Tag: defrost termination

As an A/C tech I can sometimes get the terms “defrost termination” and “defrost fail safe” mixed up because they sound pretty similar. Before we cover these terms lets set the basic defrost groundwork for refrigeration (coolers and freezers)

Defrost is accomplished in one of a few ways, these first two only apply to “coolers” where the box, air and product temperatures are above freezing but the coil temperature drops below freezing –

Off Cycle Defrost – In Medium temperature applications where the box air temperature is above freezing there is often no set defrost and instead the coil defrosts when the system naturally cycles off. This relies on appropriate over-sizing and can lead to issues when the heat or moisture load is high, especially when the door are opened a lot for loading and unloading.

Timed Defrost – In medium temperature you can use a defrost timer to simply pump down or cycle off the compressor at particular times while keeping the evaporator fan running to force a defrost  few times per day.

Next we have the methods used for defrosting low temperature applications which are below freezing and generally 0ºF to -10ºF depending on what is being stored.

Electric Heat Defrost – On a set schedule (time) the compressor is pumped down or cycled, the evaporator fan is (generally) shut off and the electric heaters are turned on inside the evaporator coil.

Hot Gas Defrost – On a set schedule the evaporator fan is shut off and hot discharge gas is pumped through the evaporator coil.

In both of these situations the goal is to get the ice off the coils as quickly as possible but to stop the defrost cycle as soon as the coil is ice free but no sooner. We don’t want to terminate or stop the defrost too early and leave ice but we also don’t want to keep adding heat to the coil for no reason.

Defrost Termination

This is where defrost termination and fail safe comes in. The evaporator coil cannot go above 32ºF so long as there is still ice in that area, so it stands to reason that if heaters are running on the coil and the coil is still at 32ºF or lower then there is still ice. A defrost termination thermostat is mounted onto the coil to detect when the coil is free of ice and will often be set to “terminate” or stop the defrost heat when the coil reaches around 55ºF – 60ºF to ensure the entire coil is ice free.

So the defrost starts on a scheduled time of 2 – 6 times per 24 hour period and terminates once the coil defrost termination thermostat ends the defrost.

It is also common for defrost cycles to have a “drip” time once defrost end to allow water to drip off the coil after defrost and then a fan delay once the refrigeration begins again to prevent the fan from blowing water off of the coil into the box. This is often set to 30º or lower before the fan can come back on.


Fail Safe Time 

There needs a to be a time limit to how long a defrost can go before it goes back into refrigeration to prevent catastrophic product loss in the case of defrost termination failure. This is part of the defrost clock and is often called the fail safe or fail safe time.

The fail safe time can be a wide range of times depending on the application and frequency of defrost but 20 to 40 minutes is common. If your fail safe time is 30 minutes this means that once a defrost cycle begins the LONGEST it will remain in defrost in 30 minutes regardless of the defrost termination thermostat.

Demand Defrost

This strategy is of using time and temperature for defrost is still the most common found in the trade. There is a more advanced strategy called demand defrost that only initiates defrost when sensors predict that defrost  is required. This is often done via trend analysis between sensors to “learn” when ice is present and when it is fully defrosted and will require some manufacturer specific understanding of the particular controls scheme.


Regardless of the strategy the goal is the same

  1. Defrost when needed to keep heavy ice buildup off the coil
  2. Stop the defrost cycle as soon as the ice is gone
  3. Don’t blow water off the coil into the box / case by starting the fans too soon
  4. Use strategies that don’t cause catastrophic product loss if a sensor fails

From a technicians standpoint its important that you fully understand the defrost strategy being used and that you fully test the defrost cycle after you make any changes.

— Bryan




The most common method to defrost appropriately in refrigeration involves both time for initiation and a combination of time and temperature for defrost termination (ending defrost).

But why can’t we just use temperature or time alone? (you may wonder).

Imagine a common freezer with a designed box temperature of -10°F and a coil temperature of -20°F. Periodically the evaporator coil will need to defrost and the amount depends on how much moisture is brought into the box from opening doors and new product being added.

How could we possibly tell when the coil needs to defrost by temperature alone? The coil is already 52°F BELOW the freezing temperature at design conditions. Whether the coil has a lot of ice on it or very little it will still be cold enough for ice to form so using the temperature of the coil alone is a poor indicator of when a defrost is needed (initiation of defrost).

So what if we used time alone? It is common to setup freezers for 2 (dry) to 6 (Humid) defrosts per 24hr period depending on the climate and how they are being used. You would then setup a length of time for the defrost as a best guess with too long wasting energy and warming the product or too short resulting in incomplete defrost and gradually freezing the coil.

With this time only strategy you are left guessing and in order to prevent progressive freeze ups you will always need to defrost a little too long.

So instead we use a combination of time to set the number of defrost occurrences per 24hr period. We use time to make a solid at how long it will take to reliably defrost the evaporator coil and then we use temperature to terminate (stop) the defrost once the coil warms up enough that we are sure it is no longer ice bound.

This combination of time and temperature for defrost termination also gives us a bit of a backup plan. If the temperature sensor or thermostat isn’t working correctly it will still go back to refrigerating once the time is complete. If the time is set way too long for defrost the temperature termination will stop the defrost and send it into refrigeration mode before the product gets too warm.

This is why it is common to initiate defrost using time and terminate defrost using temperature and time.

You can think of it like a irrigation timer that uses a time schedule to water the lawn but it can ALSO use a rain sensor to prevent it from watering when it just rained.

This same strategy of time and temperature is also used in many comfort heat pumps to defrost the outdoor coil in heat mode.

— Bryan

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