Heat Pump Defrost Mode: What’s Normal and What Isn’t
On a damp winter day you might see your heat pump stop, hear a loud whoosh, see steam rising from the outdoor unit and water pouring out underneath. It looks alarming, but it’s usually the defrost cycle doing its job. This guide explains what defrost looks like, how often it should happen, and the signs that it isn’t working.
Why heat pumps need to defrost
In heating mode the outdoor coil is colder than the air around it. When it is near or below freezing and humid, which describes much of a Lower Mainland winter, moisture freezes on the coil. Frost blocks airflow and cuts heat output. To clear it, the heat pump briefly reverses to send warm refrigerant through the outdoor coil and melt the frost.
What a normal defrost cycle looks like
- Outdoor fan stops while the compressor keeps running
- A whoosh or hiss as the reversing valve switches, at the start and again at the end
- Steam or “smoke” rising from the unit. This is water vapour, not smoke.
- Water draining from under the unit
- Indoors, airflow may slow or feel cooler for a few minutes, and some systems switch on backup heat
- Mini-splits may stop blowing and show a flashing light or a defrost symbol on the head
How long and how often?
| Typically normal | Worth a service call | |
|---|---|---|
| Length | About 2–15 minutes | Regularly longer than 15–20 minutes |
| Frequency | Every 30–90 minutes in cold, damp weather; rarely or never on dry or milder days | Every few minutes, or constantly in mild dry weather |
| Result | Coil clear after the cycle | Ice remains or keeps building |
Exact timing varies by brand. Many modern units use sensors to defrost only when needed, instead of on a fixed timer.
Signs the defrost isn’t working
- Ice keeps getting thicker and never clears. See outdoor unit frozen.
- The unit seems to be in defrost all the time
- Cold air indoors well beyond a few minutes. See heat pump blowing cold air.
- Loud banging or grinding, not just the normal whoosh
- An error code on the thermostat or indoor head
Common causes include a faulty defrost sensor or thermistor, a control board fault, a sticking reversing valve, or a low refrigerant charge. A technician needs to test these.
Helping the defrost do its job
- Keep the outdoor unit raised and the area under it clear, so melt water can drain and won’t refreeze into a block of ice
- Keep about 60 cm of clear space around the unit and fix any gutters that drip onto it
- Don’t cover the unit in winter
- Book a yearly heat pump service, which includes testing the defrost cycle
Not sure if it’s normal?
Take a short video of the outdoor unit and send it to us by text or WhatsApp at 778-798-4343. We’ll tell you whether it needs a visit. Heat pump repair · All heat pump services