Updated April 2026 • 8 min read
Why Does My Heat Pump Freeze Up? 6 Common Causes & How to Fix Them
A frozen heat pump can leave you without heat on a cold Washington night. Here's what causes it and when you need professional help.
It's a chilly morning in Olympia and you notice your heat pump's outdoor unit is covered in ice. A thin layer of frost is normal during heating mode — your unit has a defrost cycle designed to handle it. But if the entire coil is encased in ice, or the defrost cycle never seems to kick in, you've got a problem that needs attention.
Normal Frost vs. Problem Ice
Normal: A light coating of frost on the outdoor coil during heating mode, especially on cold mornings. The unit should defrost itself every 30–90 minutes.
Not normal: Thick ice covering the entire outdoor unit, ice on the refrigerant lines, ice that doesn't melt after the defrost cycle runs, or ice forming during cooling mode (summer).
6 Common Causes of a Frozen Heat Pump
1. Faulty Defrost Control Board
The defrost board tells your heat pump when to switch into defrost mode. If it fails, frost builds up unchecked. This is one of the most common repairs we handle during Tacoma and Lacey winters.
2. Low Refrigerant
A refrigerant leak reduces system pressure, causing the coil temperature to drop below normal and ice to form rapidly. Signs include hissing sounds, reduced heating output, and higher electric bills.
3. Stuck Reversing Valve
The reversing valve switches your heat pump between heating and cooling modes. If it gets stuck, the defrost cycle can't function properly. You may notice the unit blowing cool air in heating mode.
4. Dirty Air Filter
A clogged filter restricts airflow across the indoor coil, which can cause the outdoor unit to work harder and freeze. This is the easiest cause to check — and the cheapest to fix.
5. Blocked Outdoor Unit
Leaves, debris, snow buildup, or overgrown shrubs around the outdoor unit restrict airflow and trap moisture. Keep at least 18 inches of clearance around all sides — especially important in wooded areas like Yelm and Shelton.
6. Faulty Outdoor Fan Motor
If the fan motor fails or runs slowly, there's not enough airflow to prevent frost from building into ice. Listen for unusual noises or vibrations from the outdoor unit.
What to Do If Your Heat Pump Freezes
- Check your filter — Replace if dirty
- Clear the outdoor unit — Remove debris, snow, or ice from around the unit (don't chip ice off the coil)
- Switch to "Emergency Heat" — This bypasses the heat pump and uses backup heating while you wait for service
- Call a technician — If the unit doesn't defrost within 2 hours of clearing obstructions, the defrost system needs professional diagnosis
Prevention for WA Homeowners
- Schedule fall maintenance before heating season
- Keep 18" clearance around the outdoor unit year-round
- Change filters every 60–90 days
- After heavy snowfall, gently clear snow from around (not on top of) the unit
- Consider upgrading to a cold-climate heat pump if you're in the foothills near Enumclaw or North Bend
Heat Pump Frozen? Call Us Now.
Christianson's Heating & Cooling provides 24/7 emergency heat pump repair across Western Washington. Call (206) 651-4986 — we'll get your heat back on.
Schedule Emergency Repair →Frequently Asked Questions
Why does my heat pump freeze up in winter?
Some frost on the outdoor unit during heating mode is normal — heat pumps have a defrost cycle to handle it. However, heavy ice buildup indicates a problem: low refrigerant, a faulty defrost board, a stuck reversing valve, or restricted airflow from a dirty filter or blocked outdoor unit.
Should I pour hot water on a frozen heat pump?
You can gently pour lukewarm (not hot) water over the outdoor coil as a temporary measure, but this doesn't fix the underlying issue. Never use sharp objects to chip ice off — you'll damage the fins. Call a technician to diagnose why the defrost cycle isn't working properly.
How much does it cost to fix a freezing heat pump?
Costs depend on the cause. A defrost board replacement runs $250–$600, refrigerant recharge is $250–$600, and a reversing valve replacement costs $500–$1,200. A dirty filter or blocked unit is the cheapest fix at $0–$50.
At what temperature do heat pumps stop working?
Standard heat pumps lose efficiency below 25–30°F. Cold-climate models (like those we recommend for Washington mountain foothills) maintain full capacity down to 5°F. In Western WA's mild lowlands, standard heat pumps work well year-round.