Heat Pump Defrost Cycle Ontario 2026: Why Your Unit Makes Steam, Stops Heating, and What's Normal vs Broken

Why Heat Pumps Defrost in the First Place

A heat pump in heating mode works by pulling heat out of the outdoor air and moving it inside. The outdoor coil is colder than the outdoor air, by design, so refrigerant inside the coil can absorb heat from the air passing across it. The colder the refrigerant, the more heat it grabs.

The problem is that Ontario air in winter is rarely dry. Between roughly minus 8 Celsius and plus 5 Celsius, the outdoor coil is often below the dew point of the surrounding air. Water vapour in the air condenses on the coil, and because the coil surface is below zero, it freezes instead of dripping off. A thin layer of frost builds up on the fins.[1]

Frost on the coil is a problem for two reasons. It insulates the aluminum fins from the air, which cuts heat transfer. And it blocks airflow between the fins, which starves the coil of the air it needs to grab heat from. Left alone, a heating heat pump would frost completely over within an hour or two, at which point it would stop producing any useful heat at all.

The defrost cycle is the engineering answer. Every so often, the outdoor unit deliberately melts the frost off itself and goes right back to heating. Modern cold-climate heat pumps sold in Ontario use variable-speed inverter compressors and demand-based defrost logic, so cycles happen only when the coil actually needs it, not on a rigid clock.[2]

Typical Defrost Frequency in Ontario Winter

Defrost frequency depends on three things: outdoor temperature, outdoor humidity, and compressor runtime. There is no single correct interval, but these ranges are typical for a properly operating residential heat pump:

The wettest, heaviest frost builds up right around the freezing point when humid air is saturating the coil. Many Ontario homeowners notice defrost cycles running most aggressively during late-fall freezing rain, a January thaw, or an early-March wet snow. In deep cold, the air is actually drier and the unit defrosts less often, which is counterintuitive but consistent with the physics.[1]

For sizing context on what a heat pump should be doing across Ontario's full winter range, our cold-climate heat pump guide covers balance points, capacity curves, and how aux heat is configured to stay out of the way except during defrost.

What Actually Happens During Defrost (Reverse Valve)

A defrost cycle is basically a brief air-conditioning run aimed at the outdoor coil. Inside the outdoor unit there is a component called the reversing valve, a four-way solenoid valve that routes refrigerant flow. In heating mode it sends hot refrigerant from the compressor to the indoor coil. At the start of a defrost, the control board energizes the reversing valve and redirects that same hot refrigerant to the outdoor coil instead.[4]

Here is what happens in sequence over the 2 to 10 minutes of a typical cycle:

1. The control board decides defrost is needed, based on a coil temperature sensor, an ambient temperature sensor, and elapsed compressor runtime (demand defrost). Older units trigger on a simple timer plus a coil thermostat.
2. The indoor blower is locked out or switched to auxiliary heat so cold air doesn't blow into the house.
3. The outdoor fan is switched off. This lets the coil temperature rise quickly without the fan pulling cold air across it.
4. The reversing valve is energized with an audible click or whoosh. Hot refrigerant now flows to the outdoor coil. Frost melts in seconds near the refrigerant tubes and in a minute or two across the whole coil face.
5. When the coil temperature sensor rises past its termination threshold (typically around 10 to 15 Celsius, manufacturer dependent), the board ends the cycle. The reversing valve switches back, the outdoor fan restarts, and the indoor blower returns to heating mode.[5]

Inverter cold-climate units from manufacturers like Mitsubishi, Carrier, Trane, Lennox, and Daikin add a ramp-up and ramp-down to this sequence so the compressor isn't stressed by an abrupt change in load. Older single-speed heat pumps make a more noticeable thud when the reversing valve slams over.[6]

The Steam Plume Is Normal

The single most common question Ontario homeowners ask during their first heat-pump winter is some version of "why is my unit smoking." It isn't. The white plume rising off the outdoor unit during and just after a defrost cycle is water vapour.

During defrost the outdoor coil can reach 15 to 30 Celsius while surrounded by sub-zero air. The meltwater dripping off the coil flashes into steam the instant it hits the colder base pan, fan housing, and nearby air. In humid or damp conditions the plume can be thick enough to look alarming, especially against a dark sky or a snowy background. It's physically the same thing as the steam off a hot cup of coffee in a cold room, just at scale.

You can tell the difference between a normal steam plume and a real problem by a few cues:

• Steam dissipates within a few feet of the unit, has no smell, and stops within a minute or two of the cycle ending. Normal.
• Smoke that smells burnt, is dark or grey, or keeps rising after the unit shuts off. Abnormal, kill power and call for service.
• A sweet, ether-like smell anywhere near the unit. That is a refrigerant leak indicator. Abnormal.

