Quick answer
To defrost a frozen heat pump, switch the thermostat to Emergency Heat or Off, clear snow from around and under the outdoor unit, and let the ice melt. Lukewarm water from a hose or watering can speeds it up. Never use hot water, tools or a heat gun. Light frost that clears itself is normal; solid ice lasting more than about two hours means a fault.
The safe way to defrost a frozen heat pump is to stop it running, give the meltwater somewhere to go, and add nothing hotter than lukewarm water. The outdoor coil is thin aluminum fins over soft copper tubing that holds refrigerant under pressure. The damage people do while defrosting comes from chipping at that coil or heating it.
Defrost a frozen heat pump in five steps
- 1. Stop the outdoor unit. On a ducted system set the thermostat to Emergency Heat so the backup element or furnace carries the house. On a ductless system turn the head off at the remote and use another heat source.
- 2. Clear the snow. Shovel a clear space around all sides and brush snow off the top by hand. Dig out underneath so the base pan is not sitting in a drift.
- 3. Open the drainage. Find the drain holes in the base pan and clear slush or leaves from them, then break up any ice mound on the ground below so water can fall away.
- 4. Melt the ice. Above freezing, time alone will do it. To speed things up, run lukewarm water over the coil from a garden hose or watering can, top down, until the fins are visible.
- 5. Restart and watch. Switch back to Heat and stay for a full cycle. The unit should run, frost lightly and clear itself; note whether the fan spins.
What never to use on an iced-up heat pump
- Hot or boiling water: thermal shock is hard on cold plastic fan blades and coil joints. Lukewarm is enough.
- Screwdrivers, ice picks, hammers or scrapers: one slip punctures a tube or flattens the fins, and a punctured coil turns a free fix into a refrigerant repair.
- Heat guns, torches or space heaters aimed at the unit: they melt wiring insulation and plastic, and can overheat the refrigerant sealed inside the coil.
- Salt or chemical de-icer: it corrodes aluminum fins.
- Your hands in the fan opening: the fan can start without warning when the board calls for it. Switch off the outdoor disconnect before clearing ice near the blades.
Normal frost versus a heat pump that is frozen
A thin white coat of frost that comes and goes is how a heat pump works in winter. The coil runs colder than the outdoor air, moisture freezes onto it, and the control board runs a defrost cycle, typically every 30 to 90 minutes of run time, to melt it off. During defrost the fan stops, steam rises, and water runs out the bottom for 5 to 15 minutes.
The decision rule: a solid block of ice on the coil that is still there after about two hours is a fault, not weather. So is ice over the top of the unit, ice locking the fan blades, or ice climbing from the base pan into the bottom rows of the coil. Frost builds fastest in damp air between about −5 °C and 5 °C, the weather Lake Ontario humidity brings, so a unit that copes at −15 °C can still ice up on a wet night near freezing.
Rule out outside causes before you blame the equipment: a leaking eavestrough dripping onto the unit, freezing rain, or a drift blocking airflow. What to do when a heat pump is covered in snow covers clearances, and why a heat pump ices up goes through the causes one at a time.
What a technician checks when the ice keeps coming back
- Defrost board: forced into a test defrost to confirm it shifts the valve, stops the fan and times out properly.
- Coil and ambient sensors: resistance compared against the manufacturer's temperature chart. A sensor that reads warm means the board never sees a reason to defrost.
- Reversing valve: defrost depends on it shifting to cooling. A valve or solenoid that does not shift leaves the coil frozen.
- Refrigerant charge: a low charge makes part of the coil run far colder, so it ices unevenly and defrost cannot keep up.
- Outdoor fan motor: with no air moving across it, the coil ices solid quickly.
- Base-pan heater: on cold-climate models a small electric heater keeps the pan and its drain holes open. When it fails, meltwater refreezes in the pan and the ice grows upward.
Is it safe to keep running it, and what the repair costs
Do not run a heat pump that is encased in ice. The fan can strike the ice, and the compressor ends up pulling against a coil that cannot absorb heat, which is how compressors get damaged. Backup heat costs more per hour but is safe to run for days. Treat it as urgent if there is no backup heat and the house is cooling.
- Service call and diagnosis: $90–$150, which comes off the bill if you approve the repair.
- Defrost board replacement: control boards run $400–$800.
- Outdoor fan motor: $300–$700.
- Leak repair and recharge when the charge is low: $300–$800+.
- Sensors and base-pan heaters: quoted in writing once the part is identified.
If the ice returns after a thaw, the defrost system needs a technician, not another hose. ZK Mechanical's heat pump repair page lists what we test, the defrost cycle guide explains the normal sequence, and the 24/7 line at (647) 801-1252 or the booking form gets a visit scheduled.