Heat pumps share many of their components with traditional air conditioning systems. This fact should not be surprising since a heat pump is little more than a reversible air conditioner. Most of the additional electronic and mechanical components on a heat pump exist to control the direction of the system's refrigerant or manage issues that can creep up in heating mode.

The defrost board helps with the latter situation. When heating, your heat pump's outdoor coils will become very cold, allowing water to condense and freeze. The defrost board is a crucial part of the system that removes ice from your coils, and you may notice these three symptoms with a faulty defrost board.

1. Short Cycling

Short cycling may be one of the first indications you notice that something is wrong, especially if you don't frequently go outside to check your outdoor unit. Short cycles occur when the system can't run long enough to meet your thermostat's setpoint and shuts down before fully heating your home. You may notice your system frequently turning on and off.

A defrost problem can cause short cycling because ice accumulation on the outdoor coils will prevent the refrigerant from absorbing enough heat to vaporize before returning to the compressor. This condition will overwork the compressor, causing it to shut down before completing a cycle. If your system short cycles in heating mode, you may want to check for the other two symptoms on this list.

2. Excessive Ice Formation

Ice formation on your outdoor unit is normal. In air conditioning mode, your heat pump moves energy from your indoor coils to your outdoor coils. As a result, the indoor coils become cold, and the outdoor coils become hot. The reverse is true in heating mode. However, your indoor coils should always be above freezing in AC mode. In heating mode, your outdoor coils will typically be much colder.

The coils will cause water to condense and freeze, which reduces the coils' efficiency and eventually causes them to stop working. The defrost board decides when to reverse the heat pump, allowing the coils to heat up and defrost. Defrost mode should engage before the coils become too icy, so outdoor coils completely encased in ice are a common symptom of a faulty defrost board.

3. Stuck Defrost or Aux Heating Mode

Heat pump systems typically include an auxiliary (or aux) heating unit to provide heat when it's too cold outside or when the system switches to defrost mode. Your thermostat should have a light or other indicator to tell you when the system switches to auxiliary heating. When operating normally, the aux indicator will come on briefly when heating as the system switches to defrost mode.

If the aux mode does not turn off, that's a clear sign that something is severely wrong with your system unless the temperatures outside are incredibly cold. Although there are numerous reasons for a stuck aux mode, a faulty defrost board is one potential cause. Whatever the case, contact a heat pump repair service for assistance.