Auxiliary Heat During Defrost

Because the reversing valve turns the indoor coil into the cold side during defrost, the air handler would blow cold air into the house if nothing compensated. Every central heat pump in Ontario is wired to prevent this.[5]

The two standard configurations are:

• Electric-backup heat pump (all-electric or hybrid with electric strips): An auxiliary electric resistance heating element in the air handler energizes during defrost. The element is typically 5, 10, 15, or 20 kilowatts depending on system size. You may see the thermostat briefly display "AUX" or "Emergency Heat" during the cycle.
• Dual-fuel or hybrid heat pump (gas furnace backup): The gas furnace takes over for the defrost window, providing warm supply air while the outdoor unit handles its ice problem. This is the most common arrangement in Ontario because it pairs well with existing natural-gas infrastructure and Enbridge rebates.[8]

In either case, the switch is seamless and automatic. You should never feel a meaningful drop in indoor air temperature during a defrost cycle. A temporary 0.5 to 1 Celsius dip is within normal range on a mild winter day. Anything larger suggests the aux heat isn't coming on correctly, which is a control board or wiring issue.

Worth repeating for homeowners who see "aux heat" flash up and worry about electricity cost: aux heat during defrost lasts 2 to 10 minutes and runs a handful of times per day in cold weather. That's normal design behaviour, not a sign the heat pump is oversized, undersized, or losing the fight. Aux heat is only a problem when it runs for long stretches outside of defrost at mild outdoor temperatures.[3]

When Defrost Is Abnormal

A handful of patterns mean the defrost system is not working correctly and the unit needs service:

• Coil stays encased in thick ice after a cycle.The cycle is either terminating too early or not moving enough heat to the coil. Common causes: failed coil temperature sensor, low refrigerant charge, stuck or failing reversing valve, failed outdoor fan motor (if it won't restart after defrost).
• Defrost cycles run back to back every 10 to 15 minutes.Usually a short-cycling defrost timer (on older units) or a blocked outdoor coil that is re-icing immediately. Check for leaves, a snow drift against the coil, or bent fins restricting airflow. If the coil is clean, the sensor or board logic is the likely culprit.
• Cycle lasts longer than 15 minutes.Sensor failure, refrigerant charge issue, or a reversing valve that isn't fully switching. A properly running cold-climate heat pump almost never needs more than 10 minutes to clear its coil.[6]
• Unit never defrosts and ices over completely.The defrost control has failed. The unit may still try to heat through the ice, drawing power while producing almost no heat. Kill power at the disconnect to protect the compressor and call for service.
• Base pan or fan blades are encased in ice.Drainage failure. Meltwater is refreezing before it can run off because the base pan drain is blocked, or the unit isn't elevated high enough above snow line, or the base pan heater (on units equipped with one) has failed.
• Indoor air goes cold during defrost.Auxiliary heat isn't engaging. Check that the aux-heat breaker is on and that the thermostat wiring is correct (W1 or W2 terminals properly connected). This is a wiring or control board issue, not a heat pump mechanical issue.
• Very loud bang or screech at the start or end of defrost.A normal reversing valve makes a soft click or whoosh. A loud metallic bang suggests slugging (liquid refrigerant hitting the compressor) or a valve that isn't seating properly. Both shorten compressor life if ignored.

Troubleshooting Defrost Problems

Before calling a technician, the homeowner-safe checks that often resolve the issue or narrow it down:

1. Check clearance around the outdoor unit.You want at least 60 centimetres of clear space on the intake and discharge sides, and the top clear of snow. Snow drifted against the coil is the single most common avoidable cause of excessive defrost cycling and poor heating output.
2. Confirm the unit is elevated above snow line.Ontario-installed heat pumps should sit on a stand or pad that keeps the base of the unit at least 30 centimetres above expected snow depth. This lets defrost meltwater drain off instead of pooling and re-freezing under the unit.
3. Inspect for bent or damaged fins.A weed-whacker strike, a snow shovel bump, or a poorly aimed hockey net can dent the coil fins and restrict airflow enough to trigger constant defrost. Minor damage can be combed back with a fin-comb tool. Severe damage needs a technician.
4. Check the thermostat mode and setpoint.Some thermostats have a "emergency heat" or "lockout" mode that disables the outdoor compressor entirely and forces aux heat only. If that's on by accident, the unit won't heat properly and you'll see aux-heat indicators with no defrost cycles at all.
5. Listen for the reversing valve click.When a normal defrost starts, you should hear a soft click or whoosh at the outdoor unit, the outdoor fan stops, and warm refrigerant flow reverses direction. No click after a long run of heating in humid conditions suggests the valve or its solenoid coil has failed.
6. Never pour hot water, pry ice off, or use a hair dryer.Warm (not boiling) tap water can help melt a seized ice block as a last resort before calling for service. Chipping or prying bends the fins permanently and risks puncturing the refrigerant tubing.

If the behaviour doesn't match the normal patterns in this guide, shut the system off at the thermostat, set the backup to emergency heat (or gas furnace), and book an HVAC technician. Heat pump controls, refrigerant, and reversing valves are not DIY territory. A service call in the $180 to $350 range is much cheaper than a burned-out compressor.

If the outdoor unit is making unfamiliar noises that aren't defrost related, our furnace noise troubleshooting guide covers a similar symptom-to-cause breakdown for the indoor side of hybrid systems. And if you're mid-sizing a replacement and worried about winter performance, the heat pump sizing guide walks through how defrost losses are already baked into the HSPF2 and cold-climate capacity numbers you'll see on quotes.

Frequently Asked Questions