Outdoor Unit Freezing-up

Outdoor Unit Freezing-up

OUTDOOR UNIT FREEZING-UP - The outdoor unit forms an abnormal amount of ice on the outdoor coils that does not go away after an hour.

* It is normal for a heat pump to develop frost on the outdoor coil during cold temperatures.  A normal defrost cycle should take place to thaw the coil.  During defrost mode the outdoor fan motor stops, the reversing valve switches into cooling mode and the compressor continues to operate.  It is normal to see steam coming out of the outdoor unit during a defrost cycle.

POSSIBLE PROBLEM & DESCRIPTION

Ice Covering Top of Unit

  • It is not unusual after freezing rain or snow thawing for water to freeze over the top of the fan discharge of the outdoor unit.  This can either stop the fan from turning or prevent air from discharging from the unit.  In either case, the ice needs to be removed for the unit to function properly.
  • Do not try to remove ice or any other objects from the fan path with power on the unit.  Always pull the service disconnect switch near the unit or turn off the breaker before attempting any repairs.
  • It may be necessary to switch the thermostat to "Em Heat" until the unit thaws or repairs are made.

Failed Outdoor Fan - Qualified Technician Recommended

  • If the outdoor fan motor quits running while a heat pump is operating in the heating mode, the outdoor coils will quickly freeze and a defrost cycle may fail to remove all of the ice.
  • A stopped fan motor can be the result of a bad motor, bad start capacitor (below), or faulty control board or relay.
  • It may be necessary to switch the thermostat to "Em Heat" until the unit thaws or repairs are made.

Bad Capacitor - Qualified Technician Recommended

  • Most direct-drive PSC fan motors require a capacitor which provides extra power for starting.
  • A fan motor with a bad capacitor will typically attempt to start but will fail while making a louder-than-normal humming noise.
  • After several seconds of humming, the motor will typically overheat and shut-off and then retry after about 30-60 seconds of cooling.
  • Sometimes a motor will still be able to start (with difficulty) with a bad capacitor.
  • A fan motor that runs normally after helping to start by turning by hand usually indicates a bad capacitor. 

Low Freon - Qualified Technician Recommended

  • If a heat pump becomes low on refrigerant it may not be able to produce enough heat in a defrost cycle to thaw the outdoor coil.  This would be seen in a heat pump that enters and stays-in defrost mode for about 10 minutes and then exits defrost with a substantial amount of frost or ice still covering the coil.
  • It may be necessary to switch the thermostat to "Em Heat" until the unit thaws or repairs are made.

Bad Defrost Control - Qualified Technician Recommended

  • There are two types of defrost controls - Time & Temperature or Demand Defrost.
  • Time & Temperature controls use a defrost thermostat and a timer board. Time & Temperature boards typically have a setting for 30, 60 or 90 minutes.  When the coil temperature drops below 30 degrees a defrost thermostat should close, starting the timer on the board.  After the selected time period the control board should initiate a defrost cycle.
  • Demand Defrost boards measure the outside air temperature and the coil temperature to determine the optimum conditions for a defrost cycle to take place.

Bad Defrost Sensor/Thermostat - Qualified Technician Recommended

  • The defrost thermostat should "close" when the coil temperature is below 30 degrees.  A bad defrost thermostat will prevent a Time & Temperature defrost control from entering a defrost cycle.
  • Either a bad outdoor sensor or coil sensor can cause a Demand Defrost control to not defrost properly.

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