Heat Pump Sequence of Operation Explained

Heat pump sequence of operation describes how components work together to provide heating, cooling, defrost, and safety control. This article clarifies typical sequences for residential and light commercial systems, including single-stage, multi-stage, and variable-speed equipment, plus thermostat and control wiring interactions.

Mode	Main Components Activated	Key Actions
Cooling	Compressor, Outdoor Fan, Reversing Valve	Heat rejection to outside; expansion device meters refrigerant
Heating	Compressor, Reversing Valve, Indoor Blower	Heat absorbed outdoors and delivered indoors; backup heat if needed
Defrost	Reversing Valve, Defrost Control, Backup Heat	Reverse cycle or electric heat applied to melt coil frost
Emergency Heat	Aux/Emergency Heat Source	Compressor disabled; electric or gas heat provides indoor temperature

How A Heat Pump Works: Core Principles

A heat pump moves heat by circulating refrigerant between indoor and outdoor coils, driven by a compressor and controlled by a reversing valve. In cooling mode the indoor coil acts as an evaporator; in heating mode it acts as a condenser.

Modern heat pumps include components such as thermostats, contactors, reversing valves, expansion devices (TXV or fixed orifice), crankcase heaters, and defrost controllers. Sequence of operation coordinates these components to meet setpoint while protecting equipment.

Typical Cooling Sequence Of Operation

1. Thermostat Call For Cooling

When indoor temperature rises above the thermostat setpoint, the thermostat closes a call for cooling output. The control system interprets this and initiates the cooling sequence.

2. Indoor Fan And Blower Control

The thermostat energizes the indoor blower motor to circulate conditioned air. Blower typically ramps on immediately to move air over the cold indoor coil and stabilize discharge temperatures.

3. Compressor And Outdoor Fan Start

The compressor contactor closes and the outdoor fan motor starts. Many systems include a time delay (30–120 seconds) to prevent short cycling and to allow pressures to equalize.

4. Reversing Valve Position

For cooling mode the reversing valve is set so the outdoor coil acts as the condenser. Some thermostats energize the reversing valve during a call; others energize it continuously based on manufacturer wiring. Always follow equipment wiring diagrams.

5. Expansion Device And Superheat Control

The expansion device meters refrigerant into the indoor evaporator. A TXV or electronic expansion valve adjusts to maintain proper superheat. Proper metering prevents coil icing and ensures capacity.

Typical Heating Sequence Of Operation

1. Thermostat Call For Heat

When room temperature falls below setpoint, the thermostat signals a call for heat. Controls evaluate whether the heat pump can meet load or if auxiliary heat is required.

2. Reversing Valve Energizes For Heating

The reversing valve shifts refrigerant flow so the outdoor coil becomes the evaporator, absorbing heat from outside air. In many systems the reversing valve is energized for heating; wiring practice varies by manufacturer.

3. Compressor And Outdoor Fan Operation

The compressor starts and the outdoor fan runs. The outdoor coil extracts ambient heat; the indoor coil condenses refrigerant to deliver heat inside. Variable-speed compressors may modulate to match load.

4. Auxiliary Or Emergency Heat Activation

If the heat pump cannot meet the setpoint (because of very low outdoor temperature, rapid setback recovery, or defrost), the control enables auxiliary heat such as electric resistance or a gas furnace. Aux heat operates as needed to prevent short cycling and maintain comfort.

Defrost Sequence And Frost Management

Outdoor coils can form frost in heating mode. The defrost sequence restores outdoor coil performance while minimizing indoor comfort impact.

Defrost Initiation Criteria

Defrost may initiate based on time, outdoor coil temperature sensors, pressure differentials, or algorithmic controllers that use runtime and outdoor ambient temperature. Sensor-based defrost is most efficient.

Defrost Methods

• Reverse-Cycle Defrost: Temporarily switch to cooling to heat the outdoor coil, with indoor backup heat engaged if necessary.
• Electric Heater Defrost: Use trace heaters on the outdoor coil to melt frost.
• Hot Gas Bypass: Redirect hot discharge gas to the outdoor coil.

Typical Defrost Steps

Control detects frost, stops the outdoor fan, energizes reversing valve to cooling position, starts defrost timer, and may enable auxiliary indoor heat. When coil temperature rises or maximum defrost time completes, the system returns to heating.

Compressor Staging, Modulation, And Capacity Control

Compressor control strategies affect efficiency and comfort. Common approaches include single-stage on/off, two-stage, variable-speed inverter, and step-capacity compressors. Sequencing logic adapts start/stop timing and modulation targets.

On/Off Compressors

Single-stage compressors provide full capacity on call and stop at setpoint. Controls rely on time delays and anti-short-cycle protection to prevent rapid restarts.

Multistage And Staged Compressors

Two-stage systems or dual-compressor units stage a second compressor for higher capacity. The control energizes stages based on temperature differential and runtime, often with setpoint hysteresis to avoid short cycling.

Variable-Speed/Inverter Compressors

Inverter-driven compressors modulate speed to closely match load, improve efficiency, and maintain stable indoor temperatures. Control sequences include start-up ramp, speed setpoint selection, and safety limits for discharge and suction pressures.

Thermostat Logic And Control Interfaces

Modern thermostats communicate with heat pump controls via conventional wiring (Y, G, O/B, W, R) or digital protocols (Communicating thermostats like Modbus or proprietary). Proper mapping ensures correct reversing valve polarity and auxiliary heat control.

