On a cold, damp Sacramento morning, your heat pump may suddenly stop heating, blow cool air for a few minutes, make a loud whoosh, and send a cloud of steam off the outdoor unit. That alarms a lot of people. It is almost always the defrost cycle, and it is supposed to happen.
Why a heat pump needs to defrost
In heating mode, a heat pump pulls heat out of the outdoor air. To do that, its outdoor coil has to be colder than the air around it. When the air is cool and humid, moisture freezes on that cold coil. A layer of frost blocks airflow and cuts the heat pump’s ability to pull heat, so the system periodically melts it off.
What happens during defrost
- The control board decides it is time, based on a timer, a temperature sensor, or both.
- The reversing valve switches the system into cooling mode for a short time. That sends hot refrigerant to the outdoor coil to melt the frost. You hear a whoosh when the valve shifts.
- The outdoor fan stops, so the heat stays on the coil.
- Indoors, many systems turn on the electric backup heat so you do not feel a blast of cold air. If yours does not, or the backup heat is small, you feel cool air for a few minutes.
- When the coil is clear, the valve switches back with another whoosh, the fan restarts, and heating resumes.
You may see steam and water pouring off the outdoor unit. Defrost typically lasts a few minutes.
Signs defrost is working normally
- Defrost cycles happen now and then on cold, damp mornings and less often on dry or mild days
- Each one ends within a few minutes
- The coil is clear afterward, or has only a light frost building again
- Heating resumes and the house holds temperature
Signs something is wrong
- The coil stays iced over, or ice builds up on the sides and top of the unit. See heat pump freezing up.
- Defrost seems to run constantly, or the system spends more time in defrost than heating.
- The system never defrosts and the coil keeps thickening with frost.
- Cool air for long stretches, well beyond a few minutes.
- Heating in cooling mode in summer, or cooling in heating mode in winter, which points to a stuck reversing valve.
The usual causes are a failed defrost control board or sensor, a stuck reversing valve, an outdoor fan that is not working, or low refrigerant.
Why it blows cool air during defrost
Because for those few minutes the system is actually in cooling mode. Backup heat strips are there to temper that air. If you notice a cool draft for several minutes during each defrost and the house is otherwise warm, it is normal. If the air stays cool, see heat pump not heating.
Defrost and your electric bill
Every defrost cycle uses energy, and backup heat strips are expensive to run. A system that defrosts far more than it should, or leans on backup heat constantly, costs more to operate. Keeping the outdoor coil clean and making sure water from the roof is not dripping onto the unit both help.
What you should not do
- Do not chip ice off the coil.
- Do not pour hot water on it.
- Do not switch the system off every time it defrosts. Let the cycle finish.
What we do
If defrost is not working right, we test the defrost board, sensors and reversing valve, check the outdoor fan and verify the refrigerant charge. Our diagnostic is a flat $69 and it is credited toward the repair. Related: heat pump noises, how often a heat pump needs service, and our heat pump services page.
Frequently asked questions
What is heat pump defrost mode?
A short cycle in which the heat pump switches into cooling mode to send heat to the outdoor coil and melt frost, then switches back to heating.
How long does a heat pump defrost cycle last?
Typically a few minutes. It happens more often on cold, damp mornings and less often on dry or mild days.
Why does my heat pump blow cold air in winter?
During defrost the system is briefly in cooling mode. Backup heat usually tempers the air. If cool air lasts well beyond a few minutes, something else may be wrong.
Is steam coming from my heat pump normal?
Yes. Steam and water pouring off the outdoor unit during defrost is the frost melting.
Why is my heat pump always in defrost?
A failed defrost board or sensor, a stuck reversing valve, an outdoor fan problem or low refrigerant can make it defrost too often or ineffectively.
Should I turn off my heat pump when it defrosts?
No. Let the cycle finish. Switching it off interrupts the defrost and can leave ice on the coil.