Lennox Heat Pump: HP29

Re: Defrost Problem/Outdoor Coil Frozen

The Lennox HP29 Heat Pump uses a Time & Temperature Defrost Board in combination with a Defrost Thermostat (S6) a Defrost Termination Pressure Switch (S46) and Defrost Relay (K4).

Defrost Initiation

• Power goes through the Normally-Closed S46 pressure switch contacts to the S6 Defrost Thermostat.
• Once the S6 Defrost Stat senses a temperature below 30 deg, its contacts close sending power to the K4 Defrost Relay coil.
• The K4 relay has two sets of N/O contacts. One set of contacts closes to provide defrost initiation to the defrost board "DF" terminals. The other set of N/O contacts close to bypass the S6 Defrost Stat.

Defrost Termination

• The unit will stay in defrost until the S46 Defrost Termination pressure switch contacts open, breaking power to the K4 defrost relay.

CHECKOUT:

1. Check Operating Status

• Set the thermostat to call for heat.
• Ensure that the compressor and outdoor fan start and run properly.
  • Compressor & Outdoor Fan Ok → Step 3. Check Refrigerant Charge
  • No Outdoor Fan Operation → Step 2. Check Fan Motor

2. Check Fan Motor

• With the compressor operating, use a multi-meter to check for 230VAC power between the motor's Black and Orange wires.
  • No Power = Bad Defrost Board or In Defrost Mode
  • Power but No Operation = Bad Fan Motor or Capacitor

3. Check Refrigerant Charge

Hook-up refrigerant gauges to the pressure ports on the unit.

• Blue ⇒ "True Suction" Low Pressure
• Red ⇒ High Pressure (either refrigerant line)

• Monitor pressures as the system operates.

• Low Pressure Issues
  • Low Suction + Low Discharge = Low Refrigerant Charge or Bad Expansion Valve
    • Try adding refrigerant through the Low Pressure gauge.
      • Suction and Discharge Pressures should gradually increase as refrigerant is added.
      • If the Discharge Pressure increases but the Suction stays low or drops = Bad Expansion Valve

4. Check Defrost Controls

• Check for 24VAC between "C' and Terminal 9 of K4 Relay.
  • No Power = Bad S46 Defrost Termination Pressure Switch
  • Power Present ⇒ Continue
• Check for 24VAC across the coil of K4.
  • No Power + Frozen Coil = Bad S6 Defrost Stat
  • Power Present = Defrost Stat Closed → Step 5. Check Defrost Board

5. Check Defrost Board

• Check for 24VAC between "C" and "R" on the defrost board.
  • No Power = Mis-wired Low Voltage
• Check for 24VAC between "C" and "Y" on the defrost board.
  • No Power = No Call for Heat Pump
• Check for 24VAC between "C" and both "DF" terminals on the defrost board.
  • Power on One Terminal Only = Bad K4 Defrost Relay
  • Power on Both Terminals = Defrost Ready ⇒ Jump Test pins to see if unit will go into defrost
    • No Defrost = Bad Defrost Board