Ice on Your Heat Pump? Normal Defrost vs. a Problem in a Minnesota Winter

If you've recently added a heat pump, the first frosty morning can be alarming. The outdoor unit is white with frost, then it starts hissing and steaming, the fan stops, and water runs out the bottom. All of that can be perfectly normal. A heat pump that's a solid block of ice for hours is not.

More Twin Cities homes have heat pumps every year, often paired with a gas furnace in a dual-fuel system. Here's how to tell normal winter frost from a real problem, and what to do about each.

Why heat pumps frost up

In winter, a heat pump pulls heat out of the outdoor air and moves it inside. To do that, the outdoor coil runs colder than the air around it. When the outdoor temperature is near or below freezing and the air is damp, moisture freezes onto that cold coil, the same way frost forms on a car windshield overnight.

Light frost doesn't hurt anything, but it does block airflow through the coil. So every heat pump has a defrost cycle to clear it.

What a normal defrost cycle looks like

When the control senses frost building up, it briefly reverses the system, sending warm refrigerant to the outdoor coil to melt the ice. During defrost, you may notice:

• The outdoor fan stops while the compressor keeps running.
• A whoosh or hissing sound when the cycle starts and ends, as the reversing valve switches.
• Steam rising from the unit, especially on cold, still mornings. It can look like smoke, but it's just water vapor.
• Water dripping or running from the base of the unit.
• Slightly cooler air from the indoor vents for a few minutes, unless backup heat comes on.

A defrost cycle usually lasts a few minutes, up to about 10 or 15, and may happen every half hour to every hour or two in frosty, humid weather. On dry, very cold days, it may defrost less often. All of this is expected.

What happens inside the house during defrost

While the outdoor coil is defrosting, the heat pump isn't heating the house. Most systems cover that gap automatically. In an all-electric heat pump system, electric backup heat strips in the indoor air handler switch on for a few minutes so the vents don't blow cold. In a dual-fuel system, the thermostat or control usually brings on the gas furnace instead.

Your thermostat may show messages like "defrost," "aux heat" or "auxiliary heat on" during that time. A short burst of backup heat during defrost is normal. Backup heat running for hours on a mild day is not, and it can raise your electric bill quickly. If you notice that, or the thermostat shows auxiliary heat constantly when it isn't very cold, have the system checked. A setting or sensor problem may be keeping the heat pump from doing its share of the work.

When ice means there's a problem

• The whole unit, including the top and sides of the cabinet, is encased in thick ice, not just frost on the coil.
• The coil stays solid with ice for hours and never clears.
• Ice is building on the fan blades or the fan won't turn.
• Water from a roof edge or overflowing gutter is dripping onto the unit and freezing.
• The unit is buried in snow, or sitting in a puddle that has frozen around its base.
• The house is losing heat and the backup heat isn't keeping up.

Those point either to an outside cause, like dripping water or poor drainage, or to a problem with the heat pump itself, such as a failed defrost sensor or control, a stuck reversing valve, a fan motor problem or low refrigerant.

What you can safely do

• Don't chip, hammer or scrape ice off the coil. The fins and tubing are thin and easy to puncture, and a refrigerant leak is an expensive repair.
• Don't pour boiling water on the unit or use a heat gun.
• Clear snow away from the sides and top so air can move through the coil. Keep at least a foot or so of clear space around it.
• Look up. If water is dripping from the roof, fix the gutter or redirect the downspout.
• Check that the unit sits level and high enough that meltwater drains away instead of refreezing around its base.
• If the unit is iced solid and the house is cold, switch the thermostat to emergency or auxiliary heat if it has that setting, then call for service. On a dual-fuel system, the furnace can carry the house while the heat pump is checked.

Heat pump placement matters in Minnesota

Many winter ice problems start with where the unit sits. A heat pump under a roof edge without gutters gets dripped on all winter. One sitting on a pad at ground level can end up buried in drifted snow or frozen into its own defrost water. That's why heat pumps in snowy climates are usually installed on a raised stand or wall bracket, away from roof runoff, with room for water to drain.

If your heat pump was set at ground level and keeps freezing up, raising it may solve the problem. Our online shop carries heat pump and mini-split stands, and we can assess placement during a service visit.

Never cover a heat pump

It's tempting to protect the unit with a cover or tarp, but a heat pump runs all winter and needs open airflow. Covering it can stop it from heating and damage the equipment. Our post on covering your AC for winter explains the difference between an air conditioner and a heat pump in winter.

How cold-climate heat pumps handle Minnesota winters

Today's cold-climate heat pumps keep producing heat well below zero, and in a dual-fuel setup the furnace takes over on the coldest nights. Our guide to cold-climate heat pumps in Minnesota covers how they work and who they make sense for. If you're considering one, our heat pumps page explains the options we install.

When to call

Call if your heat pump stays iced over for more than a couple of hours, if the fan won't run, if you hear grinding or loud buzzing, or if the house can't stay warm. An annual maintenance plan visit checks the defrost controls, refrigerant charge and fan before the season starts.

Call Heating & Cooling Two at 763-428-3677 or request service. Office hours are Monday through Friday, 7:30 a.m. to 4:30 p.m., and after-hours emergency service is available.