Why Is My Heat Pump Covered in Ice?

You glance outside on a January morning and your heat pump looks like it was dipped in frosting. Sometimes that's exactly how a heat pump is supposed to look on a cold, damp day. Sometimes it's a sign that something has gone wrong and the system is about to stop heating. This guide shows you how to tell the difference, what you can safely do yourself, and what Denver's snow and freeze-thaw cycles have to do with it.

Why heat pumps frost up in the first place

In heating mode, a heat pump runs its refrigerant cycle backward compared with summer cooling. The outdoor coil becomes the cold side. It has to be colder than the outdoor air to pull heat out of it, so on a 35°F day the coil may sit well below freezing. Moisture in the air condenses and freezes on the fins, the same way frost forms on a cold drink can.

A little frost does no harm, but as it thickens it blocks airflow through the coil and the heat pump's output drops. So every heat pump has a way to clear it: the defrost cycle.

Frost is usually worst in the low 30s to upper 20s with high humidity, such as during a wet snow or freezing fog. In very cold, dry air there is less moisture to freeze. Denver's generally dry winter air helps, but our upslope snowstorms and foggy mornings produce heavy frosting days.

What a normal defrost cycle looks like

During defrost the reversing valve briefly switches the system into cooling mode. Hot refrigerant flows to the outdoor coil and melts the frost from the inside. The outdoor fan usually stops so the heat stays in the coil. Inside, a ducted system typically turns on backup heat for a few minutes so you don't feel cool air. Many ductless mini-splits simply pause the indoor fan.

Lennox's installation manual for one of its heat pumps gives a good look at how a typical time-and-temperature control works:

• Defrost is allowed only when the coil sensor reads 35°F or colder.
• The board checks at a set interval of 30, 60 or 90 minutes of run time. Sixty is the factory recommendation, and the manual calls for 30 or 60 in climates that spend long periods below 35°F at high humidity.
• Defrost ends when the liquid line warms to about 60°F, or after a maximum of 14 minutes.

Newer "demand defrost" controls and most inverter units compare coil and air temperatures and defrost only when frost is actually building, so timing varies. But the pattern is the same: frost builds slowly, then disappears in a few minutes.

Normal things that alarm homeowners

• Steam or "smoke" rising from the outdoor unit. That's meltwater evaporating off a warm coil.
• A loud whoosh as the reversing valve shifts at the start and end of defrost.
• The outdoor fan stopping while the compressor keeps running.
• Water dripping or a small puddle under the unit, which should drain away.
• A brief burst of cooler air indoors on systems without backup heat during defrost.

Normal frost vs. problem ice

Common causes of problem icing

1. Snow, drip lines and drainage

This is the big one on the Front Range. The National Weather Service puts Denver's average seasonal snowfall at 56.9 inches, with measurable snow possible from mid-October (average first date October 19) to late April (average last date April 28). Foothills communities get more. Our pattern of heavy snow followed by sunny days creates constant melt and refreeze.

Where that becomes a heat pump problem:

• Snow drifts around a unit sitting on a ground-level pad block airflow and give meltwater nowhere to go.
• Defrost meltwater has to drain away. If it pools under the unit, it refreezes into a growing ice block that eventually reaches the coil.
• Roof runoff from a valley or a gutterless eave drips onto the unit and freezes on the top and fan, which no defrost cycle can clear.

Manufacturers address this in their installation instructions. Lennox's manual calls for the outdoor unit to be raised above the average snow line in snowy climates, at least 3 inches above grade, with free drainage of defrost water, and kept away from walkways where meltwater can ice the path. New York's Clean Heat program commissioning checklist asks installers to record the unit's height above grade "to avoid snow line" and confirm it is not under a roof drip line.

In practice that means a sturdy stand or wall bracket that lifts the coil above typical drift depth, a clear path for meltwater (gravel rather than a sealed pad that tilts toward the house), and a gutter or diverter if the unit must sit near an eave. Your installer should choose the height based on your site. A unit in a wind-scoured side yard in Thornton and one on the north side of a house in Evergreen face very different snow.

2. Defrost controls and sensors

If the coil sensor fails, comes loose from its tube, or the control board fails, the heat pump may never start defrost. The frost thickens until the coil is a solid sheet of ice and heating output collapses. A technician can test the sensor and force a defrost to see whether the system responds.

3. Reversing valve problems

The reversing valve is what flips the system into defrost. If the valve sticks or its solenoid coil fails, defrost can't happen even when the control calls for it. Sometimes the same fault also shows up in summer as poor cooling.

4. Outdoor fan motor or capacitor

If the outdoor fan isn't turning in heating mode, the coil gets very little air and ices quickly. A humming unit with a still fan points to a failed capacitor or motor. That's electrical work for a technician.

5. Low refrigerant

A heat pump low on refrigerant runs a colder coil, which frosts faster and often unevenly, typically with ice concentrated on part of the coil. Like an air conditioner, a heat pump is a sealed system. Low charge means a leak, and the fix is to find and repair it, not just add refrigerant.

