If your heat pump is frozen or iced over right now, take a breath first: most of the time this is normal winter behaviour, not a breakdown. Your outdoor unit is designed to build a light layer of frost in cold, humid weather and clear it automatically every 30 to 90 minutes through a built-in defrost cycle. This guide walks you through how to tell normal frost from a real fault, what’s causing the ice if it isn’t clearing, what you should never do to a frozen unit, and exactly when to stop troubleshooting and call an HVAC technician.
This is written for homeowners, not technicians, so we’ll skip the jargon. In our experience servicing systems across Hamilton, Burlington, Oakville, Milton, Brantford, Cambridge, and Kitchener, roughly one in three “frozen heat pump” calls in a Golden Horseshoe winter turns out to be a thick but perfectly normal frost layer after a wet snowfall. The rest trace back to one of five mechanical causes covered below.
Before you touch the unit, run this quick safety check. Most iced-up heat pumps are a simple wait-it-out situation, but a small number point to something that needs a technician right away.
If none of those apply, it’s safe to keep reading and work through the checklist below. The flowchart gives you the same decision path our technicians walk through on a service call.
Every air-source heat pump pulls heat out of cold outdoor air, which chills the outdoor coil below the surrounding air temperature. Moisture condenses on the coil and, in freezing conditions, turns to frost. That part is expected. What separates normal operation from a fault is thickness, coverage, and whether the defrost cycle actually clears it.
| Signal | Normal Frost | Real Icing Problem |
|---|---|---|
| Coverage | Thin, patchy layer on the coil fins | Solid sheet covering the whole unit |
| Thickness | Under roughly 6 mm (a quarter inch) | Fins and fan blades aren’t visible |
| Defrost behaviour | Clears within 30 to 90 minutes | Returns in minutes, or never clears |
| Heating performance | Home stays warm through the cycle | Airflow feels cool, system stops heating |
| Sound | Normal fan hum, brief whoosh in defrost | Hissing, clicking, or grinding |
According to Natural Resources Canada, a heat pump’s defrost cycle is triggered either by a timer set to run every 30, 60, or 90 minutes, or by a sensor that reads coil temperature and starts the cycle only once frost is detected. Natural Resources Canada notes this timing varies by climate and system design. If your unit hasn’t visibly defrosted in over 90 minutes and the ice keeps thickening, that’s your cue to move from “normal” to “problem.”
A heat pump frozen after a snowstorm isn’t the same situation as one iced over during a dry cold snap, and Ontario’s weather patterns make certain types of icing more common than they’d be elsewhere in Canada. Three conditions do most of the damage:
Wind adds a secondary effect: a stiff breeze strips heat off the coil faster, which can make a borderline defrost cycle fall behind. None of this means your heat pump is failing. It means Ontario’s specific mix of humidity and lake-driven weather asks more of the defrost cycle than a drier climate would, which is also why cold-climate-rated heat pumps hold up better here than standard models. Our essential heating tips for Canadian winters guide covers broader cold-weather prep if you want the full picture beyond just icing.
Regardless of brand, this pattern and the troubleshooting steps in this guide apply the same way. Lennox, Carrier, Trane, Mitsubishi, Goodman, and every other major manufacturer builds a defrost cycle around the same core logic, so the difference is in parts and pricing, not in how you diagnose the problem. Lennox’s own guidance on frozen heat pumps confirms the same normal-frost-versus-real-problem distinction covered above.
When a heat pump won’t clear ice on its own, the cause almost always traces to one of five things. If your unit has stopped responding altogether rather than just icing over, our heat pump not working guide covers the broader checklist; this section focuses on what specifically triggers excess icing.
A slow leak drops coil pressure, chilling it further than the design allows and freezing moisture faster than the defrost cycle can keep up. This is a licensed-technician-only fix: federal ozone-layer rules and provincial halocarbon regulations restrict who can legally purchase, recover, or top up refrigerant. Environment and Climate Change Canada outlines these rules by province. Suspect a leak if icing pairs with weak heating and hissing at the lines.
The defrost board decides when to trigger a defrost cycle. When it fails, the cycle never starts or never ends properly, so ice builds evenly instead of in the light, patchy way normal frost does. It’s one of the more affordable outdoor-unit repairs, typically CAD $250 to $650 for parts and labour.
Even with a healthy board, a corroded or misreading coil-temperature sensor can tell the system the coil is warmer than it is, so defrost never triggers. It’s easy to confuse with a board failure, which is why technicians test both together rather than swapping parts on a guess.
Snow drifts, fallen leaves, a dirty return-air filter, or a coil caked in dust all force the refrigerant colder than it should run, accelerating frost. This is the one cause a homeowner can usually confirm and fix without a technician; see the safe steps below.
If your outdoor unit sits under an eave, dripping meltwater can refreeze on the coil and base pan, building an ice dam that has nothing to do with the refrigeration cycle. It’s easy to overlook because it mimics a mechanical fault, so look up before you look at the unit itself.
Answer a few quick questions about what you’re seeing and hearing, and get the most likely cause plus your next step, instantly.
Answer 5 quick questions about your heat pump symptoms and get a likely diagnosis with recommended next steps.
