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ICP · Heat pump troubleshooting

Defrost not working - test the defrost board

What is going on

Time/temperature defrost board: interval 30/60/90 min (factory 90). Defrost thermostat closes at 32F (0C) and opens at 65F (18C). Defrost limited to 10 minutes.

Likely causes

  • Defrost thermostat open or loose on its tube stub
  • Wrong interval jumper
  • Failed defrost board

Check in this order

  1. Confirm the defrost thermostat is tight on the tube stub at the coil end plate.
  2. Check the defrost thermostat 24 V across DFT = open, 0 V = closed. Or jumper the DFT terminals to bypass it.
  3. If open, force frost power off, disconnect fan lead at OF2 and tape it, run in heat until 0 V across DFT, then power off and reconnect OF2.
  4. Run in heat with DFT closed; short the SPEEDUP pins until the reversing valve shifts (normally 1-21 seconds).
  5. In defrost the reversing valve shifts, outdoor fan stops and strip heat comes on.
  6. Remove the short right away for a short defrost (ends at 65F or timer); holding it more than 2 seconds after the shift returns the unit to heating.
  7. Unit must return to heat valve shifts back, fan restarts, strip heat off. If not, replace the board.

Applies to: ICP N4H3 R-410A split heat pump (manual 428 01 5103 02); Comfort Alert codes apply only to units factory-fitted with the Comfort Alert module

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Source: ICP N4H3 Installation Instructions 428 01 5103 02, Defrost System, p.13-14. Written in our own words; check the manual for your exact model.

For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.