O/B Reversing Valve Control

Thermostats use the O or B terminal to command the reversing valve. O typically energized for cooling; B energized for heating, depending on manufacturer convention. Mistwiring can cause incorrect mode behavior.

Auxiliary Heat Control (W/E)

Auxiliary or emergency heat is signaled on dedicated thermostat terminals. Controls include lockout logic to prevent unnecessary aux heat use during mild conditions. Proper deadband setting prevents simultaneous compressor and aux heat operation.

Safety Interlocks And Protections

Safety interlocks protect equipment and occupants. Common protections include low/high pressure switches, freeze protection, crankcase heaters, anti-short-cycle timers, and flow switches for hydronic coils.

Pressure And Temperature Safeties

High-pressure switches open to prevent compressor damage during abnormal conditions. Low-pressure switches prevent operation with refrigerant loss. Temperature rollouts and flame sensors protect backup gas furnaces used as auxiliary heat.

Anti-Short-Cycle And Start-Up Delays

Controls include minimum off-time (e.g., 3–5 minutes) to allow pressures to equalize, and start-up soft-ramp for variable-speed compressors to reduce inrush and mechanical stress.

Zoning And Multiple Thermostat Systems

Zoned systems use dampers or multiple air handlers to control temperature in different areas. A zone controller manages multiple thermostats and sequences compressors, outdoor fans, and auxiliary heat to avoid conflict and maintain efficiency.

Master-Slave Coordination

In systems with multiple indoor units sharing one outdoor compressor, a master controller sequences capacity to satisfy zone calls while preventing refrigerant imbalance and compressor overload.

Priority And Staging Rules

Zone controllers implement priority rules such as emergency heat zone priority, compressor minimum runtimes, and sequencing to add capacity as more zones call for conditioning.

Common Troubleshooting And Diagnostic Clues

Understanding the sequence helps technicians interpret symptoms. Key diagnostic clues include short cycling, frost accumulation, reversal failure, high head pressure, and persistent auxiliary heat operation.

Short Cycling

Short cycling often indicates oversized equipment, thermostat differential too tight, faulty controls, or low refrigerant with inadequate pressure. Check anti-short-cycle timers and compressor run capacitors.

Frequent Defrost Or No Defrost

Excessive defrost cycles can indicate improper sensor placement or control thresholds. No defrost when needed may mean faulty reversing valve, defrost thermostat, or controller failure.

Compressor Not Starting

If the thermostat calls and the compressor doesn’t start, verify 24V control voltage, compressor contactor voltage, and safety interlocks such as pressure switches or float switches.

Wiring And Control Examples

Simple residential heat pump wiring includes R (24V), Y (compressor), G (fan), O/B (reversing valve), and W/E (aux heat). Communicating systems may use two-wire or multi-wire digital bus.

Terminal	Function	Typical Action
R	24V Power	Supplies thermostat and control relays
Y	Compressor Compressor Contact	Starts outdoor compressor and condenser fan
G	Indoor Blower	Controls air handler blower motor
O/B	Reversing Valve	Changes heating/cooling flow direction
W/E	Aux/Emergency Heat	Enables backup electric or gas heat

Best Practices For Efficient Sequencing

Good sequencing balances comfort, efficiency, and equipment longevity. Use setback strategies that respect heat pump limits and avoid relying on auxiliary heat for recovery. Implement staging, modulation, and sensor-based defrost for optimal performance.

• Enable Smart Thermostat Algorithms: Adaptive control reduces unnecessary auxiliary heat and overshoot.
• Use Outdoor Temperature Lockouts: Prevent aux heat below/above certain temperatures to optimize heat pump usage.
• Maintain Proper Controls Calibration: Verify sensor accuracy and adjust defrost thresholds based on site conditions.

Advanced Topics: Hybrid Systems And Grid-Interactive Controls

Hybrid heat systems combine heat pumps with high-efficiency gas furnaces or boilers. Sequencing manages when the heat pump operates versus combustion backup to minimize cost and emissions.

Economizer And Load-Based Sequencing

Controls may use electricity price signals or load forecasting to run heat pumps during low-cost periods and switch to backup when economical. Grid-interactive controls support demand response.

Integration With Home Automation

Modern controllers expose APIs for smart home integration, allowing sequencing adjustments based on occupancy, forecasts, or energy tariffs. Proper integration requires preserving safety interlocks and equipment protection logic.

Field Checklist For Commissioning Heat Pump Sequences

• Verify thermostat wiring and terminal polarity for O/B and W/E.
• Confirm compressor anti-short-cycle delay and minimum run time settings.
• Test defrost initiation and termination thresholds using manual commands and sensors.
• Check auxiliary heat lockout/enable temperatures and recovery behavior.
• Measure refrigerant pressures and superheat/subcooling at operating conditions.
• Document control logic and create simple operation diagrams for facility staff.

Summary Of Key Sequence Points

Heat pump sequence of operation encompasses thermostat calls, reversing valve behavior, compressor and fan starts, defrost control strategies, auxiliary heat management, and safety interlocks. Proper sequencing optimizes efficiency, comfort, and equipment life while preventing operational conflicts and damage.

For complex or communicating systems, consult manufacturer control documentation and consider professional commissioning to ensure sequences match site needs and code requirements.

Energy Star and AHRI offer additional guidance on heat pump performance and standards.