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Note: This diagnosis is based on common symptom patterns and is for informational purposes only. A certified HVAC technician should inspect your system for an accurate diagnosis.
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GET FREE QUOTESIf the ice is moderate rather than a solid block, and the system still responds to the thermostat, these steps are safe to try.
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Call the same day if the ice is a solid block, reforms within an hour or two of clearing, your home isn't heating even with backup heat, you hear hissing or clicking, or the coil ices even in mild, dry weather. Any of these point to a defrost board, sensor, or refrigerant issue outside safe DIY territory.
For budgeting, most icing-related repairs in Ontario fall into a fairly predictable range:
| Likely Repair | Typical Cost (CAD) |
|---|---|
| Defrost control board replacement | $250 – $650 |
| Defrost sensor or thermostat replacement | $150 – $400 |
| Airflow clearing / filter replacement (DIY) | $0 – $50 |
| Refrigerant leak diagnosis and recharge | $300 – $1,200+ |
| Gutter or drainage correction | $100 – $400 |
Ranges reflect typical Ontario service quotes (CAD) as of 2026 and vary by region, brand, and labour rates. Always confirm with a local estimate.
If your system is over 10 to 12 years old and keeps icing despite repairs, weigh repair against replacement rather than sinking money into an aging unit. Our heat pump buying guide for Canadian homeowners covers what a newer, cold-climate-rated system costs, and heat pumps in Canada looks at why more homes across the province are switching.
Weighing another repair against replacing an older unit? Answer a few quick questions about age, repair history, and cost to get a clear recommendation.
Answer 5 quick questions and find out if switching to a heat pump makes sense for your home, climate, and lifestyle.
Central duct systems allow for a ducted heat pump. Without ducts, a ductless mini-split is still an option but at higher cost.
Heat pumps perform best in moderate climates. Cold-climate models now work well even in very cold regions.
A heat pump is a long-term investment. Longer stays mean better return on investment.
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Note: This quiz provides a general recommendation. A certified HVAC contractor can assess your specific home and provide a tailored recommendation.
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GET FREE QUOTESMost icing problems are preventable with a small amount of routine care. These habits catch most problems before they leave you troubleshooting in the cold.
If you're weighing whether a heat pump still makes sense for a home that ices up every winter, our heat pump vs. furnace comparison breaks down how each system handles Ontario's coldest months.
If your situation is a little different from a standard winter icing call, one of these related scenarios might match what you're seeing.
If the unit won't respond to the thermostat at all and is fully iced over, don't force it. Switch it off, let it thaw completely, then check the basics in our heat pump not working guide before restarting.
Frost on the indoor coil usually points to restricted airflow, most often a dirty filter or a partially closed vent, rather than a defrost cycle problem. Shut the system off and let it thaw before restarting with a clean filter.
Wall-mounted ductless units ice up for the same core reasons as central systems, but their smaller outdoor condensers are more sensitive to snow drifts and low clearance. Check that the outdoor unit is mounted high enough to stay clear of snowbanks.
Occasional icing after a storm is normal. If you're asking "why does my heat pump keep freezing" after the third or fourth time in a couple of weeks, that pattern usually means the defrost board or sensor is degrading rather than failing outright, and it's worth a diagnostic before it strands you on the coldest night of the season.
Ice forming when it isn't cold enough for frost to make sense is one of the clearer signs of a mechanical fault, usually low refrigerant or a stuck reversing valve, rather than normal weather-driven frost. This one is a call-a-technician situation, not a DIY fix.
A frosted heat pump is normal winter behaviour; a solid, unmelting block of ice is your system telling you something specific has failed. Walk through the frost-versus-ice check, clear obvious airflow blockers, and give the defrost cycle a fair chance before assuming the worst. If ice keeps returning despite doing everything right, that's no longer a DIY problem, and a same-day diagnostic costs far less than a compressor strained through a winter of fighting solid ice.
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Usually not. Light frost is normal and clears within 90 minutes. It becomes urgent only if the ice is solid, keeps returning, or your home stops heating.
A normal cycle runs every 30 to 90 minutes and finishes within about 10 to 15 minutes once triggered, clearing visible frost from the coil.
No. Hot water can crack coils through thermal shock and often refreezes at the base. Use lukewarm water or let the unit thaw naturally instead.
Humidity, sun exposure, gutter placement, and shelter around the unit all vary house to house, so identical systems can frost differently.
Most fixes run CAD $150 to $650 for sensors or defrost boards, though a refrigerant leak can run $300 to $1,200 or more depending on severity.
Yes, switch it off or to emergency heat if it's fully iced over. Running a heating cycle against solid ice strains the compressor unnecessarily.
Yes. Emergency heat bypasses the heat pump and uses backup electric elements, keeping your home warm while the outdoor unit thaws on its own.
Don't switch it manually. The defrost cycle already borrows this same reverse-cycle trick automatically. Manually running cooling mode in winter can trigger other faults.
Yes. A clogged filter restricts airflow enough to drop coil temperature and accelerate frost, even when every other component is working correctly.
Thermostat off is usually enough. Cut power at the breaker only if you hear hissing, smell something chemical, or a technician advises it.
It depends on your policy and the cause. Sudden mechanical failure is often covered; damage from neglected maintenance usually isn't. Check with your insurer directly.
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