cover over the sides chokes it.

Deal with the gutter. If water has come off the roof onto it once, it will again.

Change the indoor filter. A blocked filter means less warm air available for the defrost cycle to work with, so defrosts take longer and clear less.

Questions people ask

How often should it defrost?

Anywhere from every half hour to every hour and a half in cold damp weather, and hardly at all when the air is dry. Every fifteen minutes all day is too often and points at the same defrost parts.

Steam is pouring off it. Is it on fire?

No. That is ice melting during a defrost cycle, and it looks far more dramatic than it is. It lasts five to fifteen minutes.

Can I pour warm water on it to speed it up?

Cool or lukewarm, yes, with the power off. Boiling, no. And nothing sharp at any point.

It iced up once after a storm and has been fine since.

Then it was the weather. Wet snow driven into the coil is a one off. It is the unit that ices again within a day of thawing that has something wrong.

Should I run it on Emergency Heat all winter to be safe?

No. It costs two to three times as much and it does not fix anything. Use it while the unit thaws or while you are waiting for a repair, and then put it back.

The unit is running but the air indoors feels cool.

That is normal for a heat pump and it is a separate thing from ice: what temperature the air should actually be.

Sources: Lennox "What to Do If Your Heat Pump Is Frozen?" (thawing time, no hot water, no chipping, gutters, causes), "Understanding the Heat Pump Defrost Cycle" and "Should I Cover My AC Unit in Winter?", Carrier "Heat Pump Troubleshooting Guide" (gutters, defrost controls), Carrier 25HCE heat pump installation instructions (defrost control, water from the roof), Trane "Is This Normal? HVAC Smells and Sounds, Explained" (two feet clear), Honeywell Home RTH6360 quick installation guide (Em Heat), the LG extended piping engineering manual (raised stand in snow) and the DOE Energy Saver Guide 2017 (heat pump efficiency)