Heat Pump Not Heating in Winter? Causes, Fixes & Real Emergency Signs

If your heat pump is running but your home still isn’t warming up, don’t assume the system has failed. A heat pump not heating winter complaint is very often a normal defrost cycle catching you at the wrong moment, not a breakdown. But persistent cold air, ice buildup, or a steadily falling indoor temperature is a different story. This guide is built for Ontario homeowners across the Golden Horseshoe. It walks through five things you can safely check right now, how to tell a normal defrost cycle from an actual fault, the six causes a technician actually looks for, what AUX and EM HEAT are doing on your thermostat, and when a cold night genuinely calls for an emergency call instead of a next-morning appointment.

This is written for homeowners across Hamilton, Burlington, Oakville, Milton, Brantford, Cambridge, and Kitchener, not technicians, so we’ll keep the jargon to a minimum. Everything below assumes your heat pump is running but not heating well. If the outdoor unit won’t start at all, the thermostat display is dead, or nothing happens when you adjust the temperature, that’s a different problem, and our heat pump not working guide covers the power, breaker, and thermostat checks for that scenario specifically.

5 Safe Checks Before Calling an HVAC Technician

Before you dig into defrost cycles or reversing valves, rule out the simple stuff first. These five checks take less than ten minutes and resolve a surprising number of heat pump not heating winter calls on their own.

1. Confirm the thermostat is set to Heat, not Cool or Fan Only, and that the display isn’t blank or frozen.
2. Raise the setpoint a few degrees above the current room temperature and wait 10 to 15 minutes for a response.
3. Set the fan mode to Auto rather than On, so it isn’t blowing unheated air between cycles.
4. Pull the air filter and hold it up to a light. Replace it if you can’t see light through it clearly.
5. Walk outside and check whether the outdoor unit is buried in snow or blocked by a drift.

Heat Pump Defrost Cycle: Normal or Actually Broken?

A normal defrost cycle lasts 5 to 15 minutes, repeats roughly every 30 to 90 minutes in cold, humid weather, and briefly blows cool air while steam rises off the outdoor unit. None of that is a fault. It becomes one when a single cycle runs past 15 to 20 minutes, the system seems to defrost almost constantly, or the coil stays coated in solid ice rather than clearing after each cycle.

Natural Resources Canada explains that heat pumps decide when to defrost one of two ways: an older time-temperature timer that fires every 30, 60, or 90 minutes whether the coil actually needs it or not, or a demand-defrost control that only triggers once sensors detect real frost buildup. Budget and older systems tend to run the simpler timer, which is one reason your unit might seem to defrost more often than a neighbour’s newer one during the exact same cold snap.

Normal defrost cycle: the outdoor coil frosts, the reversing valve sends hot refrigerant outside while the fan pauses, steam rises as the frost melts, then the valve resets and heating resumes, typically within 5 to 15 minutes total.

If your unit is stuck in this pattern for more than 20 minutes at a stretch, never seems to come out of defrost, or the coil is coated in solid ice rather than a light frost between cycles, that points to a defrost control board or refrigerant problem, not normal operation. Our heat pump frozen or iced up guide covers the specific ice patterns that separate a normal frost from a real fault in more detail.

6 Real Causes Your Heat Pump Isn’t Heating This Winter

Once a normal defrost cycle is ruled out, these six components are among the causes technicians investigate most often across the Golden Horseshoe this time of year.

• Low refrigerant or a slow leak. Refrigerant doesn’t get used up, so a low charge almost always means a leak somewhere in the line set or coil. It shows up as reduced heat output, longer run times, and sometimes ice on the indoor coil. Topping up or repairing a leak has to go through a certified technician under Canada’s provincial and territorial halocarbon regulations, not something to attempt yourself.
• A failed defrost control board. This is the sensor and relay assembly that tells the unit when to start and stop defrosting. When it fails, the system either never defrosts and ices over completely, or gets stuck defrosting and never returns to heating.
• A dirty filter or iced-over indoor coil. A clogged filter restricts airflow across the coil, which drags down heat transfer and can freeze the coil solid over time. Change the filter every one to three months regardless of how it looks, which matches Natural Resources Canada’s general maintenance guidance for heat pump systems.
• A snow-packed or blocked outdoor unit. The outdoor coil needs unobstructed airflow to pull heat from the air at all. Keep roughly two feet of clearance on every side, and clear snowdrifts and ice gently, never with a shovel or hot water against the fins.
• A reversing valve stuck in cooling position. This is the same valve that runs the defrost cycle, and when it sticks, the system runs but blows only cool air no matter what the thermostat says. Our heat pump not cooling guide shows this same valve and coil layout from the opposite season if you want to see exactly where it sits in the system.
• An undersized system for the home. Every heat pump has a thermal balance point, the outdoor temperature at which its heating capacity exactly matches the home’s heat loss. An undersized unit hits that point well before the coldest night of the year and leans on backup heat constantly rather than just during isolated cold snaps.

