Stage monitoring
Which stage is actually energised during a call, which is the fastest route to any high bill complaint.
What actually goes wrong
Three fault families unique to heat pumps, plus the one everybody blames
A heat pump is an air conditioner that can run in reverse, so everything that can go wrong with a cooling system applies here too. What makes heat pump diagnosis different is the three additional subsystems that exist only because of the reversal: the reversing valve itself, the defrost control, and the supplementary heat that carries the building when outdoor conditions fall below the point where the heat pump can keep up.
Supplementary heat is where most complaints actually originate, though the customer rarely describes it that way. The complaint is a high electricity bill, and the system appears to be working. What has happened is that electric resistance heat, which is several times more expensive to run than the heat pump itself, has been energising when it was not needed. Causes include a thermostat configured for the wrong equipment type, an outdoor sensor that has failed or was never fitted, a balance point set far too high, or a homeowner nudging the setpoint upward in steps large enough to trigger the auxiliary stage on every adjustment. None of those produce a fault code. All of them are visible if somebody monitors which stage is actually running.
The defrost cycle is the second family. Frost forms on the outdoor coil in heating mode and has to be removed periodically, which the unit does by briefly reversing into cooling mode while the outdoor fan stops. Faults here are either too much or too little: a defrost board or sensor that initiates every few minutes wastes energy and never lets the building gain, while one that never initiates lets the coil ice over completely until capacity collapses. Both are diagnosed by watching an actual cycle through to termination, which takes patience rather than cleverness.
The reversing valve is the third. It can fail to shift, shift partially, or leak internally between the high and low pressure sides. A valve that has not shifted produces cold air in heating mode, which most people interpret as a refrigerant problem. An internally leaking valve is subtler: the system works in both modes but poorly in each, with pressures that look almost right. The distinguishing test is temperature measured at the valve body ports, where an internal leak shows as a suction line that is warmer than it has any business being.
| Complaint | Most likely family | The test that settles it |
|---|---|---|
| High winter electricity bill | Auxiliary heat running unnecessarily | Monitor which stage energises across a full recovery from setback |
| Cold air from vents in heating | Reversing valve did not shift, or aux only | Line temperatures at the valve body, and compressor operation |
| Outdoor unit iced over solid | Defrost not initiating | Defrost sensor resistance and a forced defrost initiation |
| Defrosting constantly | Sensor or board initiating too often | Timing between cycles and the termination temperature reached |
| Works in cooling, weak in heating | Internally leaking reversing valve | Suction line temperature at the valve compared against the expected value |
| Auxiliary heat on in mild weather | Thermostat configuration or balance point | Equipment type setting, outdoor sensor presence and lockout setpoints |
| Loud clunk on mode change | Reversing valve solenoid and pressure differential | Usually normal, but confirmed against the shift completing properly |
One more failure mode is worth naming because it is routinely missed. Auxiliary heat elements are wired through sequencers that stage them in and out, and a sequencer whose contacts have welded closed will leave an element energised whenever the blower runs, in every season. The building stays comfortable, no fault code appears, and the only symptom is a bill that makes no sense in April. Measuring current draw on each element circuit during a cooling call is the only way that gets found.
Refrigerant charge gets assessed last, and deliberately so. In heating mode the outdoor coil is the evaporator and readings shift with outdoor conditions, which makes charge harder to judge than in cooling. Where charge genuinely is suspect, the honest approach is to verify in cooling mode where the manufacturer chart is reliable, and to treat any shortage as a leak to be located rather than a level to be topped up.
Diagnostic order
Six checks specific to heat pump faults
Ordered so the cheapest and most commonly wrong things are eliminated first.
Thermostat configuration
Equipment type, outdoor sensor, balance point and auxiliary lockout settings, all of which are frequently wrong.
Defrost observation
A full cycle watched from initiation to termination, with the sensor resistance checked against its curve.
