Coil temperature sensor
Cheap, easy to test against a resistance chart, and responsible for most defrost complaints. Checked before any board is considered.
Heat Pump Repair for the Faults a Cooling Technician Does Not See
Defrost boards, coil sensors, reversing valve solenoids and backup heat staging are the parts that fail in winter, and none of them exist on a straight air conditioner.
- Defrost circuit forced and observed
- Reversing valve checked by temperature
- Backup heat staging verified under load
- Charge assessed by the correct mode
A heat pump is an air conditioner with a reversing valve and a defrost control bolted on, and almost every winter misdiagnosis lives in those two additions.
Heat pumps attract more misdiagnosis than any other residential equipment, and the reason is structural. A technician who works on cooling systems all summer knows compressors, capacitors, contactors and charge. The components that fail on a heat pump in January are the ones a cooling system does not have, and they are unfamiliar precisely when they matter.
The result is a recognizable pattern of wrong answers. A unit iced solid gets diagnosed as low on refrigerant when the actual fault is a defrost termination sensor that has drifted out of range. A system heating poorly gets a refrigerant top up when the reversing valve is bypassing internally. A household with an enormous winter electricity bill is told the equipment is simply inefficient in cold weather, when the compressor side has failed entirely and resistance heat has been carrying the building for a month.
Every one of those is separable by test. The defrost circuit can be forced and observed. The reversing valve can be assessed by temperature readings taken across the valve body. Auxiliary heat staging can be watched at the thermostat and confirmed by amp draw on the heat strips. None of it takes long once you know it needs doing.
Heat pumps have been part of the work in Hudson, TX since 2006, and the cold weather calls are handled with that additional component set in mind rather than as air conditioning that has stopped behaving.
The defrost circuit, and how it goes wrong
In heating mode the outdoor coil is the evaporator, running below the outside air temperature so it can absorb heat from it. Below a certain ambient, running cold means running below freezing, and moisture in the air condenses onto the coil and freezes there. Frost insulates the coil and reduces its ability to absorb heat, so the system has to remove it periodically.
It does this by reversing into cooling mode briefly. Hot discharge gas goes out to the outdoor coil and melts the frost, the outdoor fan stops so the heat is not blown away, and auxiliary heat comes on indoors so the occupants do not get a blast of cold air. After a set period, or when a coil sensor reports the ice has gone, the system reverses back.
Three failure modes follow from that. The system never initiates defrost, and the coil progressively ices over until performance collapses. It initiates far too often, wasting energy and cutting heating output because it is spending its time defrosting rather than heating. Or it initiates and never terminates, leaving the system running in cooling mode in the middle of winter, which is the one that produces an urgent call.
Why the sensor is checked before the board
The defrost board takes its cue from a coil temperature sensor clipped to the outdoor coil. A sensor that has drifted, corroded at its connection or fallen off its clip feeds the board bad information, and the board does exactly what bad information tells it to.
Sensors are also far cheaper than boards, and their resistance can be checked against a temperature chart in a few minutes. Replacing a defrost board on a system whose sensor has failed is a common and expensive error, and the replacement board will misbehave in precisely the same way.
| What the household sees | Common cause | How it gets confirmed |
|---|---|---|
| Outdoor coil iced over solid | Defrost not initiating: sensor, board or timer | Force a defrost cycle and watch whether the system responds |
| Defrosting every few minutes | Coil sensor reading incorrectly, or termination setting wrong | Sensor resistance against its temperature chart |
| Cold air from vents, system stuck in cooling | Defrost cycle failing to terminate, or reversing valve stuck | Temperature across the reversing valve body during operation |
| Cools fine, will not heat at all | Reversing valve solenoid coil failed electrically | Voltage at the coil and continuity through the winding |
| Weak in both heating and cooling | Reversing valve bypassing internally | Suction and discharge lines closer in temperature than expected |
| Huge electricity bill, house warm | Compressor side down, backup heat carrying the load | Outdoor unit operation during a heating call, and strip amp draw |
Reversing valves, tested rather than assumed
The reversing valve is a four port valve that redirects discharge gas to either the indoor or the outdoor coil, which is what turns an air conditioner into a heat pump. It is moved by a pilot solenoid, and the distinction between a failed solenoid and a failed valve body matters a great deal to the invoice.
