Eric Mele explains how defrost happens on rack refrigeration systems.

The simplest form of defrost happens when we shut off the refrigeration for a set amount of time. This method works best on medium-temperature refrigeration. We rarely run medium-temp evaporators below freezing, so we can just let air move across the coil and get the ice off.

Electric defrost is also common; we use electric heat outside the coil to melt the ice off in medium-temp or low-temp refrigerators.

We can also use gas to defrost a refrigeration system, which is most common in low-temp refrigerators. The rack in this video uses gas defrost, and it has a solenoid valve for a defrost valve. We can either use hot discharge gas or with warm liquid, which is a Kool gas defrost. In either case, you’re bringing a warm fluid into the coil and melting ice from the inside out.

During a gas defrost, you’ll shut off the suction line to the suction header. Then, you’ll open the solenoid and allow the warm fluid to flow into the suction line. The gas then comes back via the liquid line and into the liquid header of the running rack. You can trace the header back to determine if a rack uses hot gas or Kool gas defrost. In this case, we can tell that this rack uses hot gas defrost because we can trace the header right back to the discharge line. (It ties in right past the oil separator.)

We can create flow back to the liquid header with a differential valve. That differential valve can be on the liquid line for hot gas or Kool gas defrost, and it may be on the discharge line for hot gas defrost only. The valve creates a pressure differential between the receiver side of the system and the liquid header side of the system. So, the liquid header will be at a lower pressure than the receiver, so the discharge line will be at a higher pressure than the liquid header. We can establish flow back from the high side of the system to our not-quite-high side of the system (the liquid header).

Typically, the pressure differential will be around 20 PSI, but that number can vary with customer expectations and manufacturer specs.

The hot gas defrost allows us to get some flow for our defrost while the system is still cooling.

Read all the tech tips, take the quizzes, and find our handy calculators at

Learn more about the 2022 HVACR Training Symposium at

Subscribe to Channel:

This video by HVAC School was liked: 218 times

If you like this video by HVAC School, please support their CHANNEL by clicking on the SOURCE link below and Subscribe.



Featuring Your Videos:

By featuring your videos on our blog, your videos will receive hundreds of views daily from our website visitors, you get a backlink to your channel for followers to subscribe to your channel. This is a win/win for SEO for both of us. If you would like your videos or channel highlighted and premiered on our blog for FREE, please contact us.

However, if you no longer want us to premier your channel, and want us to remove your video and not feature your channel anymore, please contact us.

Video Post Disclaimer:

The information contained in the multimedia content (“Video Content”) or post content in general, represents the views and opinions of the original creators of such Video Content or post, and does not necessarily represent the views or opinions of The mere appearance of Video Content or the post on the Site does not constitute an endorsement by or its affiliates of such Video or Post Content. 

The Video Content or Post has been made available for informational and educational purposes only. does not make any representation or warranties with respect to the accuracy, applicability, fitness, or completeness of the Video or Post Content. does not warrant the performance, effectiveness or applicability of any sites listed or linked to in any Video Content or Post.

Affiliate Disclosure:

This post and description may also contain some affiliate links, which means that the post creator may receive a small commission at no extra cost to you if you click on some of the product links and decide to make a purchase. This channel is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to

Copyright Issue:

If you find any of your copyrighted material in this post or video, please contact us, so we can resolve the issue.

All rights reserved by respective owners.

9 thoughts on “Rack Refrigeration Defrost

  1. HVACR Northeastern says:

    Great explaining l wish l had this about 24 year ago when l did racks Hussmann refrigeration racks had the kool /hot gas had to sit a lot of nights reading all the information about this rack in the machine room when l was there doing service we didn’t have the Internet at that time.

Leave a Reply

Your email address will not be published. Required fields are marked *