Heat Pump Defrost Mode and Winter Care: Snow, Ice and What Is Normal

Heat pump defrost mode is a normal, automatic cycle in which the outdoor unit briefly runs in reverse to melt the frost that builds up on its coil in cold, humid weather. It usually lasts a few minutes, may produce a cloud of steam and a whooshing or clicking sound, and then the heat pump returns to heating.

Ontario winters, with frequent temperatures around freezing, lake-effect snow and freezing rain, put more frost and ice on outdoor units than drier, colder climates. Knowing what is normal, and what is not, saves a lot of unnecessary worry and a few service calls.

How does heat pump defrost mode work?

In heating mode, the outdoor coil is colder than the outside air, because it is pulling heat from that air. When the outdoor temperature is roughly between −7 °C and +5 °C and the air is humid, moisture freezes onto the coil fins. Frost blocks airflow, so efficiency drops.

The heat pump's control board detects this, using temperature sensors, run time or pressure readings, and starts a defrost cycle:

1. The reversing valve switches, sending warm refrigerant to the outdoor coil.
2. The outdoor fan usually stops, so the heat stays on the coil.
3. Frost melts and drips off, often with visible steam.
4. Indoors, the system may turn on backup heat to temper the air.
5. After a few minutes (typically 2–10), the valve switches back and heating resumes, often with a short whoosh.

In humid near-freezing weather, defrost cycles might occur every 30–90 minutes. In very cold, dry weather (below about −15 °C) there is less moisture in the air, so cycles are often less frequent.

What is normal and what is not?

What you see or hear	Usually normal?	What to do
Steam rising from the outdoor unit	Yes, during defrost	Nothing
Whoosh or clunk when defrost starts or ends	Yes, reversing valve switching	Nothing, unless it is a loud bang every time
Fan stopped for a few minutes while compressor runs	Yes, during defrost	Nothing
Light white frost on the coil between cycles	Yes	Nothing
Water dripping or pooling under the unit	Yes, melt water	Make sure it drains away and does not refreeze on the unit
Coil fully encased in ice that does not clear	No	Check for snow, ice or blocked airflow; call a technician if it persists
Ice building up from the base pan or ground	No	Fix drainage or raise the unit; check the base pan heater if fitted
Thermostat showing auxiliary or backup heat constantly on mild days	No	See our backup heat guide
Fan blades hitting ice	No	Turn the unit off at the thermostat and let the ice melt

How do you keep snow and ice off a heat pump?

Placement does most of the work. A well-placed outdoor unit in Ontario should:

• Sit above typical snow depth. Wall brackets or a raised stand commonly put the unit 30–60 cm (12–24 inches) or more above grade. Snowbelt areas east of Lake Huron and Georgian Bay, and northern Ontario, may need more height.
• Stay out of roof drip lines. Water and ice falling from eaves can coat the unit in ice. Install a gutter or a small roof over the unit, leaving plenty of clearance above the fan discharge so air is not recirculated.
• Drain freely. Melt water must flow away from the unit. A unit sitting on a pad at ground level often builds an ice mound from below.
• Avoid prevailing wind and drifting. A spot where snow drifts or the plough piles snow will keep burying the coil.

Our heat pump noise and placement guide covers clearances and siting in more detail.

Caution: Do not use a shovel, ice pick, screwdriver or hammer on the coil. The aluminium fins and refrigerant tubes are easily punctured, and a refrigerant leak is an expensive repair that must be done by a technician with an ODP certificate.

Winter care checklist for your outdoor unit

1. Before winter, clear leaves and debris from the coil and around the base. Book a professional check if the system is due; see our HVAC maintenance checklist.
2. After each snowfall, gently brush snow off the top and sides with a soft broom. Clear at least 30–60 cm around the unit so air can flow in.
3. Keep the area below clear, so melt water drains away.
4. Check for ice from above after freezing rain or thaw-freeze cycles, and fix leaking gutters above the unit.
5. If the coil is iced solid, set the thermostat to emergency heat (or turn the heat pump off) and let the ice melt naturally, or pour lukewarm (not hot) water over the coil gently. Then switch back to normal mode.
6. Do not cover the unit with a tarp or plastic in winter. Heat pumps run all year and need airflow. Covers designed only for the top, from the manufacturer, are a different matter.
7. Keep indoor filters clean. Restricted indoor airflow makes the system work harder in cold weather. See our furnace filter guide.

What happens during a power outage?

When power returns after an outage, many heat pumps wait several minutes before restarting the compressor to protect it. Some manufacturers also recommend letting the compressor's crankcase heater warm up for a period after a long outage in very cold weather; check your manual. A heat pump with electric backup cannot run on a small portable generator, so plan for alternative heat in a long winter outage.

Signs it is time to call a technician

• The coil stays iced over after two or three defrost cycles.
• The outdoor fan does not run when the compressor is on (outside of defrost).
• The unit makes repeated loud bangs, grinding or squealing.
• Indoor temperatures drop steadily while the thermostat calls for heat.
• The base pan heater, if fitted, does not appear to work and ice keeps building from below.

When you call, note the outdoor temperature, what the thermostat displayed and how long the problem lasted. It helps the technician diagnose the issue quickly. If the unit is under warranty, call the installer first.

Frequently asked questions

Is it normal for my heat pump to steam in winter?

Yes. Steam during defrost is melted frost evaporating off the warm coil. It is not smoke and is not a sign of a fault.

How often should a heat pump go into defrost?

It varies with temperature and humidity. In damp weather around freezing, every 30–90 minutes is common. Modern units use sensors to defrost only when needed.

Should I cover my heat pump in winter?

No. Unlike a central air conditioner, a heat pump runs all winter and needs open airflow. A small roof or awning well above the unit is fine; a full cover is not.

Does defrost mode waste a lot of energy?

It uses some energy and briefly reduces heat output, but it is built into the seasonal efficiency ratings. Good placement and clear airflow keep defrost cycles short. Cold-climate units are still efficient overall in Ontario winters; see our cold-climate heat pump guide.