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Defrost Controls

See Also

A heat pump needs to defrost regularly when icing conditions occur. The trick is that the pump must run long enough to rid the unit of ice but not too long, so as to be an energy waster. As such, most heat pumps are equipped with a time-temperature defrost system. The time stops can be placed several intervals from 10 to 90 minutes, depending on the equipment manufacturer. The actual defrost cycle will run between 90 seconds and 3 minutes.

Types Of Defrost Controls

A defrost control device uses a timer--either an electronic version or a clock motor--which includes mechanical stops or electronic jumpers implemented via a microprocessor on a small circuit board. These stops are placed at intervals generally ranging from 30 to 90 minutes. Larger systems that go beyond residential use can be equipped with a pressure switch to detect when the coils of the heat pump have become stopped up with ice. While such a setup can be used on residential units, it's considered uncommon.

Replacing A Defrost Control

The following is taken from instructions to replace an ICM defrost control board with a UTEC defrost control board, part #917178A:

  • Disconnect power to the unit.
  • Remove the control panel cover, revealing the unit's electrical components.
  • Remove all factory and thermostat wiring at the current defrost board.
  • Snip the plastic stand-offs holding the board in place and remove the board.
  • Install the new board in the same location, mounting its plastic stand-offs in the existing holes. Field terminals connecting to the thermostat should be located within the specified low-voltage barrier.
  • Replace all factory wiring to the new board.
  • Connect the wires from the new unit to the appropriate color-coded connect terminals.
  • Connect the stripped portion of the six wires to the thermostat wires and secure in place with wire nuts.
  • Check that all connections are secure.
  • Replace the cover board and return power to the unit.

Who Makes Defrost Controls?

Among those companies making defrost control boards are:

  • Armstrong
  • Comfortmaker
  • Ducane
  • General Electric
  • Lennox
  • Nordyne
  • Rheem/Ruud
  • York
It's advisable to contact the manufacturer of the unit to see if it makes its own specific control board, as many heat pump manufacturers seem to make their own boards as well.

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