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A split system heat pump's defrost sensor is used to detect ice buildup on the heat pump's outdoor coil. When ice builds up on the coil, it reduces the heat pump's ability to transfer heat, which can cause the unit to become less productive. The exact way in which a defrost sensor works can vary (see figure 1.0).

Figure 1.



To defrost the heat pump, the defrost control must accomplish four things:

  • Shift the reversing valve from heating to cooling
  • Turn off the outdoor fan
  • Turn on the auxiliary heat
  • Energize the indoor blower motor

The most frequently used method of determining when to initiate a defrost is based on time and temperature. The defrost sensor is mounted on the tubes of the outdoor unit and it's used in combination with a timer for cumulative heat pump run time. The defrost sensor must be closed. The sensor closes when the outdoor coil temperature is cold enough to frost or freeze. Once closed, the defrost control board begins saving accumulated run time for the compressor.

For example, when the coil temperature is below 30°F ±3°F, the defrost thermostat completes a circuit to the defrost board (the sensor closes the circuit). The defrost board will then start timing. A defrost cycle is initiated when the timer reaches the elapsed time programmed by the jumper. However, if the defrost thermostat opens before the timing period is reached, the timer is reset. The system can operate for 30, 60, or 90 minutes with the coil temperature below the defrost thermostat set-point before initiating a defrost cycle. If the defrost thermostat doesn't open, the defrost cycle is terminated after approximately 10 minutes (unless the OEM specifics something different). See figure 2.0 for the timer terminals.

Figure 2.


Defrost Sensor:

Let's go through an example with a defrost sensor. In figure 3.0, this sensor will open on temperature rise and is a single pole and single throw switch. Also, there is a stamp on the sensor (L80-30). That means it's a limit switch; it opens at 80°F and closes at 30°F. So anytime this outdoor coil is 30°F or below, it will close the connections.

Figure 3.

When heat pumps run in the heating mode, the outdoor coil is about 10°F colder than the ambient air. So, for example, a heat pump that runs at 40 degrees outside will have a much colder outdoor coil. This is to absorb heat from the outside air into the refrigerant and move it to the inside.


Defrost Diagnostic: Control Board:

Let's move on to the defrost sensor diagnostic. There are a few ways to diagnose if the defrost sensor is faulty. If you suspect the defrost sensor is the culprit, there are a few things to test. First, let's ensure the defrost control board works with the defrost sensor. With the power on but not calling, remove the defrost sensor's spade connectors attached to the defrost control board. Set your meter to read volts DC and place one probe on the R (the power coming out of the control board) and the other probe on DFT (keep in each manufacturer may be different). A common reading is anywhere from 5 to 6 volts DC coming out of the defrost control board on the R terminal and into the DFT terminal on the defrost control board. (Figure 4.0). If the OEM has a particular range, always follow the OEMs guidance. For this example, we are reading 5.2 volts DC. If there were no voltage from R to DFT, the defrost control board has likely failed, or another underlying problem exists.

Figure 4.

Here is a sample defrost temperature sensor table (Figure 5.0) from an OEM. If you can locate a similar chart on the particular equipment you are working on, follow that chart.

Figure 5.


Defrost Diagnostic: Defrost Sensor:

Next, we will check if the defrost sensor is closing when it should close. Using the specs from this defrost sensor, we know the defrost sensor is supposed to close at 30°F. Let's say the outdoor ambient temperature is 41°F; you will need to lower the defrost sensor's temperature by placing it in cold water. We are trying to test whether the defrost sensor will open when it is supposed to open. In Figure 6.0, we see the defrost sensor in cold water. In figure 6.0, you will notice that the sensor has closed in the cold water, giving a resistance of .6 ohms.

Figure 6.

Moving on, we will check and see if the defrost sensor is opening when it is supposed to open. The concept is similar to putting it in cold water, but this time you will need to heat the defrost sensor with a heat gun, hot water, or some other method. As a caution, I do not recommend using your Oxy-Acetylene torches as this may damage the sensor. You can place the defrost sensor in a glass of hot water to check if the sensor will open. (Figure 7.0). When the defrost sensor opens, you will read 0.L on your meter, letting you know the defrost sensor is open. When the defrost sensor opens, the heat pump should return to its regular operation in heating mode.

Figure 7.

If you discover that the defrost sensor is receiving the correct DC voltage but is not opening and closing as it should, you will likely have a faulty defrost sensor that should be replaced. When defrosting sensors fail mechanically, they can be stuck open or closed. So first, a complete system evaluation should be performed to ensure there are no other problems and that you have diagnosed the problem correctly the first time.


Defrost Diagnosis:

Since the defrost sensor plays a critical role in heat pump operation in the heating mode, here are a few

symptoms that may indicate the defrost sensor is at fault.

The symptoms of a heat pump with a defective defrost sensor can include the following:

  • Ice buildup on the outdoor coil.
  • The outdoor fan may run constantly or intermittently.
  • The heat pump may take longer than usual to defrost.
  • The heat pump may not reach the desired temperature.
  • The outside unit may make loud noises when running.
  • The heat pump may turn on and off frequently.
  • The heat pump runs continuously without providing any heat.



Let's wrap up with a few reminders. The defrost sensor should be open during regular heat pump operation in the heating mode unless the coil is iced or frosted. The defrost sensor should be closed when the heat pump goes into defrost. This will drop out the condenser fan motor and shift the reversing valve into cooling; the blower motor should be operational and turn on the auxiliary heat strips inside to help maintain the temperature while the heat pump is defrosting. Unless the OEM says something different, look for 5-6 volts DC to the defrost sensor from the heat pump's defrost control board.

Take time to diagnose the problem correctly. Spending this time and effort up-front can help reduce callbacks, reduce your company's liability, and increase revenue by completing a system evaluation and providing repair options that will resolve the issue, not simply a band-aid fix. It's a no-brainer! Some technicians are intimidated by heat pumps, but it is critical to understand them to diagnose them accurately and thoroughly.