Residential HVAC Controls) Heat Pump Defrost Controls

 

1. Electronic Defrost Controls for Heat Pumps

• Defrost Control Board:
  • Integration: Contains a timer and defrost relay.
  • Operation: Enters defrost mode if frost is sensed on the outdoor coil by the defrost thermostat.
  • Defrost Mode:
    • The reversing valve switches to route hot discharge gas from the compressor to the outdoor coil.
    • The outdoor fan turns off.
    • Electric heaters energize to provide tempered heat.
    • Defrost cycle ends when the defrost thermostat senses 50-60°F on the outdoor coil.
  • Manual Initiation: Technicians can manually initiate defrost to check proper operation.
  • Older Systems: May use electromechanical timers and relays instead of an integrated control board.

2. Types of Defrost Controls

• Time Initiated, Time Terminated (Straight Time):
  • Operates based on time regardless of frost amount on the coil.
• Temperature Differentiated:
  • Uses temperature to signal the need for defrost.
• Time Initiated, Temperature Terminated:
  • Similar to straight time controls but terminates defrost based on coil surface temperature.
• Time and Temperature Initiated, Temperature Terminated:
  • Has a timer but will only initiate defrost if frost is sensed on the coil.
  • Terminates defrost once frost melts and coil temperature rises to the set point.

3. Key HVAC System Components

• Metering Device:

  • Function: Divides the high and low-pressure sections and regulates refrigerant flow into the evaporator.
  • Thermostatic Expansion Valve (TXV):
    • Contains a sensing bulb mounted to the suction line at the evaporator outlet.
    • Measures evaporator needs and adjusts refrigerant flow to maintain superheat.
    • Often field adjustable.
  • Electronic Expansion Valve (EEV):
    • Uses a step motor and inputs from an NTC thermistor and pressure transducer.
    • Provides precise control of refrigerant flow.

• Reversing Valves:

  • Function: Determines whether discharge gas is routed to the indoor or outdoor coil.
  • Control: Operated by the thermostat.
  • Connections:
    • Discharge Inlet: Always receives discharged gas from the compressor.
    • Suction Outlet: Always receives suction vapor from the compressor.
    • Two Outside Ports: Change functions to route gas depending on the mode (cooling or heating).

• Auxiliary Heat Relay:

  • Function: Controls high voltage components like electric heaters with low voltage signals from a thermostat.
  • Staging Strategy: Activates second-stage heat (typically electric heaters) when the heat pump cannot keep up with heat loss.
  • Location: Near or on the blower relay within the control cabinet.