Reversing valve test
Line temperatures at the valve body in both modes, which identifies a partial shift or an internal leak.
Airflow and coil
Indoor airflow and outdoor coil condition, since both imitate capacity faults in either mode.
Charge verification
Assessed in cooling mode where the chart is reliable, with any shortage treated as a leak to locate.
On every call
What a heat pump diagnostic covers
Both modes are verified on every visit, because a fault in one is often invisible in the other.
Heating and cooling verifiedBoth modes run and measured, not just the one you reported a problem with.
Control configuration auditEquipment type, staging, lockouts and any outdoor sensor, checked against the hardware present.
Defrost cycle observedThrough to termination, with sensor resistance compared against its published curve.
Electrical measurementsCapacitance, contactor condition and current draw on compressor, outdoor fan and blower.
Auxiliary heat inspectionElement condition, sequencer operation and current draw on each stage.
Written readingsRecorded in both modes so the next visit has something to compare against.
Customer testimonies
Feedback on heat pump diagnosis
My bill had doubled and everyone said the system was fine. It was fine. The thermostat was configured for the wrong equipment and running the strips constantly.
They stood outside and watched an entire defrost cycle rather than telling me it was normal and leaving.
Reversing valve was leaking internally. Two previous visits had both added refrigerant to a system that did not need it.
Heat pump questions
Auxiliary heat, defrost, cold air and winter bills
Why does the air from my vents feel cool even though the heat is on?
A heat pump delivers air at a lower temperature than a furnace does, typically in the region of body temperature rather than well above it. Against skin that reads as cool even while the room is genuinely warming. This is normal and is not a fault.
It becomes a fault when the air is actually cold rather than merely not hot, which usually means the reversing valve has not shifted into heating, or the compressor is not running and only the indoor fan is moving air. Both are distinguishable in a few minutes by measuring supply air temperature and checking whether the outdoor unit is operating.
What is emergency heat and should I use it?
Emergency heat bypasses the heat pump entirely and runs the backup heat alone. It exists for the situation where the heat pump has genuinely failed and you need the building to stay habitable until it is repaired.
It should not be used as a way to warm the house faster in cold weather, because the backup is several times more expensive to run. If your system seems to need emergency heat in ordinary winter conditions, that is a diagnosis rather than a setting, and it usually points at charge, defrost or a control configuration problem.
The outdoor unit keeps icing up. Is that normal?
Light frost forming on the outdoor coil in cold damp weather is expected, and the defrost cycle exists to clear it. You will see what looks like steam for a few minutes every so often, and the unit will then resume heating.
Solid ice encasing the coil or the fan blade is not normal. That means defrost is not initiating, not running long enough, or terminating early, and the causes are a failed defrost sensor, a board fault, or restricted airflow across the coil. Left alone it will damage the fan blade and collapse capacity, so it is worth attending rather than waiting out.
Can a heat pump really heat a building in cold weather?
Modern inverter driven cold climate equipment holds far more capacity at low outdoor temperatures than older units did, and in many buildings it carries the load without backup for most of the season. The honest answer for any specific building is the balance point, which is where the equipment output crosses the building heat loss.
Below that temperature something has to make up the difference. What matters is that somebody works that crossing out for your building instead of guessing at it, and that the backup arrangement is a decision rather than a factory default. A system that drops onto resistance heat far earlier than it needs to will produce a bill that ends the argument, and that is a settings problem rather than a technology problem.
Related services
Where a heat pump call commonly leads
Heat pump complaints frequently resolve in the controls or in the distribution rather than in the refrigerant circuit.
Furnace Replacement
Furnace Repair
Boiler Repair
Boiler Replacement
Heat Pump Installation
High winter bill? Check which stage is actually running
It is the first thing we look at, and across Aberdeen, SD it explains more heat pump complaints than every refrigerant fault combined.
Testing is quoted per visit, with any remedial work priced separately before it is carried out.