Heat Pump Defrost Cycles Ontario 2026: What Homeowners Should Expect

Why an Outdoor Coil Ices Up

A heat pump moves heat from outdoor air into the house. To do that, refrigerant in the outdoor coil must be colder than the ambient air so heat flows into it.[2]In Ontario winters the outdoor coil often operates at minus 10 to minus 20 Celsius even when the ambient air is closer to zero. Any moisture in the air that touches the coil freezes on the aluminum fins. The colder the coil and the higher the outdoor humidity, the faster frost builds.

Frost insulates the coil and blocks airflow, so heat transfer drops. Left alone, the unit would ice shut and stop heating within an hour or two. The defrost cycle is the designed response.[4]

How Reverse-Valve Defrost Works

When the defrost control board decides it is time to clear the coil, it commands the reversing valve to flip. The heat pump runs in cooling mode for a few minutes: hot refrigerant is routed to the outdoor coil instead of the indoor coil. The outdoor coil warms rapidly and the frost melts and runs off as water into the base pan.[4]During defrost:

• The outdoor fan stops so the coil heats faster and the warm air does not blow away.
• Steam (water vapour from melting frost meeting cold air) rises from the cabinet. This is not smoke or refrigerant.
• The indoor coil would blow cold air for the duration of the cycle, so electric-resistance supplemental heat energizes to keep supply air warm.

The cycle ends when the coil sensor sees the coil warm above the termination setpoint (typically 10 to 15 Celsius) or when a maximum time has elapsed. The reversing valve flips back and normal heating resumes.

Electric-Resistance Supplemental Heat

Nearly every cold-climate heat pump installed in Ontario includes electric-resistance backup heat, usually a strip heater in the air handler or the furnace's auxiliary bank. During defrost the indoor coil is being cooled rather than heated, so without supplemental heat the supply air would be uncomfortably cold for several minutes. The strip heat typically adds 5 to 20 kW depending on system size and is expensive to run. The defrost control limits supplemental heat to the defrost duration plus a short recovery.

Supplemental heat is also what carries the house through outdoor temperatures below the unit's rated low-ambient limit. A properly commissioned system switches to backup only during defrost and during design-day extremes. A system riding on supplemental heat for hours at moderate outdoor temperatures is a sign the heat pump side has failed and the defrost loop is often the cause.

Typical Defrost Duration and Frequency

A healthy defrost cycle runs 3 to 10 minutes and terminates when the coil sensor confirms the coil is clear.[4]Frequency scales with how fast frost builds, which scales with humidity and temperature:

These ranges hold for most NEEP-listed cold-climate units sold in Ontario.[1]Short bursts of wet snow followed by a drop in temperature can trigger a defrost every 20 to 30 minutes for an hour or two, which is normal for the conditions. A unit defrosting every 15 minutes continuously for an entire day in steady weather is not.

Normal Versus Abnormal Ice Patterns

Healthy frost looks like a uniform white layer across the face of the outdoor coil, 1 to 4 mm thick, with the bottom of the cabinet clear and drain holes in the base pan flowing. The fan blade spins freely with a small amount of frost on the guard. Ice patterns that warrant a service call look different:

• Solid clear ice block on the lower cabinet:drain holes are blocked, melt water cannot escape, and it refreezes between cycles. Fix the drainage; if the base pan heater has failed, that is the cause.
• Ice inside the fan shroud or on the blade:usually a failed defrost initiation (the unit never defrosts) or a stuck reversing valve (defrost runs but the coil does not warm).
• Coil iced white for hours with no signs of defrost: defrost board, coil temperature sensor, or outdoor ambient sensor has failed. Supplemental heat is carrying the load.
• One side of the coil iced, the other clear:refrigerant distribution problem, low charge, or a partially blocked distributor. Needs a pressure and superheat check.
• Ice dam on a rooftop or overhang above the unit: melt water refreezes on the unit below. Fix the drip or add a deflector; do not chip ice off the unit.

Demand Defrost Versus Timed Defrost

Older heat pumps used timed defrost: a clock initiated a cycle on a fixed schedule (every 30, 60, or 90 minutes) whenever outdoor temperature dropped below a setpoint, regardless of whether frost had actually built up. That meant unnecessary cycles on cold, dry days and unnecessary supplemental heat events.

