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Heat Pumps · RepairHeat Pump Defrost Board Replacement
Heat pump defrost board replacement is the fix when the outdoor unit ices over and will not clear itself, or goes into defrost and never comes out, and testing shows the board and not a sensor is at fault. Control board replacement runs $400–$800 after a $90–$150 diagnostic. A thin white frost that clears every hour or so is normal; a unit encased in ice is not, and should be switched to emergency heat until it is repaired.
The defrost control decides when the outdoor coil gets thawed, and there are two kinds. A timed board counts compressor run time, usually 30, 60 or 90 minutes set by a jumper, and starts a defrost at that interval if a thermostat clipped to the coil says the coil is below freezing. A demand board uses two thermistors, one on the coil and one in the outdoor air, and defrosts only when the gap between them shows frost is building. Either way the board depends on the sensor feeding it, and a sensor that has drifted or fallen off the coil produces the same symptoms as a dead board.
During a defrost the board does three things at once: it shifts the reversing valve so the outdoor coil gets hot, stops the outdoor fan so that heat stays in the coil, and energises the auxiliary heat so the air indoors is not cold. Each is a separate output, and each can fail separately. A board that never initiates leaves the unit to become a block of ice. One with a welded relay can hold the unit in defrost or keep the fan running through it, and one that drops the auxiliary heat signal sends cold air through the registers for several minutes.
The test sequence is the same on every call: we force a defrost from the board's test pins and watch for all three actions. Then we measure the coil sensor's resistance against the manufacturer's chart for the temperature it is sitting at, or check a defrost thermostat for continuity when cold. Next we confirm 24 volts on the reversing-valve output and that the fan relay opens. If the board does everything right on a forced defrost, the board is not the fault, and replacing it would not cure the icing.
Plenty of iced-up heat pumps have a good board. Low refrigerant charge, a failed outdoor fan motor, a reversing valve that will not shift, a coil matted with leaves, or an eavestrough dripping onto the unit all bury a coil in ice. Near Lake Ontario the heaviest frosting comes in damp weather around freezing, not in deep cold, so a unit can ice up on a mild, foggy January day. How the cycle is meant to work is explained in the heat pump defrost cycle, and the wider causes in why a heat pump ices up.
Signs the defrost control has failed
- The outdoor unit is encased in ice top to bottom, not just frosted, and stays that way for hours
- The unit runs in heating for hours in freezing, damp weather without a single defrost: no steam, no pause in the fan
- The unit goes into defrost and stays there, with the outdoor fan stopped and cool air indoors
- Cold air from the registers for several minutes at regular intervals: defrost is running without auxiliary heat
- The outdoor fan keeps running during defrost, so the coil never gets warm enough to clear
How it works
If the unit is iced solid and your system has backup heat, set the thermostat to emergency heat so the backup carries the house and the compressor stops working against the ice. Do not chip at the coil; the fins and tubing are thin. Tell us the make, the model and how long it has been frozen.
We clear the ice safely, force a defrost, and check the sensor readings and each board output with a meter. Refrigerant pressures and the outdoor fan are checked too, so a charge or airflow problem is not mistaken for a control fault.
The new board is wired terminal for terminal and its defrost interval or mode is set to match the unit. We run a full defrost and confirm it starts, the fan stops, auxiliary heat comes on and the cycle ends by coil temperature, not by the time limit.
Brands: We replace defrost controls on conventional heat pumps from Lennox, Carrier, Trane, Goodman, Rheem, York, Amana and KeepRite, and diagnose defrost faults on Mitsubishi, Fujitsu and Daikin inverter units.
Common questions
How do I know it is the board and not the sensor?
You cannot tell from the symptoms; both leave the unit iced or stuck. A technician reads the sensor's resistance and compares it with the chart for its temperature, then forces a defrost. A sensor out of range with a board that responds correctly is a sensor repair. A sensor in range with a board that will not start or end a defrost is a board.
Is it safe to run a heat pump that is covered in ice?
Not for long. Ice blocks airflow, so the unit makes little heat while running constantly, and a thick build-up can bend fan blades and crush coil fins. Switch to emergency heat if you have it and book a repair. Light frost that clears itself is normal operation. For thawing a frozen unit without damaging it, see how to defrost a frozen heat pump.
Can a universal defrost board replace the original?
On many conventional single-stage and two-stage heat pumps, yes: universal timed boards exist and work well when the wiring and sensor type are matched carefully. On demand-defrost systems and inverter units the original part is usually the only correct choice, because the logic and sensors are specific to that model. We tell you which applies before ordering anything.
Why does my heat pump blow cold air every hour or so?
That is a defrost cycle. For a few minutes the system runs in cooling mode to heat the outdoor coil, and the board is supposed to bring on auxiliary heat to cover it. If the air turns properly cold each time, the board's auxiliary output, the wiring to the air handler or the backup heat itself has failed. On a hybrid system the gas furnace should fire during defrost.
Should I replace the board or the whole heat pump?
A defrost board is a mid-priced repair on a system that costs many times more, so on a heat pump under 12 years old with a sound compressor it is worth doing. If the unit is older and the icing has already damaged the coil or fan, compare the total with replacement. Our heat pump repair page covers the other faults, or book a diagnostic.