The solenoid is an electrical component and it is tested electrically: voltage present at the coil when the system calls for the relevant mode, and continuity through the winding. A solenoid that has gone open circuit is a modest part and a straightforward replacement.
The valve body itself is brazed into the refrigerant circuit, so replacing it means recovery, cutting, brazing with heat shielding to protect the internal seals, evacuation and recharging. It is a substantial job, which is why it is only undertaken once the solenoid has been eliminated and the temperature readings across the valve body support the conclusion.
Those readings are the key test. On a correctly operating valve, the line temperatures at each port follow a predictable pattern. A valve leaking internally allows hot discharge gas to bleed into the suction side, which shows up as a suction line warmer than it has any business being and a system that underperforms in both modes without any obvious component fault.
Defrost control board
Replaced only when the sensor is proven good and the board fails to respond to a forced defrost cycle. Boards are frequently blamed for sensor faults.
Reversing valve solenoid
An electrical coil that either has voltage and continuity or does not. Eliminating it first separates a modest repair from a major sealed system job.
Auxiliary heat staging
Strip heat, its sequencers or relays, and the thermostat configuration controlling when it engages. Verified by amp draw rather than by what the display claims.
Charge, and why winter readings mislead
In heating mode the coils swap roles. The outdoor coil becomes the evaporator and the indoor coil becomes the condenser, which means superheat and subcooling are now being measured in completely different places than they are in summer. Applying cooling mode targets to heating mode readings produces confident nonsense.
Where conditions allow, charge is verified by running the system in cooling mode. Where that is not practical in the middle of winter, the reliable approach is recovery and weighing in the manufacturer's specified charge, or using the manufacturer's heating mode charging chart which accounts for outdoor ambient and indoor conditions together.
This is the single most common way a heat pump gets overcharged. A technician reads a low suction pressure in January, adds refrigerant, and leaves a system that will flood the compressor with liquid the following summer.
If the outdoor unit is not running at all during a heating call and the house is still warming up, the compressor side has failed and resistance heat is carrying the building. The comfort is real and the running cost is not sustainable.
Backup heat, and the bill that reveals it
Almost every air source heat pump installation includes some form of supplementary heat, usually electric resistance elements in the air handler. Their job is to cover the gap below the balance point, where the heat pump alone can no longer meet the building's heat loss, and to maintain comfort during a defrost cycle.
Resistance heat is expensive to run compared with the compressor, which is the entire economic argument for a heat pump in the first place. A system that leans on it unnecessarily is quietly costing several times what it should, and the household usually has no way of knowing. The house is warm. Nothing sounds wrong. The only indication is a bill.
Diagnosis is a matter of watching what engages and measuring it. Strip element amp draw confirms how many stages are actually energizing. Thermostat configuration reveals whether backup is being called on every large setpoint recovery, which is a common misconfiguration. Outdoor unit operation during a heating call establishes whether the compressor side is contributing at all.
Where the compressor side has failed outright, that combination is the tell. The building stays comfortable, so nobody calls, and the system runs on resistance heat for weeks before anyone investigates.
How a cold weather visit is structured
Confirm what is actually running
Whether the outdoor unit operates on a heating call, whether the indoor blower runs, and whether auxiliary heat is engaged, before any panel comes off.
Read the thermostat configuration
Equipment type, stage settings, balance point and any lockouts, since a misconfigured control produces faults that look mechanical.
Measure temperature rise
Return and supply air across the indoor coil, which immediately separates a heat pump that is producing heat from one that is not.
Test the defrost circuit
Coil sensor resistance against its chart, then a forced defrost cycle observed from initiation through to termination.
Assess the reversing valve
Solenoid voltage and continuity first, then line temperatures across the valve body during operation in each mode.
Verify auxiliary heat
Strip element amp draw and sequencer operation, confirming the staging engages when it should and does not when it should not.
Then the shared components
Capacitors, contactor, motor amp draws and charge, exactly as they would be checked on a cooling system.
Quote, repair and prove
The fault explained against the readings, quoted flat and approved, then the system run through a full cycle and the readings retaken.
- Coil sensor tested before any defrost board is condemned
- Solenoid eliminated before any reversing valve is replaced
- Charge never adjusted on heating mode pressures alone
- Auxiliary heat confirmed by amp draw, not by the display
- Findings written down and left with the homeowner
Cold weather questions
Heat pump behavior that is normal and behavior that is not
A good deal of what worries heat pump owners in winter is the system working correctly. These separate the two.