Not sure which of these matches what you’re seeing, hearing, or smelling? The free tool below asks the same questions a technician would over the phone before dispatching anyone.

Heat Pump Problems During Ontario Winters

The Golden Horseshoe's freeze-thaw swings and lake-effect humidity make several of the six causes above show up more often here than in a drier, steadier cold climate. Wet snow packs harder around outdoor units than dry powder, and damp air puts more frost on the coil in the first place, which is why demand-defrost controls tend to earn their keep in this region more than in a place like the Prairies.

Older, single-speed heat pumps generally lose more capacity in this kind of weather than newer cold-climate models built with variable-speed compressors and demand-defrost controls. If you're chasing a not-heating problem every time the temperature swings through freezing, that pattern itself is worth mentioning to your technician, since it can point toward equipment age as much as a specific broken part.

Auxiliary Heat vs Emergency Heat: What's the Difference?

Auxiliary heat (AUX) turns on automatically, running alongside the heat pump when the system can't close the gap between room temperature and the thermostat setting fast enough, commonly during a defrost cycle or a hard cold snap. Emergency heat (EM HEAT) works differently. It's a setting you switch on manually, and it shuts the heat pump off completely, running only the backup furnace or electric elements until you switch it back.

Many Canadian ducted installations use what Natural Resources Canada calls a two-stage thermostat: stage one calls on the heat pump alone, and stage two brings in the backup system only if the room keeps falling behind after that. Seeing AUX light up occasionally on a cold night is that second stage doing exactly its job, not a fault worth worrying about.

Should You Call for Repair Now, or Wait Until Morning?

Call now if indoor temperature is dropping steadily with no backup heat running, or if you notice burning smells, sparking, or grinding from the outdoor unit. If your home is holding a comfortable temperature on AUX or a furnace backup, a same-day or next-morning appointment is almost always the cheaper, equally safe choice.

We cover the full emergency triage checklist, after-hours pricing, and repair cost by part in our heat pump emergency repair guide. The short version for a winter not heating call specifically is that a normal defrost cycle and the six causes above explain most of what looks alarming late at night but genuinely isn't.

Do You Have Backup Heat for Nights Like This?

Whether tonight's issue turns into an emergency call comes down almost entirely to what backup heat your system has, more than how cold it actually is outside. Every air-source heat pump has manufacturer-specified operating and capacity limits, and Natural Resources Canada notes that newer cold-climate models typically hold a minimum operating temperature of -15°C to -25°C, though heating capacity generally declines well before that cutoff. Check your specific model's performance data rather than assuming a single number. Whatever that limit is for your system, a supplemental source has to carry the load below it, and that same backup kicks in the moment the heat pump itself has a problem rather than just a cold night.

• Hybrid (heat pump + gas furnace). These two systems can't run at the same time, but the thermostat can typically be set to run the furnace alone, so a heat pump fault rarely turns into an emergency call.
• Ducted with electric backup elements. These strip heaters can run at the same time as the heat pump and keep the ductwork warm automatically, at a higher cost per hour, until the unit is repaired.
• Ductless mini-split, no backup. There's no fallback heat source at all, so a failure during real cold weather is a genuine emergency. Electric baseboards in one or two key rooms are a relatively inexpensive way to close that gap.

If you're adding electric backup heat to a system that doesn't currently have it, any new circuit or panel work needs to go through a licensed electrician under Ontario's electrical safety rules, not a DIY breaker swap. And if you're weighing whether a hybrid setup makes sense for your next system altogether, our heat pump vs furnace comparison breaks down when each one wins on its own.

Frequently Asked Questions

Sources & References

• Natural Resources Canada: Heating and Cooling with a Heat Pump
• Environment and Climate Change Canada: Provincial and Territorial Halocarbon Acts and Regulations