Heat pumps freeze up when moisture freezes on a coil and the system cannot remove the ice through its normal defrost process. A light layer of frost on the outdoor unit during heating is usually normal. Thick ice, ice that keeps growing, or ice that remains after defrost points to a problem such as restricted airflow, low refrigerant, a failed outdoor fan, or a faulty defrost system.
Heat Pump Freezing Up at a Glance
| What you see | Is it normal? | Likely explanation |
|---|---|---|
| Thin frost on the outdoor coil in winter | Usually | Moisture is freezing during normal heating |
| Frost that disappears periodically | Yes | The automatic defrost cycle is working |
| Thick ice covering the outdoor coil | No | Airflow, refrigerant, fan, drainage, or defrost fault |
| Ice remains after defrost | No | The system needs professional diagnosis |
| Ice on the indoor coil or refrigerant line in summer | No | Restricted airflow or low refrigerant |
Why Does a Heat Pump Frost Over During Normal Heating?
A heat pump frosts over during normal heating because the outdoor coil becomes colder than the surrounding air. Moisture in humid air condenses on the coil and freezes. The U.S. Department of Energy explains that frost forms when the coil is below the air's dew point and its surface temperature is below freezing.
The system should remove this frost through an automatic defrost cycle. During defrost, the heat pump temporarily reverses the refrigeration cycle and sends hot refrigerant through the outdoor coil. The outdoor fan may stop. You may also see steam or hear the system change sound. A short period of cooler indoor air can occur during the cycle.
A normal defrost cycle clears the frost. Ice becomes a problem when it builds up faster than the system can remove it.
What Causes a Heat Pump to Freeze Up?
1. Restricted Airflow
Blocked airflow prevents the heat pump from transferring heat properly. Possible causes include:
- A dirty indoor air filter
- Leaves, grass, or snow blocking the outdoor coil
- A dirty outdoor coil
- Blocked return or supply vents
- A malfunctioning outdoor fan
- Ice covering the coil
Carrier recommends checking the air filter and keeping the outdoor unit clear of debris. The required clearance depends on the model, but Carrier gives approximately 24 inches around many outdoor units as a general guideline.
2. Low Refrigerant or a Refrigerant Leak
Low refrigerant can make the coil operate at an abnormally low temperature. That encourages more ice formation and reduces the heat pump's ability to heat or cool the home.
A homeowner cannot safely diagnose or recharge refrigerant. An HVAC technician must test the system, locate any leak, and correct the refrigerant charge.
3. A Failed Defrost Control or Sensor
The defrost system uses temperature sensors and control components to determine when the outdoor coil needs to be cleared. A failed sensor, control board, or defrost timer can prevent the system from entering defrost mode or make it leave the cycle too early.
If the outdoor coil remains iced over even when airflow is clear, a defrost-control problem becomes more likely.
4. A Faulty Reversing Valve
The reversing valve changes the direction of refrigerant flow. During defrost, it must switch the system into a cooling-like operating mode so hot refrigerant reaches the outdoor coil.
If the valve is stuck or malfunctioning, the heat pump may not defrost correctly. This problem requires professional service.
5. Water Freezing on the Outdoor Unit
Water from a leaking gutter, roof edge, condensate drain, or melting snow can drip onto the outdoor coil and freeze. The resulting ice may build up faster than the normal defrost cycle can remove it.
Redirect water away from the heat pump, but do not chip ice from the coil. Chiseling can damage the fins and refrigerant tubing.
Why Is My Heat Pump Frozen in Summer?
A heat pump freezes in summer when too little warm indoor air passes over the indoor coil. The coil can then fall below freezing, causing moisture to freeze on the coil and restrict airflow further.
In cooling mode, the ice usually forms on the indoor evaporator coil or refrigerant line rather than across the outdoor coil.
Common causes include:
- A dirty or clogged air filter
- Blocked return airflow
- A dirty indoor coil
- Low refrigerant
- A blower-fan problem
- Ductwork or other airflow restrictions
Turn the system off and let the ice melt. Running a heat pump with a frozen indoor coil can damage the compressor or other components.
What Should You Do If the Outdoor Heat Pump Is Frozen?
If the outdoor heat pump is frozen, first determine whether the frost is light and temporary. Then:
- Check the frost. Thin frost that disappears during an automatic defrost cycle is generally normal.
- Check the air filter. Replace a dirty filter with the correct size and type.
- Clear visible debris. Remove leaves, grass, and snow from around the outdoor unit without opening the cabinet.
- Look for dripping water. Check nearby gutters, downspouts, roof edges, and other sources of runoff.
- Leave the ice alone. Do not chip or scrape it. Sharp tools can damage the coil.
- Call an HVAC technician if the ice remains. Persistent ice can indicate low refrigerant, a failed fan, a defrost-control fault, or another mechanical problem.
Carrier states that ice thicker than approximately 1/4 inch, or ice that does not clear during defrost, is not normal. The instructions for your specific heat pump take priority.
When Is a Frozen Heat Pump an Emergency?
Turn the system off and arrange professional service if:
- Ice completely covers the outdoor coil
- The heat pump never enters or completes defrost
- The outdoor fan is not running when it should
- The system blows cold air continuously in heating mode
- Ice appears on the indoor coil or refrigerant line
- The system freezes again after you replace the filter
- The compressor makes unusual noises or the breaker trips
A small amount of winter frost is part of normal heat pump operation. Ice that persists, spreads, or returns quickly is not normal. It usually means the heat pump needs repair, not more frequent defrosting.