Heat Pump Defrost Cycle Explained
On a cold, damp morning your outdoor unit stops its fan, makes a whoosh, and starts pouring steam. Inside, the air from the vents goes lukewarm for a few minutes. Most homeowners who call about this are watching a heat pump do exactly what it was designed to do.
This guide explains what the defrost cycle is, why BC weather makes it happen more often than you might expect, and the specific signs that mean something is actually wrong.
The Problem: Your Outdoor Coil Is Colder Than the Air
In heating mode, the outdoor coil is the part that pulls heat out of the outside air. To do that, the refrigerant inside it has to run colder than the air around it — often 5–10°C colder.
When the coil surface drops below 0°C and below the dew point of the air, moisture in the air freezes onto the fins as frost. Frost insulates the coil and blocks airflow, so heating output falls. Left alone, it would build into solid ice.
The defrost cycle clears that frost before it costs you real capacity.
What Happens During a Defrost Cycle
- The reversing valve shifts. The system briefly runs in cooling mode, sending hot refrigerant gas to the outdoor coil. That shift is the "whoosh" or hiss you hear at the start and end of the cycle.
- The outdoor fan stops. With no air blowing across it, the coil heats up quickly and the frost melts.
- Steam rises from the unit. Warm coil plus melting frost in cold air makes a visible cloud. It is water vapour, not smoke.
- Water drains out of the base. Meltwater runs out of the bottom of the unit. On a cold, humid day that can add up to several litres.
- Indoors, the heat dips. Ducted systems usually turn on backup heat (electric strips or the furnace) so you don't feel cold air. Ductless heads typically slow or stop their fan until the coil is warm again.
A normal cycle lasts roughly 2–10 minutes. Then the valve shifts back, the outdoor fan restarts, and heating resumes.
Why BC Homes See More Defrost Cycles Than You'd Think
Frost builds fastest when it is near freezing and humid — roughly −7°C to +2°C with damp air. That describes a large share of winter hours on the coast.
- Lower Mainland, Victoria, and the coast: long stretches of 0–5°C fog and drizzle. Expect frequent but short defrost cycles. This is normal.
- Okanagan and Thompson: valley cloud and inversion fog bring humid near-freezing days, then colder, drier spells. Defrost frequency swings with the weather.
- Kootenays, Cariboo, and the North: at −20°C and below the air holds very little moisture, so frost builds more slowly — but snow and drifting become the bigger risk to the outdoor unit.
Technical detail: how the unit decides when to defrost
Time-temperature defrost (common on older and entry-level units): when a sensor on the outdoor coil reads below a set point (often around −1°C to 0°C), a timer starts. Every 30, 60, or 90 minutes of run time, the unit defrosts whether it needs to or not. The interval is usually set on the control board.
Demand (adaptive) defrost (most modern inverter units): the control watches the gap between outdoor air temperature and coil temperature, and in some designs refrigerant pressure or compressor data. As frost builds, that gap widens. The unit defrosts only when the reading says frost is actually there, and it learns from how long the last cycle took to clear. Fewer unnecessary cycles means less wasted energy.
Termination: a cycle ends when the coil sensor reaches a set temperature (often around 10–20°C) or a maximum time limit, whichever comes first.
Efficiency cost: each cycle borrows heat from your house to warm the outdoor coil. In humid near-freezing weather, defrost can take a noticeable bite out of seasonal efficiency. That is already reflected in HSPF2 ratings, which include defrost losses in their test conditions.
Normal vs. Needs Service
| What you see | Normal | Call a technician |
|---|---|---|
| Steam from the outdoor unit | Yes, for a few minutes | — |
| Whoosh/hiss at start and end | Yes | Loud banging or grinding |
| Outdoor fan stops briefly | Yes, during defrost | Fan never restarts |
| Light frost on the coil between cycles | Yes, in damp cold | — |
| Coil fully encased in ice for hours | — | Yes |
| Ice building from the bottom up | — | Yes — drainage or base problem |
| Defrosting every 15–20 minutes in dry, very cold weather | — | Yes — possible sensor or charge issue |
| Never steams, ice keeps growing | — | Yes — defrost isn't starting |
Common causes of a unit that ices up solid:
- Low refrigerant charge (a leak — this needs a licensed refrigeration mechanic, not a top-up)
- Failed coil or ambient temperature sensor, or a faulty defrost control
- Reversing valve that sticks or doesn't shift fully
- Outdoor fan motor problems
- Water dripping onto the unit from a roof edge or gutter, refreezing on top
- Meltwater with nowhere to go, freezing under the unit and growing back up into the coil
Installation Details That Prevent Most Ice Problems
Many "defrost problems" are really installation problems. When you get quotes, ask about these:
- Mounting height. The unit should sit above your area's typical snow depth on a stand or wall bracket, so snow can't block the coil or bury the base. In the Interior and North, that often means a raised stand rather than a ground pad.
- Drainage path. Meltwater needs to drain away freely and must not run across a walkway, driveway, or stairs, where it will refreeze into a slip hazard.
- Roof protection. Keep the unit out from under roof drips and valleys, or add a manufacturer-approved cover that doesn't restrict airflow.
- Base pan heater. Many cold-climate models have, or can add, a small heater that keeps the base pan from freezing so meltwater can always drain.
- Clearances. Follow the manufacturer's minimum clearances on all sides so airflow isn't restricted. Restricted airflow means more frost.
What You Can Do Yourself
- Keep snow cleared from around and on top of the unit, and keep the base and drain area open.
- Make sure nothing is stacked against it — firewood, bins, or plowed snow.
- Don't chip ice off the coil. The aluminium fins bend and puncture easily, and a punctured coil is an expensive repair. If the unit is iced solid, switch the thermostat to emergency or backup heat and call for service.
- If you have ductless heads, a few minutes of cool air or no airflow during defrost is normal. Don't turn the setpoint up to compensate.
Frequently Asked Questions
Is it normal for my heat pump to steam in winter?
Yes. Steam during a defrost cycle is water vapour from frost melting off a warm coil. It usually lasts a few minutes and then stops when normal heating resumes.
How often should a heat pump defrost?
It depends on the weather and the control type. In humid near-freezing weather, cycles every 30–90 minutes are common. In dry, very cold weather, they should be much less frequent. Frequent cycles in dry cold can point to a sensor or refrigerant problem.
Why does cold air blow from my vents during defrost?
During defrost the system is briefly running in cooling mode to warm the outdoor coil. Ducted systems normally switch on backup heat to cover this. If you feel genuinely cold air, the backup heat may not be staging on — have it checked.
Should I pour hot water on an iced-up heat pump?
It's better not to. It can refreeze on the base and drain, and it doesn't fix the cause. Switch to backup heat and have a technician find out why the defrost cycle isn't clearing the ice.
Does defrost use a lot of electricity?
Each cycle uses some energy, and in damp coastal weather it adds up over a season. Modern demand-defrost controls keep this to a minimum, and published HSPF2 efficiency ratings already account for it.
Next Steps
- Backup heat for cold climates — how strips, furnaces, and controls cover defrost and the coldest nights
- HSPF2 and COP explained — comparing real cold-weather performance between models
- Cold-climate heat pumps for the BC Interior — equipment and installation for colder, snowier regions
- Heat pump noise and your neighbours — placement that keeps defrost noise and steam away from property lines
Before you get quotes, run a free panel check to see whether your electrical service can carry a heat pump and its backup heat: