Heat Pump Flowcharts

A heat pump is an AC plus three additional layers: a reversing valve, a defrost board, and auxiliary heat. Everything that can fail on an AC can fail on a heat pump — contactor, capacitor, compressor, fan, refrigerant circuit — and those failures look identical to the AC failures. What’s added is a set of failures unique to heat-pump operation: reversing valve stuck in one mode, defrost stuck on or stuck off, aux heat running when it shouldn’t or not running when it should.

The flowchart for a heat pump no-heat call starts by asking whether the system is actually broken or merely in a transient state (defrost, cold-start delay) that looks broken. A lot of heat-pump service calls end with “it’s operating correctly, this is what a defrost cycle looks like” — which is a frustrating diagnosis to deliver but a real one.

Before you start — is it actually broken?

Only after ruling these out should you start diagnosing components.

Heat pump no-heat branches

Branch A — compressor won’t run at all

Identical to the AC “blower runs, outdoor silent” branch. Check Y at the outdoor contactor. 0V → wire break or indoor panel issue. 24V but contactor not pulling → contactor failure. Contactor pulls but compressor doesn’t run → capacitor, compressor protection, or compressor itself.

Heat pump specific twist: in heat mode, the compressor start is slightly more stressed than in cool mode because the system is starting against a reversed pressure gradient and a cold outdoor coil. A compressor that starts fine in summer may be marginal in deep winter — fails hard-start or draws lockup current. If a heat pump compressor runs fine in the shoulder seasons but buzzes on January calls, suspect a start capacitor issue or aging compressor windings.

Branch B — compressor runs but system produces no heat (cool air at registers)

This is the reversing valve branch. With compressor running but cold air blowing, the valve is either:

• Stuck in cool mode — valve solenoid de-energized correctly (O terminal 0V during heat call), but the internal slide didn’t move. Mechanical failure.
• Getting wrong signal — O terminal energized during a heat call (thermostat mis-configured, wiring error, or defrost board stuck in defrost output).

Measure O at the reversing valve solenoid during a heat call. Should read 0 VAC. If 24V is present, track backward: is the thermostat erroneously commanding O? Is the defrost board stuck outputting defrost command?

If O correctly reads 0V and the valve still won’t shift to heat position, the valve is mechanically stuck. See Chapter 44 for replacement considerations.

Branch C — compressor runs, faint heat, aux runs constantly

The heat pump is producing some heat but not enough. Aux heat is compensating. Causes in order of frequency:

1. Low refrigerant charge — a heat pump is more sensitive to low charge in heat mode than in cool mode because the outdoor coil is the evaporator and already fighting for heat in cold air. 10% low charge that’s unnoticeable in cooling becomes obvious in heating.
2. Dirty outdoor coil — restricts airflow over the evaporator side, reducing heat absorption. Winter operation also plasters snow, leaves, and debris onto the coil.
3. Iced outdoor coil — defrost isn’t keeping up, ice is accumulating, capacity is dropping.
4. Compressor capacity loss — older compressor with internal wear producing less refrigerant flow than rated.
5. Aggressive thermostat aux-heat staging — system is producing adequate heat but the thermostat brings in aux heat too readily (low delta-T trigger, unrealistic balance point).

Check the easy ones first. Is the outdoor coil iced or visibly clogged? Is the air filter clean? When’s the last service? Has the customer noticed bills creeping up for months (long-term refrigerant leak) or is it sudden (recent failure)?

Branch D — stuck in defrost

The outdoor unit has been in defrost-looking state for more than 10–15 minutes, well past a normal termination. Symptoms: outdoor fan off, compressor running in cool direction, strip heat running. Cause: the defrost board’s termination logic isn’t working. Possibilities:

• Defrost thermistor failed open or shorted — board thinks coil is still cold, keeps running defrost indefinitely.
• Defrost board failed — relay welded closed, output stuck on.
• Thermistor lead disconnected — reads open-circuit, board defaults to “keep defrosting.”

Test the thermistor: pull the lead, measure resistance. Heat pump defrost thermistors are typically 10K or 30K NTC. Expected resistance at 32°F is somewhere in the range of 25K–35K (depends on type). Open-circuit reading means failed or disconnected thermistor.

Branch E — never goes into defrost

Opposite problem: outdoor coil is icing up visibly, but defrost never initiates. The board isn’t commanding defrost when it should. Possibilities:

• Defrost thermistor reads artificially warm — board thinks coil doesn’t need defrost.
• Defrost initiation timer disabled or maladjusted — on time-temp boards, the 30/60/90-minute runtime jumper may have been pulled.
• Board output relay stuck open — defrost logic is commanding but the output relay won’t close.

Branch F — aux heat running when it shouldn’t

The system is heating fine (compressor running, decent warm air at registers) but aux heat is also running, driving up power bills. Causes:

• Thermostat aux staging too aggressive — delta-T trigger too low, staging timer too short, or outdoor balance-point set too high.
• Thermostat misconfigured for “heat pump + aux” — some thermostats treat W as first-stage heat, running aux with no compressor. Configuration error.
• Emergency heat mode latched — customer or someone bumped the E mode on and forgot.
• Compressor protection fault — compressor is locked out (pressure or thermal protection), thermostat is staging aux to compensate. Check for error codes or contactor behavior during a call.

Written decision tree — heat pump no heat

Until this tree is authored into the interactive widget, here’s the logic to run mentally:

Common misdiagnoses on heat pumps

• “Reversing valve bad” when actually the O terminal was mis-wired at install (common on replacement thermostats).
• “Compressor bad” when actually the start capacitor is degraded and compressor starts fine with a boosted cap.
• “Low refrigerant” when actually the outdoor coil is iced because defrost isn’t running.
• “Thermostat bad” when actually the thermostat was correctly staging aux heat but the customer didn’t know heat pumps use aux heat normally in cold weather.

In each case, the fault is “upstream” of where the tech first looked. The diagnostic discipline pays off — a systematic run through the flowchart would have caught each of these before a wrong part was ordered.

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Check your understanding

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01You arrive at a heat pump no-heat call on a 25°F day. The outdoor unit has the compressor running but the fan is off, vapor is rising from the coil, and warm air is coming from the registers inside. What's happening?

AThe reversing valve has failedBThe system is in a defrost cycle — normal operationCThe compressor is overheatingDThe outdoor fan motor is bad

02A heat pump is producing cold air from the registers on a heat call. You measure 24 VAC at the reversing valve O terminal during the call. What's the problem?

AThe reversing valve's internal slide is mechanically stuck in cool positionBThe thermostat is wired incorrectly OR configured for the wrong O/B convention — O should be 0V during a heat call on a standard heat pumpCThe compressor is badDThe outdoor fan is bad

03Heat pump is running fine in heat mode, producing warm air, but the homeowner complains of a huge electric bill. Strip heat indicator light is on much of the time. What's the first thing to investigate?

AReplace the compressorBAdd refrigerantCReview thermostat configuration: aux-heat staging parameters, balance point setting, mode (emergency vs normal heat)DReplace the outdoor coil