How does Dometic or Norcold RV Fridge Work?

This video shows how both Dometic & Norcold RV refrigerators work.

How Dometic & Norcold Fridge Works Video

NOTE: Click icon in player to view video full screen.

The How Dometic & Norcold Fridge Works video* teaches why heat is needed to drive the cooling process. This video teaches the fluid flow in the system and why the fluids cannot stop flowing without stopping the refrigeration process and/or damaging the system.

This video was made in cooperation with RV Productions by Bill Gehr. Please use our Contact Us form if you wish to have Bill help you with your RV endeavors.

Dometic or Norcold Cooling Unit Function

Distillation Process

NOTE: Right click the still image and 'Open image in new tab' for clear view.

In a nutshell, Dometic and Norcold fridge are driven by a distillation process, the working fluid (orange) is separated into two fluids. The heat, either electric heating elements or LP gas flame drive the distillation process. The ammonia in the system boils at a lower temperature than the water, thus the heat turns the ammonia into gas and the water remains as a liquid. This is the same way alcohol is made, heat the mash and the alcohol turns to gas and the remaining water is a liquid. Please note that the Fridge Defend monitors the boiler to make sure that this process is functioning as described.

Please click here to view detailed boiler video and why heat can destroy the fridge.

Example of 'How The Process Can Stop and Destroy the Fridge':

One example of how the Fridge Defend protects the cooling unit is freezing weather fridge failures. As viewed in the video, the boiler separates the ammonia from the water in the system. It is important to note that an ammonia water mixture is an antifreeze, that is the mixture of ammonia and water will not freeze. By viewing the video and image above, the Weak Solution is pure water, there is no ammonia mixture. Therefore the Weak Solution will freeze when temperatures drop below 32°F (0°C). The result is disastrous! When the Weak Solution freezes, the ammonia is not absorbed in the Absorber Coil, therefore the ammonia never makes it back to the boiler to drive the process. The result is that the boiler overheats, if and only if you have a Fridge Defend, the heat source is turned off. This in turn will usually thaw out the Weak Solution. Thus, the fridge is protected from failure and will automatically restart when safe. Without the Fridge Defend, the cooling unit can fail, possibly leading to a fire that may be blamed on a heater rather than the fridge.
Click here to see freezing weather fire in Dometic RV Fridge.

Heater Separates Flow

At the top of the boiler pump tube, the weak solution liquid (yellow) flows to the absorber coil. The ammonia gas (purple) rises to the condenser as is represented by the arrows in the image.


Once the ammonia is separated in the boiler, it is cooled again in the condenser. The pure liquid ammonia (blue) is what cools your fridge. Thus, the ammonia is truly the refrigerant.


The evaporator is where the liquid ammonia (blue) will, what else, evaporate :). It is the evaporation of the ammonia that moves the heat from within the refrigerator cabinet to outside the fridge.

Absorber Coil

To complete the process, the ammonia vapor (purple) is reabsorbed in the absorber coil. Please note that ammonia is converted to vapor twice in this process, once by separating it from the working fluid (orange) in the boiler, and once by the evaporation of the ammonia in the evaporator.

Thus, the absorber coil is where the heat from inside of the fridge is sent to the outside. Also, the absorber coil is the completion of the coolant circuit, this is why this is the best place to measure the heat to controls the fans ( Click to go to ARP Fan Controller ).

Holding Tank - Summery

Staring at the absorber coil, the working fluid (orange) is made out of liquid weak solution (yellow) and ammonia gas (purple), the working fluid is stored in the holding tank. A rust preventer must be in both the working fluid and the weak solution. The solutions will rust the cooling unit from the inside without an active rust preventer, the Fridge Defend monitors and protects the rust preventer.

The boiler separates the working fluid into ammonia gas (purple) and weak solution (yellow). The ammonia gas is cooled in the condenser converting the ammonia to liquid (blue) which is in turn send to the evaporator where the fridge is cooled.

Fridge Defend by ARP on the Job

As long as the boiler is not allowed to overheat, the rust preventer will not be destroyed. The Fridge Defend prevents boiler overheating, thus, not much to go wrong with the cooling unit because there are no mechanical parts to break, other than the walls of the cooling unit being destroyed by rust.

Assistant Gas Side-Note

The type of assistant gas is a matter of opinion, thus it is personal choice. We at Fridge Defend have the opinion that hydrogen makes a better assistant gas for 3 reasons:
1) Hydrogen (H2) is a larger molecule than the helium (He) atom. Thus, it does not escape from the cooling unit as easily. The hydrogen gas molecule is approximately twice the size of the helium atom.
2) Hydrogen is approximately half the weight of helium, therefore it generally has a wider range of ambient temperature cooling efficiency.
3) Use of helium does not negate the fire hazard because ammonia is a flammable gas. NFPA 704 flash point is below normal cooling unit boiler temperature if there is a leak .

Questions? Please use Contact Us Form

The RV refrigerator and how it works is simple once understood, but because the process is so dynamic, it can be hard to see this dynamic process in your mind's eye! Please use the Contact Us form to ask any questions, we are here to help!

* The animations on this page are not to scale so as to better clarify the events that occur. The events depicted in these videos are not sequenced as they actually occur, rather they are presented in steps so the viewer can better understand the science behind the method of failure. Most of the events in this video occur simultaneous, this is why the Fridge Defend has a 2 second reaction time to thermal events and approximately +/-2ºC accuracy at typical boiler temperatures.