Troubleshooting Defrost Timers

A common problem found when troubleshooting a freezer that uses an electric heater as its supplemental heat source is a frozen evaporator coil. Although there are several possible causes for this issue, one common cause is a faulty defrost timer. Either the defrost timer fails to initiate a defrost cycle or the defrost cycle is terminated too quickly.

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A defrost timer initiates a defrost cycle by an internal time clock: if the timing mechanism fails, a defrost cycle will never be initiated. The defrost timer is also part of the termination controls. If the defrost timer or an associated termination control fails, the system may not stay in defrost long enough to adequately defrost the evaporator.

To test the defrost timer and its associated termination controls, manually advance the timer into defrost. Most electromechanical timers can be advanced into a defrost cycle by rotating a dial on the front of the timer. For digital timers or controllers, follow the instructions by the control manufacturer to initiate a defrost cycle.

DEFROST CYCLE: Most electromechanical timers can be advanced into a defrost cycle by rotating a dial on the front of the timer. (Courtesy of Joe Marchese)

Next, check and see if the defrost cycle has been initiated. Referencing the wiring diagram of the system or of the defrost timer, identify the defrost circuit and place an ammeter on that circuit. The ammeter should measure the amperage of the energized electric heaters. If no amperage or very small amperage value is measured, then the problem may not lie with the defrost timer but with a component of the defrost circuit, such as the electric heaters or safety switch. If a manual defrost cycle has been initiated and the system seems to be defrosting the evaporator normally, then the likely problem is the defrost timer not initiating a timed defrost.

If, when manually advancing the timer into defrost, the defrost cycle is terminated immediately or soon after the cycle is initiated, then the issue may lie with the terminating components. Most mechanical timers will have an internal solenoid that when energized, will switch the system back into its cooling cycle. This internal solenoid is energized by a temperature switch mounted on the evaporator. It is designed to close at a specific temperature (typically around 55°F) to ensure the evaporator is void of ice. It is not uncommon for these temperature switches to be stuck closed and cause the defrost cycle to be terminated too quickly.

A quick test to prove this is the issue is to electrically disconnect the termination switch at the timer and again, manually advance the timer into a defrost cycle. If the system stays in defrost, then the problem lies with the temperature termination switch. Most defrost timers will have a backup time period, which will terminate the defrost cycle by time if the temperature termination switch fails to end the defrost cycle.

It is possible to leave the defrost cycle energized and verify the evaporator is defrosting normally. While monitoring the defrost cycle, time the cycle and see when it terminates. It should terminate based on its backup timer setting. This will also verify if the clock on the defrost timer is or is not working. Do not leave the system unattended during this test, just in case the defrost fails to terminate. If needed, as a temporary fix, leave the temperature switch disconnected until it can be replaced. However, do not leave it disconnected permanently and solely rely on the backup timer to terminate the defrost cycle. This can cause the evaporator to be over defrosted.

Many reach-in freezers use an electronic temperature controller, which will also control the defrost cycle of the freezer. There is no separate defrost timer. They typically control the defrost cycle in the same manner as a separate defrost timer. The cycle is initiated by time and terminated by temperature; however, instead of a temperature switch terminating the defrost cycle, it will use a temperature sensor.

The electronic controller will terminate the defrost cycle at a temperature measured by the temperature sensor attached to the evaporator coil. When these sensors fail, they can also cause the defrost cycle to terminate too quickly. These tend to be a little more difficult to troubleshoot, and it may not be possible to simply disconnect the sensor and test the cycle. Instead, it will be necessary to get into the programming menu of the controller and see the actual temperature of the sensor to verify if that is the issue. Also, some electronic controllers will have a minimum defrost cycle, which means they may not immediately terminate the defrost cycle when manually advanced but will not run long enough to adequately defrost the coil.

If the termination sensor is found to be the issue, adjust the termination value on the controller to a very high value to allow it to stay in defrost until its backup time expires. This will allow technicians to get the system running again until the sensor can be replaced. Again, remember to come back and replace the temperature sensor and not leave it terminating on a timed cycle.

Joe Marchese is an author, instructor and HVACR service contractor. He can be reached at [email protected].