Almost certainly not. That is a defrost cycle finishing. In heating mode the outdoor coil runs colder than the outside air and accumulates frost, so periodically the system reverses briefly to melt it off. The steam is that ice turning to water and evaporating off a suddenly warm coil.
What is not normal is the unit becoming encased in solid ice, or defrosting every few minutes, or never defrosting at all while frost builds up over days. Each of those points at a specific fault in the defrost circuit rather than at normal operation.
Supply air from a heat pump in heating mode is genuinely cooler than air from a furnace. A furnace delivers a hot blast; a heat pump delivers a large volume of moderately warm air over longer run times. Air at a temperature below skin temperature feels cool even while it is heating the room.
That said, there is a threshold. If the temperature rise between return and supply is very small, or the auxiliary heat is running constantly to keep up, that is a fault rather than a characteristic. The distinction is a measurement, not an impression.
The classic presentation is a system that cools perfectly and will not heat, or heats and will not cool, because the valve is stuck in one position. Sometimes it is the solenoid coil that has failed electrically rather than the valve body, which is a much cheaper repair and gets tested first.
A valve that has partially shifted produces a subtler fault: poor performance in both modes, with suction and discharge pressures closer together than they should be, because refrigerant is bypassing internally. That one is diagnosed by temperature readings across the valve body rather than by pressures alone.
Sometimes correctly, sometimes not. Below the balance point of the system, the heat pump alone cannot meet the building's heat loss and backup is supposed to engage. That is designed behavior.
Incorrectly, it happens when the thermostat is configured wrongly and calls backup on any large setpoint change, when the outdoor sensor or balance point setting is off, when the heat pump has lost charge and cannot deliver its rated output, or when a defrost board is faulty. Resistance heat is expensive to run, so a system leaning on it unnecessarily is worth diagnosing promptly.
Yes, and this is where a lot of heat pump work goes wrong. Charging by the summer method in heating mode gives meaningless numbers, because the roles of the coils have swapped: the outdoor coil is now the evaporator and the indoor coil is the condenser.
Charge is best verified in cooling mode where conditions permit, or by weighing in a known charge after recovery, with manufacturer charging charts for heating mode used where a check is genuinely needed in winter. A technician adding refrigerant on the basis of a suction pressure read in January is guessing.
It is the control that decides when to run a defrost cycle and when to end it. It takes input from a coil temperature sensor and usually a timer, reverses the system, stops the outdoor fan and brings on auxiliary heat so the indoor air does not turn cold during the cycle.
Failures split into three. Never defrosting, which lets the outdoor coil ice over completely. Defrosting too often, which wastes energy and reduces heating output. Or defrosting and never terminating, which leaves the system stuck in cooling mode in the middle of winter. A failed coil sensor produces most of these, and it is a far cheaper part than the board.
Yes. In heating mode the outdoor unit is the part extracting heat from outside air, so it runs whenever heating is called for. It should also be clear of snow and ice on all sides and should not be sitting in standing water or a drift.
Clearance matters more in winter than summer. Defrost water needs somewhere to go, which is why these units are set on raised stands or brackets. A unit that has been allowed to freeze into a block of its own defrost water is a genuine problem.
It is the same machine, so yes. A heat pump is an air conditioner with a reversing valve and a defrost control added. Everything that fails on a cooling system, from capacitors and contactors to compressors and coils, fails here too and is diagnosed the same way.
What is added is the heating side: the reversing valve and its solenoid, the defrost board and sensor, and the auxiliary heat staging. A technician comfortable with cooling work is only partway to being comfortable with heat pumps, which is why cold weather complaints on these systems are so often misdiagnosed.
Run a heating call and measure. If the outdoor unit runs and the temperature rise across the indoor coil is reasonable, the heat pump is working. If the outdoor unit does nothing while the building still gets warm, backup heat is carrying the load on its own and the compressor side has failed.
That second case is easy to miss because the house stays comfortable. The first indication for most households is a heating bill that has jumped sharply, by which point the system may have been running on resistance heat for weeks.
Related heat pump and cooling work
The same machine handles both seasons, so these pages overlap with the cooling side of the catalog.
Say whether the outdoor unit is running and whether it has ice on it
Those two observations separate most heat pump faults into categories before a technician arrives, and they cost nothing to check from a window.
Heating and cooling coverage across Hudson, TX, with dispatch staffed at every hour including holidays.