Heat pumps in Ontario winter routinely build up frost on the outdoor coil — that's normal, and modern systems handle it automatically through periodic defrost cycles. The question isn't whether you see frost; the question is how much and whether it's clearing. A heat pump should never be encased in ice to the point where the fan blades are blocked, the entire coil is covered, or ice has built up in a thick block on top of the unit. That level of ice means the defrost cycle isn't working — usually a sensor failure, a stuck reversing valve, or a refrigerant problem — and the system is running inefficiently or not heating at all.
Most common causes
The likely culprits, roughly in order of how often we see each one:
- Normal frost during cold weather
A thin layer of frost on the coil during cold operation is normal. Heat pumps run defrost cycles every 30-90 minutes to clear it. After defrost, the coil should look mostly clear again.
- Defrost sensor failure
The defrost sensor tells the system when to run a defrost cycle. When it fails, defrost cycles either don't happen or don't happen often enough. Ice builds up over hours or days. Replacement: $300-$450.
- Reversing valve stuck
During defrost, the reversing valve temporarily switches the system to AC mode to push hot refrigerant through the outdoor coil and melt the frost. If the valve sticks, defrost can't happen. Replacement: $900-$1,800.
- Low refrigerant
Low refrigerant impairs both heating capacity and defrost effectiveness. Often paired with reduced heating output even when the coil isn't obviously iced.
- Drainage blocked
Some ice buildup happens when defrost meltwater can't drain from the bottom of the unit and refreezes. Often a clogged drain pan or a unit installed too close to the ground.
- Unit installed too low or in a snow-prone spot
Heat pumps need clearance — typically 12+ inches above ground level — to handle defrost meltwater and snow. Installation issues in some Halton Hills properties cause recurring ice problems that have to be addressed structurally.
What to check yourself first
Before calling, walk through these — sometimes the fix is something simple:
- Wait through one defrost cycle and observe
A heat pump entering defrost reverses temporarily — you may hear a "whoosh" as the reversing valve switches, see steam from the unit, and the indoor air briefly goes cooler. If this happens within 30-90 minutes of you noticing ice, the defrost is working.
- Brush off snow accumulation
Use a soft broom or your hand (gloved) to clear snow off the top and sides of the unit. Don't use sharp tools that could damage the fins. Don't pour hot water on the unit (rapid temperature change can damage components).
- Check that nothing is blocking airflow
Snow drifts, ice fall from the roof, or debris piled near the unit can prevent normal operation. Clear at least 2 feet of space on all sides.
- Look for ice in unusual places
Frost on the coil and bottom panel is normal. Ice on the fan blades, ice covering the entire grille, or ice forming above the unit are not normal — service call.
- Check indoor performance
Is the heat pump still keeping the house warm? If it's iced AND not heating effectively, that's a higher priority than iced and heating fine.
What NOT to do
- Don't pour hot water on the unit — rapid temperature change can crack components
- Don't chip ice off with sharp tools — easy to damage refrigerant lines or coil fins
- Don't cover the unit with a tarp or wrap — heat pumps need airflow even in winter
- Don't shut off power to "let it thaw" without confirming with us first — some systems need power to manage condensate pumps and electronics
When to call us
Stop troubleshooting and pick up the phone if:
- Ice covers the entire outdoor coil and isn't clearing after multiple defrost cycles
- Ice has built up to the point where the fan can't spin freely
- The heat pump isn't producing enough heat to keep up
- Auxiliary heat (electric backup) is running constantly
- You see steam coming from the unit at unusual times (could indicate refrigerant issues)
- The unit is cycling on and off rapidly without producing heat
- You've had recurring ice problems in past winters
Frequently asked questions
How much ice is "normal" on a heat pump in winter?
A thin layer of frost on the coil is normal and clears with each defrost cycle. After defrost, you might still see some moisture or remaining frost — also normal. What's NOT normal: thick ice covering the entire coil, ice on or above the fan, ice that doesn't clear over multiple defrost cycles, or ice formations growing larger over hours.
Can I just turn the heat pump off and let the ice melt?
Sometimes — but check with us first. Some systems need to stay powered to run condensate pumps or controls. Better approach: switch your thermostat to "Emergency Heat" or "Aux Heat" mode (uses backup heat instead of the heat pump), which lets the outdoor unit defrost while you still get heat. Then call.
Will running my heat pump in extreme cold damage it?
Modern cold-climate heat pumps (Mitsubishi Hyper-Heat, Daikin Aurora, etc.) are rated to operate to -25°C and below without damage. Older non-cold-climate heat pumps weren't designed for prolonged operation in extreme cold and can ice up more readily. If your heat pump struggles below -10°C and you don't have a cold-climate model, that's a sizing/equipment issue rather than a one-time fix.
My heat pump iced up once last winter too. Should I worry?
A single ice event in extreme weather is usually fine. Recurring ice problems (multiple times per winter) indicate something systemic: defrost sensor wearing out, slow refrigerant leak, installation issue with drainage. Worth a diagnostic visit before next winter to identify the root cause.
Related services
If you need professional repair, the relevant services for this issue: