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Heat Pump Defrost Mode

Why frost forms

To absorb heat from outdoor air, the outdoor coil must be colder than that air. When outdoor air is between roughly 20 and 40°F and humid, moisture condenses and freezes on the coil fins. Frost acts as an insulating blanket and blocks airflow, so the unit's output and efficiency drop. Defrost clears it.

How defrost works

Most air-source heat pumps reverse the refrigerant flow for a short period, sending hot refrigerant to the outdoor coil to melt the ice. The outdoor fan stops so the heat goes into the coil rather than the air. Indoors, the system may turn on backup heat to avoid blowing cool air. Some mini-splits stop the indoor fan instead. When the coil is clear, the system switches back to heating.

How defrost is triggered

Older units used fixed timers, such as every 30, 60 or 90 minutes of runtime below a set temperature. Modern units use demand defrost, sensing coil temperature, outdoor temperature, pressure or airflow to defrost only when needed. Demand defrost saves energy by skipping unnecessary cycles.

What normal defrost looks like

• Outdoor fan stops; you may hear a whoosh or click as the reversing valve shifts.
• Steam or vapour rises from the outdoor unit; this is melting frost, not smoke.
• Water drips from the coil and runs out beneath the unit.
• Indoor air may feel cooler or the indoor fan may pause briefly.
• After a few minutes, normal heating resumes.

When defrost is not normal

• A thick block of ice that does not clear after several cycles.
• Ice building on top of the unit from roof drip.
• Defrost happening every few minutes in mild dry weather.
• The outdoor fan running while the coil stays iced.

These signs suggest a sensor, control, airflow or refrigerant issue, or a drainage problem. Turn the system to emergency heat if it is not heating, and call a licensed technician. Do not chip ice off the coil with tools; fins and tubes are easily damaged. Gently clearing loose snow around the unit is fine.

Installation that helps defrost

Raise the outdoor unit on a stand or bracket so meltwater drains away and does not refreeze around the base. Keep it out from under roof edges without gutters. In heavy snow regions, a small roof over the unit can help, provided it does not restrict airflow; follow the manufacturer's clearances. Some units have optional base pan heaters to keep drain areas clear. See cold-climate heat pumps.

Defrost and energy use

Defrost cycles use energy and, when backup heat tempers the indoor air, they add to costs. In damp climates near freezing, defrost can lower seasonal efficiency noticeably. It is one reason real-world COP is lower than lab values. See COP explained.

Worked example

Figures below are illustrative examples (2026-10-29), not quotes or guarantees.

A unit runs 4-minute defrosts about once an hour on a damp 32°F day. That is about 7 percent of each hour without heat to the house. If the indoor heat strips run at 5 kW during each defrost, they add about 0.33 kWh per hour, roughly $0.05 at $0.15/kWh, or about $1.20 over 24 hours.

Why frost forms

In heating mode the outdoor coil runs colder than the surrounding air so it can absorb heat. When the air is near freezing and humid, moisture in the air freezes onto the coil. Frost blocks airflow and lowers output, so the unit periodically reverses to warm the coil and melt it. This is most frequent between about 25°F and 40°F in damp weather, not on the coldest dry days.

Demand versus timed defrost

Older units start defrost on a fixed timer, for example every 60 or 90 minutes of run time, whether the coil needs it or not. Newer units use sensors to start defrost only when frost is detected. Demand defrost wastes less energy and keeps the house more comfortable. If you are comparing units, ask which method each uses.

Keeping the house comfortable during defrost

While the unit defrosts, it briefly stops delivering heat indoors, and some systems bring on backup heat to avoid blowing cool air. A cycle normally lasts a few minutes. If the house feels chilly during defrost, the installer can adjust the backup staging or fan delay.

Drainage and ice buildup

Meltwater must drain freely away from the unit. If it refreezes under the coil, ice can build up from the base and eventually block the coil. Raising the unit on a stand, keeping the area clear and ensuring the ground slopes away help. Never chip ice off the coil with tools; switch the system off and call a technician if ice will not clear.

When to call for service

Call a technician if the unit defrosts constantly, never seems to defrost while ice builds, or blows cold air indoors for long periods. These can indicate a faulty sensor, low refrigerant, a stuck reversing valve or a control board issue.

Defrost and efficiency

Each defrost cycle uses energy, so frequent cycles lower seasonal efficiency. Clean coils and correct refrigerant charge keep defrosts short.

Sounds during defrost

A hiss or click as the reversing valve shifts and a short pause of the outdoor fan are normal parts of the cycle.

FAQ

Is steam from my heat pump a fire?

Almost always not; it is defrost vapour. Smoke with a burning smell is different and needs immediate attention.

Why does my house feel cooler during defrost?

Heat to the house pauses briefly. Backup heat may soften this.

Can I trigger defrost manually?

Some units have a service mode for technicians; homeowners should not bypass controls.

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