Tech Tip: Defrosting During The Cold & Understanding The Defrost Cycle

In the depth of winter, the weather outside is frightful and the heat pump is “chugging” away to keep the inside delightful. Without our trusty defrost cycle in these conditions our heat pump would quickly begin to look like Frosty the Snowman…minus the jolly part. There’s nothing jolly about a broken heat pump.

The defrost cycle is what allows for our heat pumps to continue efficient heating operation in cold weather. However, the actual operation of the defrost cycle is often not understood as well as desired. This leads to improper settings as well as misdiagnosis. Let’s dig into the basic concept of a heat pump operating in heat and the defrost cycle that protects it.

Understanding The Cycle

During the heating mode, refrigerant leaves the compressor as a superheated hot gas. It reaches the reversing valve which is de-energized, and it flows directly up our “vapor” line to the indoor coil. As the hot gas refrigerant flows through the indoor coil it is transferring heat from the hot gas refrigerant to the air running across the coil. This heat transfer provides heat to the conditioned space while at the same time allowing our refrigerant to cool.

This cooling of the refrigerant condenses the refrigerant from a hot gas to a liquid state. The liquid refrigerant makes its way back to the outdoor coil as a high-pressure liquid on the “liquid” refrigerant line. The refrigerant enters the outdoor unit and is metered through a metering device, the reduces the refrigerant pressure dramatically and causes the refrigerant to begin boiling off from a high-pressure liquid to a low-pressure vapor.

This also makes the outdoor coil very cold. During this refrigerant cycle, the outdoor coil will continually be cold. Depending on the ambient temperature and the humidity level there may be some frost that will begin to form on the outdoor coil. As frost forms, it blocks air from flowing across the outdoor coil which in turn makes the coil even colder and creates more frost. This is normal, and therefore we must have a way to defrost the outdoor coil. Otherwise, the refrigerant cycle will become ineffective at generating energy to use as heat.

Understanding The Defrost Cycle In Standard Equipment

Defrost on Amana equipment is initiated by time and temperature. To begin with, we look at temperature. There is a defrost thermostat or sensor (depending on the model) attached to the lower circuits of the outdoor coil. This thermostat or sensor is used to trigger a signal to the defrost control when the coil is close to freezing temperatures.

  • Thermostats, which are used on single-stage models will be normally open and close on a temperature drop.
  • Sensors, which are used in two-stage models provide a resistance reading back to the main control board — indicating the coil temperature.

In either scenario, once the sensor has signaled that the coil is at freezing temperature, then the timer begins. Timer periods may be 30 min | 60 min | 90 min | 120 min depending on the model. This timer setting is to be chosen by the installing contractor. It is important to choose a setting that does not create defrosting too often, which will reduce heating efficiency, but we also must make sure we do not postpone defrost too long or the system may be incapable of fully thawing. In many areas, 60 minutes is a great general setting as it allows for a happy medium.

When the defrost timer has reached its set time, the control board will look to see that either the thermostat is still showing closed or the sensor is still reading a freezing temperature. If this is still so then the unit will switch into defrost – and, if not, then the timer will reset and begin again once it receives a starting signal.

Defrosting Begins

If defrost is initiated, the first step is the reversing valve will be energized allowing for the unit to go back into a traditional cooling mode refrigerant cycle. The outdoor fan motor and compressor will be disengaged. After a 30-second time delay, the compressor only will cycle on while the outdoor fan motor maintains no operation. We are now essentially in the cooling mode where our hot gas refrigerant is running through the outdoor coil and the fan motor is not circulating outside to allow for the heat created to thaw the outdoor coil faster. We will stay in this mode until one of two things happen:

1). The defrost sensor or thermostat indicates the outdoor coil has reached a factory specified temperature (around 60*F).

Or

2). We achieve the maximum allowable defrost time of 12 minutes.

Whichever of the above occurs first will bring the unit out of defrost mode. The fan motor will come on, the reversing valve will de-energize back into the heating mode, and the system will begin normal heating operation. At this point, if everything has occurred properly then the outdoor coil should be free of any ice. During defrost, the control board outside will energize the W2 terminal which if properly wired will send voltage to bring the heat strips on, this is done to prohibit cool air from circulating through the ductwork during defrost.

What If Defrost Won’t Cycle?

Common issues that can prevent defrost from cycling include

  • an issue with the defrost thermostat/sensor,
  • the defrost board lacking 24-volts on the “R” input (this is not needed for actual system operation …. Just for the low voltage defrost relay),
  • or a faulty defrost control board.

These items should always be closely examined if a defrost cycle does appear to be functioning correctly. Testing the defrost cycle is a great first step toward examining its function. To initiate a test defrost cycle on a single-stage unit we will first begin by:

  • placing a field “jumper” across the R | R-DFT terminals on the defrost control board (this provides a signal for a closed defrost thermostat),
  • then, we will remove the factory installed jumper from our 30 | 60 | 90 timer pins,
  • and lastly, we will place a “jumper” across the test pins just long enough to begin defrost initiation.

Once defrost has begun the cycle will continue:

  • until we remove our field “jumper” from the R | R-DFT terminals
  • or until the 12-minute maximum defrost cycle time is met.

To initiate defrost on a two-stage product we will merely hold the “Test” and “Recall” buttons down simultaneously on the outdoor unit UCB. The defrost cycle will initiate and continue until either the outdoor coil sensor reaches the satisfying temperature or the 12-minute maximum defrost cycle time is met.

The defrost cycle is merely a sequence of inputs that will result in defrost initiation if they are properly received. The cycle itself is vital to the proper operation of a heat pump in the coldest parts of the winter season. It will greatly benefit everyone to be familiar with how the cycle works and how to properly test it. No good ever comes from a frozen heat pump!