On cold, wet mornings around here, it's normal to see a light coat of white frost on your heat pump's outdoor coil. The heat pump is built for that: every so often it runs a defrost cycle, melts the frost off, and goes back to heating. You might see steam and hear a whoosh while it does it.
What's not normal is ice that keeps building: a thick, clear or white shell covering the coil, ice on top of the unit, or the fan blades frozen in place. That means something is keeping the unit from defrosting or draining. Here's what to do.
Safety first
- Smell gas (rotten eggs)? Don't touch switches or the thermostat. Get everyone out, then call your gas company or 911 from outside.
- Carbon monoxide alarm going off? Get everyone outside into fresh air and call 911. Don't go back in to check the furnace.
- Turn the power off at the furnace or air-handler switch before you open any panel.
- Never bypass a safety switch (door switch, float switch, pressure switch, limit switch). They shut the system off for a reason.
What to check
Tell frost from ice
Frost is a thin, fuzzy white layer you can still see the coil fins through, and it comes and goes. Ice is thick, often clear or glassy, covers the fins completely, and doesn't go away after 30 to 90 minutes.
If it clears up on its own after a defrost cycle, it's working normally.
Never chip, scrape or hammer the ice
The coil fins and the copper lines behind them are thin. Breaking ice off with a tool, scraper or ice pick can puncture the coil and cause a refrigerant leak, which turns a service call into a major repair.
Turn the system to Off or Emergency Heat while you wait
If the unit is a solid block of ice, set the thermostat to Off (or Emergency Heat if you need heat in the meantime) so the outdoor unit stops running while it's frozen. Emergency heat uses more electricity, so treat it as temporary. See emergency heat explained.
Melt it with lukewarm water, gently
If you want to speed things up, turn the outdoor unit off at the thermostat first, then pour lukewarm (not boiling) water over the ice, or rinse it with a garden hose. Don't aim a pressure washer at the coil.
Check what's around and under the unit
The unit needs to drain the melt water away. Clear leaves, pine straw and mulch from around the base, and make sure the unit sits up on its pad or risers, not sunk into the ground. A gutter dripping onto the top of the unit can also cause a freeze-up. If yours does, that's worth fixing.
Check the filter inside
Low airflow indoors makes a heat pump work harder and can contribute to icing. Replace the indoor filter if it's dirty.
Turn it back on and watch it
Once the coil is clear, set the thermostat back to Heat and watch for the next hour or two. If ice builds right back up, the unit has a problem it can't fix on its own, and it needs a tech.
Why heat pumps frost up in the first place
In heating mode, the outdoor coil gets colder than the outside air, because that's how it pulls heat from it. When the air is cold and damp, which describes a lot of winter mornings in Raleigh and Durham, moisture in the air freezes onto that cold coil, the same way frost forms on a car windshield.
The heat pump's defrost control watches for that buildup using a temperature sensor and a timer. When it decides the coil needs it, it briefly reverses the system to send warm refrigerant to the outdoor coil, melts the frost, and the water drains out of the bottom of the unit. That's the steam and the whoosh you see and hear.
A freeze-up happens when that process breaks down: the defrost board or sensor fails, the outdoor fan doesn't restart, the system is low on refrigerant, or the melt water can't drain and refreezes at the base. Freezing rain and ice storms can also overwhelm a perfectly good unit for a while.
Prevention is mostly about drainage and maintenance. Keep the base clear, make sure the unit sits level on its pad or risers, redirect any gutter that drips onto it, and have the defrost controls and refrigerant charge checked during a fall tune-up.
When to call a pro
- Ice comes back after you've cleared it.
- The outdoor fan doesn't spin, or the unit hums but nothing moves.
- The unit never seems to go into defrost (no steam, no fan stop) even when frosted.
- Ice is on the copper lines or the top of the unit.
- You've been running on emergency heat for more than a day.
Still not working?
Frost Fire Heating & Cooling: $59 diagnostic (waived with repair), same-day service, no weekend surcharge, 4.8-star rated. Licensed & Insured. Serving Raleigh, Durham, the Triangle, Clayton and surrounding areas.
Common questions
Why does my heat pump freeze up?
Common causes include a failed defrost control or sensor, low refrigerant, a fan motor problem, poor drainage around the unit, or water dripping onto it from a gutter. A tech can test the defrost board and sensors and check the refrigerant charge.
Is it OK to run my heat pump while it's frozen?
No. Running a heat pump that's encased in ice can damage it. Turn it off or use emergency heat and get it checked.
How long should a defrost cycle take?
Usually several minutes, rarely more than 10 to 15. If it seems to run longer or happen constantly, mention that to your technician.
Can I put a cover on my heat pump in winter?
No. Unlike an AC-only unit, a heat pump runs all winter, so it needs open airflow. Covering it causes problems.
Related guides
Services: