HVAC Technician Academy
Learn how to diagnose reversing valves, O/B control signals, defrost systems, auxiliary heat, heating-mode refrigeration behavior, thermostat controls, and common heat-pump failures without guessing at parts.
Heat pumps use the same refrigeration circuit for both heating and cooling, but the reversing valve changes refrigerant direction. Technicians must understand O/B control logic, defrost operation, auxiliary heat, outdoor-coil frosting, and how pressures and line temperatures change with operating mode.
Before interpreting pressures or temperatures, confirm:
Step 1: Understand reversing-valve operation
Step 2: Learn O/B thermostat logic
Step 3: Understand heating-mode refrigerant flow
Step 4: Learn normal outdoor-coil frosting behavior
Step 5: Understand defrost initiation and termination
Step 6: Learn auxiliary and emergency heat operation
Step 7: Diagnose sensors, boards, and control signals
Step 8: Separate refrigeration faults from control faults
Prove the O/B command, coil operation, temperature response, pressure behavior, and whether the reversing valve actually shifts.
Heat Pump Defrost Diagnostics →Diagnose defrost sensors, board logic, coil conditions, outdoor fan behavior, reversing-valve operation, and defrost termination.
24-Volt Control Circuits →Trace thermostat commands, reversing-valve circuits, relays, boards, and safety inputs.
The reversing valve changes which coil functions as the evaporator and which functions as the condenser.
A proper diagnosis separates:
The presence of 24 volts at the solenoid does not prove the valve shifted.
O/B is commonly used to control the reversing-valve solenoid.
However, the required energization mode is manufacturer-specific.
Some systems energize the valve in cooling; others may use different logic.
Continue: Heat Pump O/B Thermostat Wiring Guide
Measure control voltage directly across the solenoid coil when the valve should be energized.
If correct voltage is present but the valve does not respond:
Reversing valves are not limited to only fully good or fully failed.
Possible faults include:
A partial shift or internal bypass can create unusual temperature and pressure patterns.
In heating mode, the indoor coil generally functions as the condenser and rejects heat into the building.
The outdoor coil generally functions as the evaporator and absorbs heat from outdoor air.
This means the outdoor coil can operate below outdoor-air temperature and accumulate frost.
Some outdoor-coil frost during heating operation can be normal.
Problems arise when frost accumulates excessively and the system cannot clear it properly.
Before blaming the defrost board, inspect:
Exact logic varies by manufacturer, but a typical sequence may involve:
1. System operates in heating mode.
2. Control detects conditions requiring defrost.
3. Reversing valve shifts temporarily.
4. Outdoor fan may stop depending on design.
5. Auxiliary heat may energize to reduce cold-air delivery indoors.
6. Defrost terminates based on manufacturer control logic.
Many modern systems use thermistors to determine coil or outdoor temperature.
Diagnose thermistors by comparing measured resistance with:
Do not condemn a sensor because of resistance alone without knowing its actual temperature.
Before replacing a defrost board, verify:
A board can only make the correct decision if it receives accurate inputs.
Auxiliary heat may operate when:
Depending on the equipment, auxiliary heat may be electric resistance heat or another backup heat source.
Emergency heat is typically a user-selected operating mode that disables normal heat-pump compressor operation and relies on the backup heating source.
Exact thermostat and system behavior varies, so follow manufacturer controls documentation.
Possible directions include:
Refrigerant pressure relationships change when the system changes modes.
Use manufacturer heating-performance information and understand which coil is functioning as the evaporator and condenser.
In heating mode, restricted indoor airflow can increase indoor-coil temperature and affect system pressures.
Verify:
Technician Case File
Thermostat: calling for heat.
Compressor: running.
Indoor air: cold.
O/B command: incorrect for this equipment configuration.
Thermostat setup: programmed for opposite reversing-valve logic.
Lesson: A refrigeration symptom can be created entirely by incorrect control configuration.
Initial assumption: bad defrost board.
Outdoor fan: not running.
Coil airflow: nearly zero.
The icing was driven by an outdoor-airflow problem. Defrost controls were not the first fault.
Defrost sequence: initiates normally.
Reversing valve: shifts correctly.
Auxiliary heat: not energizing.
Diagnosis: control problem in the auxiliary-heat circuit.
The defrost system itself was functioning. The comfort complaint was caused by a separate backup-heat fault.
1. Verify thermostat mode and actual call.
2. Identify required O/B logic for the equipment.
3. Verify reversing-valve control voltage.
4. Confirm that the valve actually shifts.
5. Identify whether the unit is in normal heating or defrost.
6. Verify indoor and outdoor airflow.
7. Check defrost sensor inputs and board outputs.
8. Verify auxiliary heat when required.
9. Evaluate refrigerant pressures and temperatures for the actual operating mode.
10. Verify delivered heating performance.
11. Retest the complete heat-pump sequence after repair.
Understand reversing-valve thermostat configuration and control logic.
Heat Pump vs AC →Review the operating differences between heat pumps and straight-cooling systems.
Heat Pump Installation →Apply control, airflow, and commissioning principles to installed heat-pump systems.
Heat-pump diagnosis can involve energized line voltage, electric heat strips, high refrigerant pressures, rotating fans, hot refrigerant lines, and defrost-mode changes. Use properly rated instruments and PPE, verify equipment state before testing, and follow manufacturer procedures for forced-defrost or diagnostic test modes.
Reversing valves, thermostat configuration, defrost controls, outdoor airflow, refrigerant performance, auxiliary heat, and duct airflow can create overlapping symptoms. Diagnose the operating sequence before replacing parts.
HVAC Diagnostic Services HVAC Technician Academy