Heat Pump Not Defrosting: Why Ice Keeps Building Up

Frost that clears on its own is normal. Ice that keeps growing means the defrost cycle is not running, and that is a fault rather than a cold snap.

First, is it actually failing

Frost on the outdoor coil in cold damp weather is expected. The system collects it and periodically melts it off. Before treating this as a fault, confirm which of these you are looking at:

What you see	Verdict
Light frost on the coil that disappears every hour or two	Normal
Steam rising from the unit periodically	Normal, that is a defrost cycle working
Ice thickening over hours with no cycle observed	Fault
Ice encasing the coil, or sitting on top of the unit	Fault
A growing slab of ice underneath the unit	Usually drainage rather than defrost

The single most useful observation you can make is whether a defrost cycle ever happens. During one, the outdoor fan stops while the compressor keeps running, and steam is often visible. If you watch for an hour in freezing damp weather and that never occurs while ice accumulates, the control is not initiating defrost. Our guide to the defrost cycle covers what a healthy one looks like in detail.

Why defrost fails to start

Defrost is initiated by a control that decides when the coil needs clearing. On older systems that decision comes from a timer combined with a coil temperature sensor; on newer systems, from sensors comparing coil and outdoor air temperature. Either way, three things can break the chain:

The coil sensor. If the sensor that reports coil temperature has failed or has come loose from the coil, the control never learns that conditions warrant defrost. This is a common and relatively inexpensive failure.

The defrost control board. The board itself can fail, or fail partially, so the timer runs but the command never issues.

The reversing valve or its solenoid. Defrost works by shifting the same valve that switches heating and cooling. If the valve will not shift, the control can call for defrost all it likes and nothing happens. A useful clue: this failure often shows up alongside problems switching between heating and cooling generally, covered in running in the wrong mode.

All three require a technician. What you can do is narrow which one it is by observing whether the valve makes its characteristic whoosh at any point.

Causes that are not the control at all

Several things produce heavy icing on a system whose defrost function is working perfectly. These are worth ruling out first because some are free to fix.

Restricted airflow across the outdoor coil. Leaves packed into the fins, snow drifted against the unit, shrubs grown in too close, or anything stacked around it. The coil runs colder than it should and frosts faster than defrost can clear.

Water that cannot drain away. This is the one people misread most often. Melt water from a normal defrost has to escape. If the base of the unit sits in a depression, or on the ground in a snowy climate, that water refreezes and the ice column grows upward until it reaches the coil. The defrost cycle is running correctly and losing ground each time. Units in snowy regions belong on a raised stand for exactly this reason.

A leaking gutter or roof runoff above the unit. Constant dripping onto a cold coil defeats any defrost cycle.

Low refrigerant charge. An undercharged system runs its coil colder than designed, which promotes frosting. Icing plus weak heat output together point here, and it needs a certified technician because it means a leak.

What to do right now

1. Switch the system off if the coil is encased in ice. Running a heat pump against a solid block of ice achieves nothing and risks the compressor.
2. Clear snow and debris from around and above the unit, leaving generous clearance on all sides.
3. Check whether melt water can drain away from the base rather than pooling and refreezing.
4. If you must remove ice, use cool or lukewarm water. Never hot water on a cold coil, and never a pressure washer.
5. Never use anything metal. No screwdriver, no hammer, no ice scraper. The fins and the refrigerant tubing are soft, and a puncture turns a defrost complaint into a refrigerant leak and a far larger bill.
6. Do not switch to emergency heat as a fix. It stops the icing only because it shuts the outdoor unit off entirely, and it runs on resistance heat that costs substantially more. It is a way to stay warm while you wait for a visit, not a repair.

The opposite problem: defrosting too often

Worth mentioning because it presents as a complaint about defrost and has different causes. A system cycling into defrost every twenty minutes or less, repeatedly, is not behaving normally either.

The usual explanations are a failed sensor calling for defrost that is not needed, low refrigerant charge, or restricted airflow across the outdoor coil. Each defrost cycle costs you heat you already paid for plus a burst of backup heat, so excessive defrosting shows up on the bill as well as in comfort.

If the complaint is really that the system runs briefly and stops repeatedly rather than defrosting specifically, see short cycling, which is a different fault with a different cause list.

Common questions

How do I know if my heat pump is defrosting at all? Watch the outdoor unit during freezing damp weather. In a defrost cycle the outdoor fan stops while the compressor keeps running, steam is often visible, and the valve makes an audible shift at the start and end. If ice is accumulating and none of that ever happens, defrost is not initiating.

Can I pour hot water over a frozen heat pump? Use cool or lukewarm water rather than hot. Hot water on a very cold coil is a thermal shock, and the refreezing meltwater can make the situation worse if it cannot drain away. Never use anything metal to chip ice.

Is ice underneath the unit the same problem as ice on the coil? No, and this distinction matters. Ice underneath usually means melt water is not draining and is refreezing, which is a siting or drainage problem. Ice on the coil itself points at the defrost function or airflow.

Will running emergency heat fix the icing? It stops it, because emergency heat shuts the outdoor unit off entirely. It does not fix anything, and it costs considerably more to run than normal heat pump operation, so treat it as a way to stay warm until a technician can attend.

How this guide is put together

This page separates normal frosting from genuine defrost failure first, because most calls about ice turn out to be the system working correctly. Causes are then ordered by how often each is the answer, and the boundary between homeowner checks and technician work is stated rather than implied.

• Sources. Manufacturer documentation, industry pricing surveys and the references linked at the foot of this page.
• The DIY line. Anything involving refrigerant is legally restricted to an EPA Section 608 certified technician. Sealed systems, capacitors and line-voltage wiring are called out as technician work rather than left ambiguous.
• Prices. Given as ranges, because regional labour rates and local competition move them more than any other factor. Confirm against written quotes.

Sources

Primary references for the figures and procedures on this page. Where a number here is a range drawn from field experience rather than a published measurement, it is labelled as such in the text.

• EPA Section 608 Technician Certification — The federal requirement governing refrigerant handling, relevant to any suspected leak.
• ENERGY STAR: Air-Source Heat Pumps — Federal efficiency criteria and general cold-weather operating guidance.
• AHRI Directory of Certified Product Performance — Confirm the certified low-temperature performance of your specific matched system.

Last checked September 2026. Procedures are general to residential air-source systems; confirm against the manual for your exact model.