There are many different ways to control defrost but, in light commercial refrigeration applications a defrost control timer is most often used. In low temperature applications where the product is kept below 32°F, heat is needed to make sure the ice is completely removed during the defrost cycle. The standard timer has a timer wheel that represents the 24 hours in a day, it also has 12 threaded holes and 4 matching screws (defrost initiation pins). See below.

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The defrost initiation pins are screwed into the threaded holes that correspond with times the freezer will be defrosted. The defrost is usually initiated 4 times a day, and should be set for times when the freezer is not in use or immediately following normal stocking times. The timer also has an inner wheel that is settable for up to 120 minutes of defrost. This timer is not the primary means of terminating defrost, but acts as a back-up should the primary defrost termination device fail. This back-up defrost termination is usually set for 20 minutes.

*Please refer to your specific timer’s wiring diagram as different manufacturers may use different terminals than this example.

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Most timers have a copper jumper between two of the terminals, in this instance between 2 and 3. This jumper can be discarded but, in most cases, it is not. The timer motor itself is powered by terminals 3-2 and X. There are 2 normally closed (NC) contacts and 1 normally open (NO) contact. The NC contacts are connected to the compressor and the evaporator fans and the NO contacts are connected to the defrost heater.

Finally, there is a defrost termination solenoid connected between terminals 1 and X. This solenoid is the primary means of defrost termination and, when energized, kicks the timer out of defrost. The defrost termination solenoid is controlled by a defrost termination fan delay stat often called a Klixon named after a manufacturer of this stat.

Defrost Termination Fan Delay Stat

Before we can discuss properly operating commercial defrost timers we have to understand the defrost termination fan delay stat. This device has a common lead (usually 120 Volts) and a close on temperature rise lead and a close on temperature fall lead. Let’s break this into its 2 sections:

  • First, we have the close on temperature rise lead. During the defrost cycle, the defrost timer de-energizes the fan(s) and turns on the defrost heaters. As the frost begins to melt and the temperature of the evaporator coil rises to the close on rising setpoint, 120 Volts is applied to the defrost termination solenoid, ending the defrost cycle.
  • As the defrost cycle ends 120 Volts is applied to the fan(s) but the close on temperature fall lead is wired in series with the fans. The fan(s) will not come on until the temperature of the coil falls to the close on fall set point.

If you want to learn more about HVAC and Commercial Defrost Timers check out our online HVAC classes.

by Ron Walker


Ron Walker
Ron Walker

After retiring from the U.S. Marines and achieving his B.S. degree, Ron Walker entered the HVAC field. He has been an HVAC technician, service manager, and business owner. Working as a service manager, he spent many years training HVAC technicians to be more technically competent and really understand their trade. His passion for teaching and helping others resulted in the creation of HVAC Training Solutions, LLC.