6. Dirty coil or low indoor airflow

A coil clogged with summer cottonwood fluff and dirt frosts faster and defrosts less effectively. On the indoor side, a clogged filter or closed registers reduce heat transfer and can upset system pressures. Both are easy to rule out.

7. Freezing rain and ice storms

Freezing drizzle can glaze a unit in ice that the normal defrost cycle can't keep up with, especially on the cabinet and fan guard. Switch the heat pump off and let backup heat carry the house until the ice can be cleared safely or melts.

What to do when you find your heat pump iced up

Emergency heat: when to use it and when not to

Ducted heat pumps usually have auxiliary heat, either electric resistance strips in the air handler or a gas furnace in a dual-fuel system. In normal operation the thermostat brings it on automatically for defrost, on very cold days, or when you raise the setpoint a lot. You'll often see "Aux" on the display. That's normal.

Emergency heat ("Em Heat") is a manual setting that turns off the heat pump and runs only the backup. Use it when:

• The outdoor unit is encased in ice or buried in snow.
• The outdoor unit has failed and you're waiting for a repair.
• A technician tells you to.

Don't leave it on longer than necessary if your backup is electric strips. Electric resistance heat has a coefficient of performance (COP) of 1.0: one unit of heat for each unit of electricity. In NREL lab tests in Golden, one cold-climate mini-split measured a COP of 2.77 at 47°F and still about 1.3 at -22°F, as our article Do heat pumps work in a Denver winter? explains with NREL test data. A week on emergency strips can show up clearly on your Xcel bill. With a gas furnace as backup, the cost difference is smaller, but the heat pump should still be fixed promptly.

Ductless mini-splits generally have no built-in backup. If one ices up and stops heating, use another heat source safely until it's repaired, and never use an oven or an outdoor-rated heater indoors.

Denver winter specifics

• Typical winter nights are not extreme. NOAA's 1991–2020 normals for Denver International Airport put average December and January lows around 18–19°F. Most of the season is spent in the temperature range where frost and defrost are routine, so expect frequent cycles on damp days.
• Snow arrives early and late. With an average first freeze around October 7 and last freeze around May 5 (NWS), your unit sees freeze-thaw for seven months. Check its surroundings after every big storm, not just midwinter.
• Chinook and sun. Warm, sunny afternoons melt roof snow that refreezes at night. Watch for new drips above the unit after storms.
• Altitude. Thinner air carries less heat per cubic foot, so outdoor coils move more air for the same output. Proper sizing accounts for it. See our altitude guide and Manual J sizing guide.

Preventing ice problems

• Have a fall tune-up check the defrost control, sensor, reversing valve, fan, capacitor and refrigerant charge.
• Rinse the outdoor coil in late summer after cottonwood season (power off, garden hose only).
• Keep 2 feet of clearance around the unit, and never stack shoveled snow next to it.
• Fix gutters above the unit and divert roof runoff.
• Replace the indoor filter on schedule.
• If your unit sits on a ground pad, ask about a stand or wall bracket before the next big storm.

When to call for service

Call if ice doesn't clear after a couple of hours, the coil is a solid block, ice covers the top or fan, the fan doesn't spin, the house can't keep up without emergency heat, or defrost seems to happen constantly. MrFixer Heating & Cooling repairs heat pumps, mini-splits and dual-fuel systems across the Denver metro. See our heat pump repair and mini-split repair pages, or call (720) 466-7983. If you've been told your heat pump needs replacing because of icing, a free second opinion can confirm whether it's a repairable control or placement problem. If you are replacing, our heat pump installation team plans for snow and drainage from day one. Check our service areas.

Frequently asked questions

• Is it normal for my heat pump to have ice on it? A thin, even layer of frost on the outdoor coil is normal in cold, damp weather, and the defrost cycle clears it regularly. A thick block of ice, ice on the top or fan, or frost that never clears is not normal and needs service.
• How long does a heat pump defrost cycle last? Usually a few minutes. One common Lennox control checks for frost every 30, 60 or 90 minutes and limits defrost to 14 minutes, ending sooner once the coil warms. Demand-defrost and inverter systems vary but follow the same pattern.
• Why is steam coming out of my heat pump? That's a defrost cycle. Warm refrigerant melts the frost and the water evaporates off the coil as steam. It's normal, as is a whooshing sound and the outdoor fan stopping for a few minutes.
• Can I pour hot water on my frozen heat pump? It's not recommended. The water can refreeze in the base and stress components, and chipping ice can puncture tubing. Brush away snow, clear drainage, switch to emergency heat if needed and call a technician.
• Should I switch to emergency heat when my heat pump is iced? Only if the outdoor unit is encased in ice, buried in snow, or not running. Emergency heat turns the heat pump off and runs backup heat only, which costs more if your backup is electric strips. Switch back once the unit is repaired or cleared.
• How high should a heat pump sit off the ground in Denver? Above the typical snow depth for your site, with room for defrost water to drain and freeze below the coil. Lennox's manual calls for at least 3 inches above grade and elevation above the average snow line in snowy climates. Your installer should set the height for your yard.