How to Keep Your Heat Pump Running Through Ontario Extreme Weather

If you moved to a heat pump in the last few years, you already know Durham Region weather does not read the manufacturer’s brochure. A humid 32°C afternoon in Ajax and a −22°C morning in Uxbridge are the same machine’s problem to solve. When a heat pump struggles in extreme conditions, the cause is almost never the technology itself — it is a defrost cycle blocked by drifted snow, a summer coil coated in cottonwood fluff, an undersized backup heat strategy, or a maintenance interval that quietly slipped. This guide is what our techs actually check on service calls across Whitby, Oshawa, and Bowmanville, and how homeowners can keep a heat pump running through the seasons Ontario keeps handing us.

Understand what “extreme weather” means for a heat pump

A standard air-source heat pump loses capacity as outdoor temperatures drop. A basic single-stage unit may only deliver a fraction of its rated heat below −10°C, while a modern cold-climate heat pump (ccASHP) with an inverter-driven compressor can hold usable capacity down to −25°C or −30°C. Summer is the mirror image: as ambient temperatures push past 30°C with high dew points, the same outdoor unit has to reject heat into hotter, denser air, and any restriction — dirty coil, blocked airflow, low refrigerant — shows up as long run times and warm supply air.

Extreme weather in Durham Region, for planning purposes, means design lows around −22°C to −25°C for the older builds inland toward Port Perry and Uxbridge, and summer design conditions in the low 30s °C with high humidity along the lake in Ajax and Pickering. A properly specified cold-climate heat pump is engineered for that envelope, but only if it is sized, installed, and maintained with those conditions in mind.

Winter: keeping defrost cycles healthy

Every air-source heat pump running in freezing weather will frost up. Warm refrigerant carries humidity from outdoor air onto the coil, and that moisture freezes as the coil surface drops below 0°C. The system compensates by running a defrost cycle: it briefly reverses into cooling mode, sends hot refrigerant into the outdoor coil, and melts the frost off. You will see steam rising off the unit, hear a whoosh, and — if you have electric backup — feel the auxiliary heat kick in for a few minutes to keep supply air warm. This is normal.

What is not normal is a defrost cycle that runs every 20 minutes, or one that never seems to finish. On service calls in Whitby and Oshawa, the culprits are almost always physical: drifted snow packed against the coil, ice built up on the base pan because the condensate cannot drain, or leaves and debris trapped under the fan grille from fall. A cycle that cannot complete because it cannot shed water will re-freeze, choke airflow, and eventually trip the unit on high pressure or low suction.

Snow clearance rules that actually matter

Keep at least 60 cm of clearance around the outdoor unit and 90 cm above it. After a heavy snowfall or freezing rain, brush snow off the top grille and away from the sides with a broom — never a shovel or ice pick against the fins. If drifting is chronic on the north or west side of the house, a simple wind baffle or a properly designed roof over the unit (with clearance) prevents most winter service calls. Do not enclose the unit or wrap it in a tarp; a heat pump needs to breathe in winter, unlike a summer-only AC condenser.

Elevate the unit above design snowfall

Older heat pump installations in Durham are often mounted 10–15 cm off the ground on a standard pad. For year-round operation, we recommend mounting on a bracket or stand 45–60 cm above the highest expected snow line. That single change eliminates most defrost drainage problems and makes February service work possible without excavating the unit.

Backup heat: the honest strategy

Even a cold-climate heat pump rated for −30°C loses efficiency at the extreme end of its range. The coefficient of performance (COP) that might be 3.5 at +5°C typically drops toward 1.5–2.0 near −25°C. It still heats the house, but each kWh of electricity delivers less heat. A well-designed system uses backup heat intelligently — not as a crutch, but as insurance for the coldest 20 to 50 hours of the year.

There are three common backup configurations in Durham Region homes: electric resistance strips inside the air handler, a dual-fuel setup pairing the heat pump with a gas furnace, and a hybrid where an existing high-efficiency furnace stays in place as the backup. Each has a place, and the right choice depends on your electrical panel, your Enbridge gas service, and how the house is built.

• Electric resistance backup — simple, no gas required, but expensive to run at Ontario time-of-use rates during a cold snap. Best for tight, well-insulated homes with modest heat load.
• Dual-fuel with a new gas furnace — a heat pump handles milder weather (typically above −5°C to −10°C), and the gas furnace takes over below the balance point. Lowest operating cost in most Durham homes still on Enbridge.
• Hybrid with existing furnace — a common retrofit: pair a new heat pump with a working furnace to spread the capital cost. Eligible for Enbridge Home Efficiency Rebate Plus pathways when installed to program specs.

Whichever route, your thermostat’s balance point and lockout settings need to be configured for your specific system and utility rates — not left on factory defaults. This is one of the most common gaps we find on second-opinion visits: the equipment is capable, the settings are not.

2026 Durham Region heat pump pricing

Below are installed price ranges we see across Whitby, Oshawa, Ajax, Pickering, and Bowmanville in 2026. Ranges reflect equipment tier, ductwork condition, electrical work, and whether backup heat is included. HST is extra. Federal and provincial incentives can meaningfully reduce net cost — see the incentives section below.

