HVAC Technician Academy
Learn how HVAC technicians diagnose reversing valves, O/B control circuits, heating-mode refrigeration problems, auxiliary heat, emergency heat, defrost systems, thermostat configuration, and low-temperature heat-pump performance.
Heat pump diagnosis combines refrigeration, electrical, airflow, thermostat, and control testing because the same refrigeration circuit operates in both heating and cooling modes. The technician must determine whether the problem is caused by the refrigerant circuit, reversing valve, defrost system, auxiliary heat, airflow, controls, or system configuration.
Heat pumps use the same basic refrigeration principles as air-conditioning systems, but they add controls that change refrigerant-flow direction and manage heating performance during cold outdoor conditions.
That means a heat-pump problem can originate from:
Strong heat-pump diagnosis requires proving which subsystem is failing before parts are replaced.
Changes refrigerant-flow direction between heating and cooling modes.
Commands the reversing-valve solenoid based on thermostat and equipment design.
Manages outdoor-coil frost during heating operation.
Supplements heat-pump output when additional heating capacity is needed.
Typically disables or bypasses normal heat-pump operation and uses backup heat.
Requires technicians to understand which coil is acting as evaporator and condenser.
1. Confirm operating mode. Verify whether the thermostat is calling for heating, cooling, auxiliary heat, or emergency heat.
2. Verify thermostat configuration. Confirm system type, O/B setting, staging, and auxiliary-heat setup.
3. Verify airflow. Check filter, blower operation, static pressure, and indoor-coil condition.
4. Verify reversing-valve command. Check whether O/B is energized when expected.
5. Confirm actual refrigeration mode. Determine whether the refrigerant circuit is physically operating in heating or cooling.
6. Evaluate refrigeration performance. Check temperatures, pressures, charge, and compressor operation using manufacturer procedures.
7. Verify auxiliary heat. Confirm backup heat energizes only when commanded and produces expected temperature rise.
8. Evaluate defrost operation. Inspect sensors, board logic, outdoor coil, and defrost initiation/termination.
9. Verify full operation after repair. Confirm heating capacity, auxiliary staging, airflow, thermostat control, and defrost behavior.
The reversing valve changes the direction of refrigerant flow so the indoor and outdoor coils exchange roles between heating and cooling.
Before condemning the valve, verify:
Heat-pump systems may energize the reversing valve in cooling or heating depending on equipment design.
An incorrect O/B setting can create symptoms such as:
Study: Heat Pump Thermostat O/B Wiring Guide
Auxiliary heat supplements heat-pump capacity when the heat pump alone cannot satisfy the heating demand quickly enough or when controls command supplemental heat.
When auxiliary heat runs excessively, possible causes include:
The presence of auxiliary heat is not automatically evidence of a failure. The technician must determine why the thermostat or control system is requesting it.
| Mode | Typical Function | Diagnostic Question |
|---|---|---|
| Auxiliary Heat | Supplements normal heat-pump operation | Why is additional capacity being requested? |
| Emergency Heat | Uses backup heat when normal heat-pump operation is intentionally disabled or unavailable | Is emergency mode being used intentionally and is backup heat operating correctly? |
During heating mode, the outdoor coil acts as the evaporator and may operate below freezing temperature.
Moisture in outdoor air can freeze on the coil.
The defrost system periodically removes that frost so airflow and heat transfer can continue.
Depending on equipment design, defrost control may use:
During defrost, the heat pump may temporarily shift into cooling mode while auxiliary heat operates indoors to reduce the chance of blowing cold air into the home.
Heat-pump supply air often feels cooler than furnace discharge air even when the heat pump is operating normally.
The correct question is not simply:
"Does the air feel hot?"
The technician should measure:
Measured performance is more useful than judging comfort from hand temperature alone.
In heating mode, the indoor coil functions as the condenser and releases heat into the home.
The outdoor coil functions as the evaporator and absorbs heat from outdoor air.
This changes how technicians interpret temperatures, pressures, and refrigerant flow.
Do not use cooling-mode expectations blindly when diagnosing a heat pump in heating mode.
Use manufacturer heating-mode charging and diagnostic procedures whenever available.
| Symptom | Possible Direction | Next Test |
|---|---|---|
| Cooling during heat call | O/B configuration or reversing-valve control | Verify thermostat setup and valve command |
| Aux heat runs continuously | Low heat-pump output or control issue | Measure heat-pump capacity and thermostat logic |
| Outdoor coil heavily iced | Defrost problem or refrigeration/airflow issue | Check sensors, board, coil temperature, refrigerant conditions |
| No heat from electric backup | Heat-strip, relay/sequencer, breaker, control problem | Verify call, voltage, amperage, and temperature rise |
| Heat pump runs but capacity is poor | Refrigerant, airflow, compressor, outdoor conditions | Verify airflow and heating-mode refrigeration performance |
Diagnostic Case
Thermostat is set to heat.
Outdoor unit is operating.
Auxiliary heat is energized.
Yet indoor temperature continues falling.
The heat strips test correctly.
Refrigeration circuit is operating in cooling mode.
Thermostat was recently replaced.
O/B configuration does not match the equipment.
The heat pump is cooling the house while the auxiliary heaters try to overcome that cooling load.
Lesson: Multiple components can operate correctly individually while the complete system operates incorrectly because of control configuration.
Electric backup heat should be diagnosed as an electrical heating system.
Verify:
Do not diagnose excessive auxiliary-heat operation by looking only at the heat strips. Determine why the control system believes supplemental heat is needed.
Master O/B control, thermostat calls, and low-voltage logic.
Refrigeration Diagnostics →Strengthen pressure, temperature, superheat, and subcooling diagnosis.
Heating & Combustion →Continue into furnace heating sequence and safety diagnostics.
Heat-pump supply air may be cooler than furnace discharge air while still adding heat to the home. Measure return and supply temperatures, airflow, and operating conditions instead of judging performance by feel alone.
The reversing valve may be energized in the wrong mode, causing the system to cool during a heat call or heat during a cooling call.
In heating mode the outdoor coil operates as an evaporator and may accumulate frost. Defrost removes that frost so airflow and heat transfer can continue.
Auxiliary heat may energize when additional heating capacity is needed, during defrost, during large thermostat recovery, or because a control or heat-pump performance problem is reducing normal heating output.
No. Icing can also involve sensors, refrigeration problems, outdoor airflow, fan operation, controls, or operating conditions. The complete defrost system should be diagnosed.
Heat-pump systems contain high voltage, electric heating elements, pressurized refrigerant, rotating equipment, and hot surfaces. Testing should be performed only by qualified personnel following manufacturer procedures, appropriate PPE, electrical safety practices, refrigerant requirements, and applicable codes.
Heat-pump problems may involve refrigeration, reversing valves, thermostats, defrost controls, airflow, auxiliary heat, or electrical faults. Proper diagnosis tests the complete system before parts are replaced.
Heat Pump Services Technician Academy