Heat Pump Defrost Cycle: What’s Normal in a Canadian Winter

A heat pump defrost cycle is a normal, automatic process that melts frost off the outdoor unit in winter. If you see your heat pump pause, release steam and make a whooshing sound for a few minutes, it’s most likely defrosting, not breaking down. Knowing what normal defrost looks like helps you tell it apart from a problem that needs a technician.

Why heat pumps need to defrost

In heating mode, the outdoor coil gets colder than the outdoor air so it can absorb heat. When the air is cold and damp, moisture freezes onto that coil. Frost blocks airflow, so the heat pump gets less heat out of the air and runs less efficiently.

To fix this, the heat pump briefly reverses. Natural Resources Canada (NRCan) explains that during defrost, the reversing valve switches the system to cooling mode and the outdoor fan shuts off. Warm refrigerant then flows through the outdoor coil and melts the ice. Read NRCan’s explanation of heat pump defrost.

What a normal heat pump defrost cycle looks like

During a typical cycle you may notice:

• the outdoor fan stops while the compressor keeps running
• steam or mist rising from the outdoor unit
• a whoosh or hiss when the system switches modes, and again when it switches back
• water dripping from the coil
• slightly cooler air from indoor vents for a short time, especially with ducted systems

Many systems turn on backup heat during defrost so your home stays comfortable. When the coil is clear, the heat pump switches back to heating automatically.

How often does a heat pump defrost cycle run?

There’s no single right number. How often a heat pump defrost cycle runs depends on the weather and the controls in your system:

Condition	What to expect
Around freezing with high humidity, fog or wet snow	More frequent defrosting, because frost builds up quickly
Very cold, dry air	Often less frequent, because there’s less moisture to freeze
Snow or ice blocking the unit	More frequent or longer defrosting, which is a sign to clear the unit

NRCan describes two types of control:

• Demand defrost watches conditions such as coil temperature and airflow, and defrosts only when frost has built up. This is the more efficient approach.
• Time-temperature defrost runs on a preset schedule, such as every 30, 60 or 90 minutes, when it’s cold enough for frost to form.

Your owner’s manual will tell you which control your system uses and how long a typical cycle lasts for your model.

Signs of a defrost problem

A heat pump defrost cycle that doesn’t clear the ice is a warning sign. Call a qualified heat pump technician if you notice any of the following:

• Thick ice stays on the coil after a defrost cycle, or the whole unit is encased in ice.
• The unit seems stuck in defrost, or defrosts much more often than usual in similar weather.
• Cold air blows indoors for long periods while the system is set to heat.
• You hear grinding, banging or metal-on-metal sounds.
• Water pools and refreezes around or under the unit.

These can point to a faulty sensor, a low refrigerant charge, a fan problem or poor drainage. Don’t chip ice off the coil with tools. The fins are thin and easily damaged.

If the unit is only noisy, not iced up, our guide to why a heat pump is loud in winter explains which sounds are normal.

How to help your heat pump defrost properly

• Keep it clear. After a storm, gently clear snow from around and on top of the outdoor unit. Keep the area around the base open so meltwater can drain away.
• Mind the roofline. Water or ice falling from the roof onto the unit can cause heavy icing. Ask your installer about a cover or a different location if this happens.
• Keep it raised. The unit should sit on a stand or wall bracket above typical snow levels. Clearances depend on the model, so follow the manufacturer’s installation manual.
• Book annual maintenance. NRCan recommends having a qualified contractor service your heat pump every year. A pre-winter visit can check sensors, controls and drainage.

If you’re thinking about screens or landscaping near the unit, see how to hide a heat pump without blocking airflow.

The bottom line

A heat pump defrost cycle is a sign your system is protecting its efficiency, not a sign it’s failing. Short pauses, steam and a whoosh are normal. Persistent ice, cold air for long periods or harsh mechanical noises are not. When you’re choosing a new system, a correctly sized cold-climate model with demand defrost will handle Canadian winters best. Our cold-climate heat pump guide can help.

Frequently asked questions