System type	Typical size	Installed price (2026 CAD)	Notes
Standard ducted heat pump	2–3 tons	$8,500–$12,500	Above-freezing efficient; needs backup for −20°C+ snaps
Cold-climate ducted heat pump	2–4 tons	$12,000–$17,500	Rated to −25°C or −30°C; inverter compressor
Dual-fuel: ccASHP + gas furnace	Matched sizing	$14,500–$22,000	Lowest operating cost with Enbridge gas service
Ductless mini-split (single zone)	9k–24k BTU	$4,800–$7,500	Common for additions and basements
Multi-zone ductless (3 heads)	24k–36k BTU	$12,500–$18,500	Retrofit for older Durham homes without ducts
Annual maintenance visit	—	$180–$280	Included in most protection plans

Summer: the problems no one warns you about

A heat pump in cooling mode is the same machine as a central air conditioner, and the failure modes in summer look identical. The three things that will take a heat pump out of service on a 32°C day in Pickering are the same three that take AC out: dirty outdoor coil, restricted indoor airflow, and refrigerant charge problems. None of them are catastrophic on their own — but combined with a hot day and a full load, they add up to warm supply air, long run times, and eventually a compressor lockout.

Dirty outdoor coils

The outdoor coil rejects heat, and it can only do that if air flows through the fins freely. After a Durham spring — cottonwood fluff, pollen, mower clippings, dandelion seed — the coil surface can be visibly matted. Gently rinse the outdoor coil with a garden hose (no pressure washer) from the inside out if you have safe access, or have a tech do it during your annual visit. Any restriction here raises head pressure, drops capacity by 10 to 30 percent, and shortens compressor life. If cooling problems persist after a coil rinse, an air conditioner repair diagnostic is the next step.

Refrigerant charge

Modern systems using R-410A or R-454B are precision-charged at installation. If the system is undercharged from a slow leak or was never charged correctly, summer performance falls off first. Symptoms include ice forming on the indoor coil or the outdoor suction line, warm supply air despite the compressor running, and cycling on high or low pressure. Adding refrigerant without finding the leak is not a repair — it is a delay. A licensed tech will identify the leak, repair or replace the failed component, and recharge to manufacturer spec.

Oversized units and short cycling

Oversizing is the most common installation error we see on replacement calls in Durham Region. A heat pump sized for the coldest winter hour is often too big for the summer load, especially with a variable-speed compressor set up incorrectly. The result is short cooling cycles that pull temperature down without removing humidity, leaving the house feeling clammy at 22°C. Fixing this after the fact usually means a thermostat and control revision, not new equipment — but it takes a proper load calculation, not a rule of thumb.

Maintenance frequency that actually prevents breakdowns

Because a heat pump runs year-round, it accumulates roughly twice the operating hours of a furnace or AC alone. The maintenance schedule should reflect that. A once-a-year AC tune-up is not enough. In our maintenance and protection plans, heat pump customers get two visits: a spring cooling-mode check and a fall heating-mode check, each with distinct tasks.

• Homeowner monthly — check and change the indoor filter (MERV 8–11 for most systems; MERV 13 only if the blower is rated for it). Walk around the outdoor unit and clear obstructions.
• Homeowner seasonal — rinse the outdoor coil in spring, brush snow off after storms in winter, verify the condensate drain runs freely.
• Professional spring visit — coil cleaning, refrigerant pressure check, capacitor test, blower amp draw, condensate treatment, thermostat calibration.
• Professional fall visit — reversing valve operation, defrost board test, backup heat verification, safety controls, electrical connections retorqued.

Homes with pets, gravel driveways, or heavy tree cover benefit from a mid-summer coil rinse between visits. The 15 minutes it takes saves compressor amperage all summer long.

Ontario incentives worth checking in 2026

Two programs are still meaningfully changing heat pump economics in Durham Region: the Enbridge Home Efficiency Rebate Plus (HER+), which offers rebates for eligible heat pump installations paired with a pre- and post-retrofit energy assessment, and the Canada Greener Homes Loan, which provides interest-free financing up to $40,000 for eligible upgrades including heat pumps. Program details, eligibility, and rebate values change — confirm current terms with a registered contractor before you sign a proposal. Any gas line disconnect, capping, or venting change performed as part of a full electrification also has to be handled by a TSSA-certified gas tech, not the electrician.

When to call for service versus wait it out

Some symptoms can wait until business hours; others should not. During a −20°C stretch, if the heat pump has locked out and only backup heat is running, the house is still safe but your hydro bill is climbing every hour — book a diagnostic within 24 to 48 hours. If you smell gas near the furnace on a dual-fuel setup, leave the house and call Enbridge’s 24-hour emergency line at 1-866-763-5427 before anyone else. If the outdoor unit is buried in an ice shell after freezing rain, do not chip at it; shut the system off at the thermostat, let auxiliary heat carry the house, and call for service.

For anything more routine — a whistling supply register, thermostat that lost its schedule, humidifier that stopped filling — batch it into a scheduled visit. If you are not sure, call us at 905-576-7600 and describe what you are seeing. Half of the questions we get on the phone from Whitby, Bowmanville, and Courtice homeowners are answered without a truck roll, and the other half get triaged to the right technician the first time. For any repair need across the region, contact our team and we will get you scheduled.

The short version

Ontario weather is hard on heat pumps, but the equipment is not the fragile part of the equation. Sizing, backup heat strategy, snow clearance, coil cleanliness, and two maintenance visits a year cover almost every extreme-weather failure we see across Durham Region. If your system was installed more than five years ago and has never had a professional check, that is where to start — a $200 tune-up now is cheaper than a compressor replacement in February.

References and further reading

• Natural Resources Canada — cold climate air-source heat pump specification
• Energy Star Canada — cold climate heat pumps
• Enbridge Gas — gas emergency (1-866-763-5427)

Frequently asked questions