Quick answer
A light coat of frost on the outdoor coil in damp weather near freezing is normal; the defrost cycle clears it, usually within an hour or two. A heat pump icing up into a solid block — ice on the top grille, the fan or the whole cabinet — means the defrost cycle has failed, refrigerant is low, the fan has quit, or water is dripping onto the unit from above.
Frost on a heat pump in winter is expected; a heat pump icing up until it looks like a freezer nobody has defrosted is not. The difference is where the ice is and whether it comes back within an hour of clearing. Here is how to tell the two apart, and what actually causes the bad kind in Burlington and Hamilton homes.
Normal frost versus a real icing problem
Normal: a thin, even white coating on the coil fins, heaviest on damp days just around freezing, that disappears when the unit runs a defrost cycle. You may see steam rise and hear a whoosh as the reversing valve flips. Not normal: solid ice rather than frost, ice on the top grille or the fan blades, icicles inside the cabinet, or a coil still iced an hour or more after a defrost should have run.
Lake Ontario makes frost more common here than inland. Humid air off the lake at -2°C carries far more moisture than dry air at -15°C, so a heat pump in Bronte or Roseland frosts faster on a grey January afternoon than one in Binbrook on a clear, cold night. Extra defrost cycles on those days are the unit working properly, not failing.
Cause 1: the defrost cycle is not running
The heat pump decides when to defrost using a coil temperature sensor and a control board; older units use a timer and a defrost thermostat clipped to the coil. If the sensor drifts, the board fails or the reversing valve does not shift, frost never clears and builds into ice. This is the usual cause of a fully iced unit, and it is a sensor, board or valve repair rather than a replacement. How the cycle is supposed to behave is covered in the heat pump defrost cycle explained.
Cause 2: low refrigerant
A low charge from a leak makes the outdoor coil run colder than it should, so frost forms even in dry weather and defrost cannot keep up. The tell is often ice concentrated where the small copper line feeds the coil, along with weak heat indoors. A technician confirms it with gauges and a leak search; adding refrigerant without finding the leak repeats the problem next season. Leak repair and recharge runs $300–$800+ depending on where the leak is.
Cause 3: airflow through the coil is blocked
- Outdoor fan motor or its capacitor has failed, so the coil sits in still air and ices within hours.
- Snow drifted against the coil, a shovelled bank of driveway snow, or a unit set at grade in a corner where lake-effect snow piles up.
- Matted leaves from the fall, or a winter cover somebody fitted in good faith. A heat pump must breathe all winter; never cover it.
- Less than the manufacturer's minimum clearance to a fence, wall or cedar hedge — the figure is on the installation sheet and differs by brand and by side of the cabinet — so the unit re-breathes its own cold discharge air.
Cause 4: water dripping from above
An eavestrough that overflows, or a roof valley that drips straight onto the unit, will freeze layer on layer until the fan is locked solid. We see this on 1970s–90s subdivision homes where the outdoor unit was tucked under a roof edge with no gutter above it, and on pre-war Aldershot houses with eavestroughs full of maple keys. Moving the unit, adding a gutter section or fitting a small drip shield fixes it. Freezing rain will ice any unit; that clears on its own once temperatures climb.
What to do when it is iced right now
- Switch the thermostat to emergency heat, or off, so the compressor is not fighting a blocked coil. A solid casing of ice is one of the few good reasons to do this — should I turn off my heat pump in extreme cold lists the others.
- Do not chip ice off the fins with a tool; the aluminum bends and the copper underneath is easily punctured. If you want to speed the thaw, turn the unit off at the disconnect first, then run lukewarm — not hot — water over the coil, and remember that below freezing the runoff will refreeze around the base.
- Clear snow and debris around the unit, then look up for a drip source.
- If it re-ices within a day of a full thaw, book a repair; see heat pump repair for what a visit involves.
New installs should sit on a stand or wall bracket well above the snow line, with the coil clear of any drip line. That one placement decision heads off most snow- and drip-related icing before it starts.
If your unit is a block of ice and the house is running on backup heat, treat it as an urgent call; ZK Mechanical's 24/7 line carries no evening or weekend surcharge. See heat pump installation in Hamilton for placement done properly the first time, or contact us to book a repair.