Demand defrost (also called smart or adaptive defrost) uses sensors to decide when to defrost. The control board watches coil temperature against outdoor ambient and may also watch pressure drop across the coil as frost thickens. It triggers a cycle only when the coil shows measurable frost loading. Most NEEP-listed cold-climate heat pumps (Mitsubishi Hyper-Heat, Daikin Aurora, LG, Fujitsu, Carrier Greenspeed, and others) ship with demand defrost as standard.[6]In Ontario conditions demand defrost typically cuts cycle counts 30 to 50 percent compared to timed defrost, with a corresponding drop in supplemental-heat runtime and winter hydro consumption.[8]

Why NEEP-Listed Cold-Climate Units Defrost More Efficiently

NEEP maintains the Cold Climate Air Source Heat Pump product list, which certifies units that deliver specified capacity and COP at minus 15 Celsius and below.[1]Cold-climate units use larger outdoor coils with wider fin spacing, inverter-driven compressors that can modulate capacity instead of cycling on and off, and demand defrost control logic. Larger coil surface area means each cycle melts more frost in less time. Wider fin spacing means frost accumulates more slowly and takes longer to block airflow. Variable-speed compressors can run at partial capacity on mild days, keeping the coil warmer and delaying frost formation.[6]The combined effect is fewer, shorter defrost cycles and less supplemental heat, which is the difference that shows up on the January hydro bill.

Steam, Fog, and What Is Not Smoke

The plume that rises from the outdoor unit during defrost is water vapour condensing in cold air. The melting frost becomes water, the warm cabinet evaporates some of it, and the vapour meets cold outside air and forms visible fog. It has no smell, dissipates in seconds, and stops when the cycle ends. It is not smoke and not refrigerant leakage.[2]

Actual warning signs that involve the outdoor unit: an electrical or burning-plastic smell, black smoke rather than white vapour, a hissing sound that persists after the cycle ends (possible refrigerant leak), or oil streaks near line-set connections. Any of those warrants shutting the unit off at the disconnect and calling service.

Snow Drift and Clearance Requirements

Manufacturers publish clearance requirements so defrost melt water drains, airflow is unobstructed, and the unit is above the typical snow line. A common Ontario spec:

• Minimum 12 inches above grade or snow line:the unit must sit on a riser or pad tall enough to stay above drifts. Snow melting from a drift directly on the cabinet will re-ice the unit.
• Minimum 24 inches of clearance around the sides and front: airflow into the coil cannot be blocked by fences, shrubs, wall returns, or snow piles.
• Minimum 48 inches above the unit if there is an overhang: roof drip lines that drop water or snow onto the unit cause the most expensive ice damage.
• Not under a rainspout or downspout: the unit is not a splash block.

If the installer placed the unit in a spot that violates these clearances, that is an install defect, not a homeowner problem. Short of relocating the unit, a snow riser and an overhang deflector are usually the fix.

Signs the Defrost Is Actually Broken

Three signs together, not one in isolation, mean call service:

• Coil stays solidly iced for hours: you can see the outdoor coil is white or blue-ice and no defrost cycle is audibly starting. The fan keeps trying to spin against frost or is stuck.
• Supplemental or auxiliary heat runs continuously: the thermostat shows "AUX" or "Em Heat" on for hours instead of minutes. The heat pump side has effectively stopped contributing and the house is being heated by electric strips only.
• Hydro bill jumps 40 to 100 percent in January or February compared to the same month in the previous year. Electric-resistance heat uses roughly 2 to 3 times as much electricity per BTU as a working heat pump.[8]

Individually these can be weather, thermostat settings, or a dirty filter. Together they mean the defrost control board, a coil sensor, the reversing valve, or the outdoor fan motor has failed. Running on supplemental heat costs money every hour it continues, so this is not a wait-and-see diagnosis.

Typical Defrost Repair Costs

Ontario 2026 pricing for the most common defrost-related repairs on a residential air-source heat pump:

Parts are often covered by manufacturer warranty on units under 10 years old; labour is usually not unless you bought an extended labour warranty. That is why most out-of-pocket defrost repairs land between $200 and $500 even on warrantied units.

When to Call Service Versus Wait It Out

Wait it out: a defrost cycle running right now with steam rising and the fan stopped; thin white frost across the coil between cycles; more frequent cycles than usual after a wet-snow event (give it a day); house maintaining setpoint with supplemental heat cycling only briefly during defrost.

Call service: coil solidly iced for hours with no defrost activity; Aux or Em Heat on the thermostat for hours at a time; house not reaching setpoint on a day that is not extraordinarily cold; electrical smell, oil streaks, or persistent hissing after a cycle; visible ice inside the fan shroud or on the blade; a winter hydro bill more than 40 percent above the same month last year.

On a multi-zone ductless heat pump, a failure on one outdoor condenser can still leave other zones heating, which sometimes masks the problem. Check every indoor head during a suspected defrost failure, not just the one you notice. See our related guide on cold-climate heat pump selection for NEEP-listed units that are designed for the conditions that drive defrost the hardest, and our HVAC commissioning guide for the measurements that catch a failing defrost loop at install time.

Frequently Asked Questions

Related Guides

• Cold Climate Heat Pump Ontario 2026
• HVAC Commissioning and Testing Ontario 2026
• HVAC Noise Bylaws Ontario 2026