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Ice on the outdoor unit

A light frost that clears during defrost is normal; heavy ice that covers the coil, blocks airflow or does not clear within hours needs attention, and you should never chip ice or pour hot water on the unit.

Editorial staffEdited by heatpumpwiki

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Contents

Some frost on the outdoor coil in winter is normal. The heat pump is designed to build it up and melt it off with Heat pump defrost cycles. Lennox's guidance draws the line: "A light layer of frost on the outdoor coils is completely normal... heavy ice buildup that covers the entire unit or blocks airflow signals a problem."Source 1

Normal vs not normal

Normal Needs attention
Thin white frost on the coil fins in cold, damp weather Solid ice covering the whole coil or cabinet
Frost that disappears during a defrost cycle, with steam Ice that stays after defrost cycles, or keeps growing
Water running off during and after defrost Ice building up under the unit or on the fan
Ice on the top of the unit from a dripping gutter or roof edge

Why it happens

NRCan explains that in heating mode, when the outdoor temperature is near or below freezing, "moisture in the air passing over the outside coil will condense and freeze on it," and the amount "depends on the outdoor temperature and the amount of moisture in the air."Source 2 Defrost normally clears it. Heavy ice usually points to something stopping that from working, such as:

  • Snow around or against the unit blocking airflow or drainage. Lennox's user guide says to "keep the snow level below the louvered panels."Source 3
  • Meltwater with nowhere to go. If the unit sits too low, defrost water can refreeze under and around it. This is why installation manuals and codes call for elevation above snow. The DOE and New Buildings Institute checklist requires that "in locations susceptible to snow, outdoor units must be installed on stands above the snow line,"Source 5 and Daikin's manual suggests minimum elevations from 2.5 to 12 inches depending on design temperature, "or above the annual snow fall level."Source 4
  • Water from above. Roof runoff or a gutter dripping onto the unit can freeze on top of it.
  • A defrost control, sensor or refrigerant problem. These are for a technician.

What you can do safely

  1. Clear snow from around and on top of the unit with a broom or by hand. Keep the area around it open so air can flow.
  2. Do not chip or break the ice, and do not pour hot water on it. Lennox: "never pour hot water on the unit... don't try to chip or break the ice."Source 1
  3. Give it time. Lennox suggests calling a technician "if ice doesn't melt within 6-8 hours or refreezes quickly."Source 1
  4. Use backup heat if needed. If the house is getting cold while you wait for service, systems with backup heat can run on emergency heat. See Heat pump backup heat and lockout temperatures.
  5. Fix the cause where you can. Redirect a gutter downspout or roof drip away from the unit.

When to call a technician

  • Ice does not clear within the time Lennox suggests, or comes back right away.Source 1
  • The outdoor fan is blocked by ice or not turning.
  • The unit never seems to go into defrost (no steam, no fan stop) while ice grows.
  • The unit sits below the snow line and needs a stand. See What a good heat pump installation looks like.

Diagnosing a defrost control or a refrigerant problem, and any refrigerant work, requires an EPA Section 608 certified technician.Source 6 Do not attempt to add refrigerant or open refrigerant connections.

Sources

  1. 1.What to Do If Your Heat Pump Is Frozen. Lennox. Accessed Oct 25, 2026
  2. 2.Heating and cooling with a heat pump. Natural Resources Canada. Accessed Oct 25, 2026
  3. 3.Heat Pump System Information, User Guide (507657-01). Lennox. Accessed Oct 25, 2026
  4. 4.Condensing Unit / Heat Pump Installation Instructions (P/N IOD-4002G). Daikin Comfort Technologies. Accessed Oct 25, 2026
  5. 5.Heat Pump Space Heating Permitting & Inspection Checklist. U.S. DOE / New Buildings Institute. Accessed Oct 25, 2026
  6. 6.Section 608 Technician Certification Requirements. U.S. Environmental Protection Agency. Accessed Oct 25, 2026

More in Troubleshooting

  • Heat pump not heatingCheck the thermostat mode and setpoint, breakers and disconnects, the filter, vents and the outdoor unit first; anything involving refrigerant, electrical components or repeated faults needs a technician.
  • Heat pump short cyclingShort cycling means the heat pump turns on and off in short bursts, commonly because it is oversized for the load, and also because of thermostat placement, airflow or refrigerant problems.