By Hood Service Company · Corsicana, TX · Updated
The short version
- In heating mode the outdoor coil runs colder than the air, so frost forms in damp winter weather.
- The defrost cycle briefly reverses the system to melt frost, and steam and a whoosh are normal.
- Thick ice that covers the coil, the top panel or the fan for hours is not normal.
- Never chip ice off the unit; switch to emergency heat if you have it and call for service.
Why does a heat pump frost over in winter?
In heating mode, the outdoor coil has to be colder than the outdoor air to absorb heat from it. When the air is cold and damp, moisture freezes on the coil, much like frost on a windshield.
North-Central Texas winters often bring damp days near freezing, which are the conditions where frost tends to build quickly. A thin, even layer of frost is part of normal operation. Our guide to how heat pumps work explains the heating cycle.
What is the defrost cycle?
Defrost is a short, automatic cycle that switches the heat pump into cooling mode so warm refrigerant melts frost off the outdoor coil. It usually runs for a few minutes, then heating resumes.
A control board and sensors decide when to start and end defrost, based on coil temperature, elapsed time or both.
- The outdoor fan stops while the coil warms.
- You may see steam rising from the unit and water running off it.
- You may hear a whoosh or a click as the reversing valve shifts.
- Backup heat may come on indoors so the air from your vents doesn't feel cool.
What kind of icing is not normal?
Ice that covers the whole coil, builds on the top panel or around the fan, or stays for hours means defrost isn't keeping up. So does ice on the unit in mild, dry weather.
Ice on the refrigerant lines in summer is a different problem; see our guide to a frozen evaporator coil. The HVAC troubleshooter can help you sort out other symptoms.
- A solid sheet of ice hiding the coil fins.
- Ice on the top panel or fan blades, or a fan frozen in place.
- Frost that never clears after several defrost periods.
What causes a heat pump to ice up?
Common causes are a failed defrost control or sensor, a low refrigerant charge, a failing outdoor fan, and water dripping or pooling on the unit from gutters, roof runoff or poor drainage.
Leaves or debris packed around the base can trap meltwater so it refreezes. A refrigerant charge that's low from a leak makes the coil run colder and frost faster. Defrost controls, sensors and the reversing valve all need a technician to test.
What should you do if your heat pump is iced over?
Don't chip or pry at the ice, because coil fins and tubing damage easily. Clear debris from around the unit, check for dripping gutters, and switch to emergency heat if the unit is encased.
- Clear leaves, snow and debris from around the unit by hand.
- Check whether a gutter or roof edge is dripping onto it.
- Don't pour hot water on the unit or use a heat gun or tools on the ice.
- If your thermostat has an emergency heat setting and the unit is encased in ice, use it to stay warm until service. It runs backup heat only and uses more electricity.
- Schedule heat pump repair service and describe what you see.
How do you keep a heat pump from icing up?
A fall tune-up, which typically tests the defrost controls, checks the charge and cleans the coil, goes a long way. Keep the unit level, clear of debris and out of roof runoff.
Every Hood Comfort Club tier includes a fall tune-up each year; see our maintenance plans or schedule heat pump maintenance.
When should you call a technician?
- The outdoor unit stays encased in ice for hours.
- The outdoor fan won't spin or is frozen in place.
- The unit never seems to go into defrost, or seems to defrost constantly.
- Aux or emergency heat runs for long stretches in mild weather.
- You hear loud grinding or banging from the outdoor unit.
Frequently asked questions
How often does a heat pump go into defrost?
It varies with the weather and how the control is set. In cold, damp conditions it defrosts more often; in dry or milder weather it may rarely need to. A unit that seems to defrost nonstop should be checked.
Why does cool air come from my vents during defrost?
During defrost, the system is briefly in cooling mode. Many systems bring on backup heat to temper the air, but you may still notice a short stretch of cooler air. If it lasts longer than a few minutes, call for service.
Is steam from my heat pump a sign of a fire?
Usually not. In winter, steam is typically meltwater from the defrost cycle evaporating off the warm coil. Smoke with a burning smell is different: turn the system off at the thermostat and call.