Why Is My Heat Pump Steaming in Winter?

Twice on a cold afternoon, a heat pump will stop blowing warm air, go quiet outside, make a sound like a large sigh, and then start steaming as if something is burning. Homeowners who are new to a cold-climate heat pump installation almost always call about this the first winter. It is the defrost cycle, it is supposed to happen, and the steam is the most visible part of a process that mostly takes place inside a coil.

What happens during a heat pump defrost cycle?

During defrost, a heat pump temporarily runs backwards. In heating, the outdoor coil is the cold side: it absorbs heat from outdoor air, and its surface sits below the outdoor temperature. Below roughly freezing, any moisture in that air condenses on the coil and freezes there. Once enough frost accumulates, air can no longer pass through the fins and the unit loses capacity. The control responds by reversing the refrigerant flow so the outdoor coil becomes the hot side and the frost melts off.

1. The control decides frost has built up, using coil temperature, outdoor temperature and run time.
2. The reversing valve shifts. That is the whoosh or thud people hear, and it is normal.
3. The outdoor fan stops, so the coil is not being cooled by the air it is trying to warm.
4. Hot refrigerant floods the outdoor coil. Frost melts, water runs down through the base and out, and the warm wet coil throws off visible vapor.
5. Supplementary heat may come on indoors to keep the supply air from feeling cold while the coil is busy.
6. The valve shifts back, the fan restarts, and normal heating resumes — usually after a few minutes.

How often should a heat pump go into defrost?

How often a heat pump defrosts depends on humidity and temperature, not on a fixed schedule. The most frost-heavy conditions here are not the coldest ones — they are the damp, near-freezing days that make up a great deal of a Twin Ports shoulder season, when there is plenty of moisture in the air to deposit on the coil. Expect more frequent defrosts in wet snow and freezing fog, and fewer in the dry, deep cold of a clear January night.

What matters is that the cycle ends. A defrost that runs, clears the coil and returns to heating has done its job, whether it happens twice an hour in a wet snow or twice a day in dry cold. Cold-climate equipment is built to keep working through this: these units hold roughly 70–85% of rated capacity at 5°F and continue producing useful heat down to about −13°F to −15°F, which is the point covered at length in our guide to heat pumps in Duluth winters.

When is ice on a heat pump a problem?

Ice on a heat pump is a problem when it stops looking like frost and starts looking like a casting. Frost is white, fuzzy and evenly spread across the coil fins, and it disappears at the next defrost. Ice is clear or milky, hard, and it stays. If you can see solid ice that survived several hours, the defrost cycle is not doing what it should, and running the system that way risks the outdoor fan blade and the coil itself.

Normal winter heat pump behavior versus a service call

What you see or hear	Normal	Call for service
Steam rolling off the outdoor unit for a few minutes	Yes — defrost melt water evaporating	Only if it never stops
A whoosh or thud, then the outdoor fan stops	Yes — the reversing valve shifting	If it happens every few minutes, all day
A thin even layer of white frost on the coil	Yes, between defrosts	If it is still there hours later
A solid block of clear ice on or through the coil	No	Yes — shut off heating and call
Ice growing upward from the base of the unit	No	Yes — the drain path is blocked
Water dripping and freezing on the pad below	Some is normal	If it builds into a mound touching the unit

Why does ice build up from the bottom of the unit?

Ice building upward from the base of an outdoor heat pump means the melt water is not getting away. Every defrost releases water that has to leave through the base pan and drain down and out. If the unit sits flat on a pad or directly on the ground, if the base drain holes are plugged with leaves and needles, or if the water lands somewhere it cannot soak away, it freezes under the unit and the next defrost adds to it. The ice climbs into the coil and the fan, and eventually the defrost cycle cannot win against it.

This is a mounting problem, and it is the single most common installation mistake in this region. An outdoor unit up here belongs on a stand that holds it well above the snow line for its actual location — not average snowfall, but the drift that forms in that corner of that yard. Out in the open country toward Sawyer and Atkinson, that can be a good deal higher than a standard pad puts it.

What should a homeowner do to help?

Keep the outdoor unit clear. A heat pump needs unobstructed air on all sides and above it, and in winter the things blocking it are snow, a drift off a roof, and the pile the plow or snowblower leaves. Clearing that is the whole of the homeowner's role.

• Shovel snow back from all sides of the unit and keep the top clear, using a broom or your hands near the fins.
• Never chip ice off the coil with a tool — the fins and the tubing puncture easily, and a refrigerant leak is a repair that dwarfs the problem you were solving.
• Do not pour hot water over the unit; it refreezes in the base and makes the blockage worse.
• Keep the unit out from under a roof drip line or a valley that sheds snow onto it.
• Do not wrap, box in or cover a heat pump that is running — it has to breathe all winter.
• If the coil is iced solid, switch the system to emergency or backup heat if you have it, then call.

What causes a heat pump to defrost constantly?

A heat pump that defrosts constantly is telling you that either it thinks there is frost when there is not, or frost is forming faster than it should. A failed or loose coil temperature sensor makes the control call defrost on a schedule with nothing to melt. Low refrigerant charge drops the coil temperature further below ambient, so frost builds faster than normal. A reversing valve that leaks internally will not shift cleanly, so the coil never gets hot enough to actually clear. Restricted airflow through a snow-packed or dirty coil does much the same.

None of those are diagnosable from the yard, because all of them look identical from outside: lots of defrosts, less heat, and a unit that is icing anyway. They are separated by refrigerant pressures, temperatures and electrical readings. The same applies to the outdoor sections of ductless mini-split systems, which run the same defrost logic and ice up for the same reasons.

When to call, and what a technician will check

Call when ice persists through several hours, when the unit defrosts continuously, when the heat pump never defrosts even though the coil is frosted, or when the unit is running but the house is not keeping up in cold weather. A technician checks the defrost sensor and control, refrigerant charge and pressures, the reversing valve's operation, the base pan drainage, and the unit's height and clearances — because a correct cold-climate heat pump installation prevents most of this before it starts. Homes on exposed ground around Cloverland usually need more stand height than the same equipment would need in town, and that decision is made at install time.