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10 Steps to a Perfect Refrigerant Charge

Ben Waller Frigoboat MS160 marine refrigeration cabinet

NOTE: This refers to small, capillary tube (cap tube) systems only.

1. Do not have someone attempt to assess refrigerant charge level by pressures

Although the suction and discharge pressures will give a savvy technician a rough idea of what’s going on, more importantly it will give them an indication if anything is amiss. Refrigeration parameters are very dynamic, and the pressures, temperatures, and current draw are changing constantly, second by second. Oh, if only it were a simple matter of pumping these things up to certain pressure and Bob’s your uncle, but it just ain’t so.

2. Do not have someone attempt to asses refrigerant charge level by weight.

These small systems contain a ridiculously small amount of refrigerant, and it is impossible to weigh in refrigerant accurately, especially if the boat is rocking. Moreover, the correct weight of refrigerant charge will vary from system to system and will be dependent on what components are installed and the application, so the weight of refrigerant charge is unknown and is not published for any of our systems, unlike residential appliances. Oh, if only it were a simple matter of dumping in a certain weight of refrigerant and all’s peachy keen, but it just ain’t so.

3. Do not allow any additives, like leak locating dye, “conditioner”, extra oil, etc. to be added to the system.

I know it’s very tempting to pick up a can of R134a refrigerant at the local auto store with one or more of those additives, but just keep in mind that they are intended for use in automobile air conditioning, not marine refrigeration. In auto air conditioning, the mass flow around the system is like a hurricane compared to the drip, drip, drip of a low temperature refrigeration system. In the auto AC system, those additives will be whipped up together with the refrigerant, but in a boat refrigeration system, they will separate and settle in the lowest point of the system where they can clog things up. Make sure no one puts anything but pure, unadulterated refrigerant R134a in these systems.

4. Do not try charging a system without the compressor functioning normally.

You can’t charge a refrigeration system or determine the refrigerant charge level without the system being operational. The static pressure in the system, i.e. without compressor running, will be determined by the average temperature, and it will be at the same pressure whether the system is almost empty or almost completely flooded with liquid refrigerant.

5. Do not add any additional pipe insulation to that supplied with the system.

On our Frigoboat systems we supply only about 1m of pipe insulation, and that should be positioned starting where the tubing exits the insulated box. This is installed only to prevent nuisance condensation from a cold copper pipe entering a potentially warm, humid atmosphere. Adding more insulation will only make the system less efficient, and will mask the effects of the system being overcharged with refrigerant.

6. Do not rush.

These systems take a long time to settle down after making any adjustments to the refrigerant charge. You should wait 20 minutes or so before determining the next step. Relax, take it easy, read a book or magazine, go on Facebook, call your chums, etc. Yes, being a refrigeration technician can be a truly fulfilling occupation.

7. Do not allow any air to get into the system.

If you’ve had no training on refrigeration system, you should not be messing about with them. There are dangerous high pressures involved, and having liquid refrigerant boil on your skin or eyeballs at -23°F is no laughing matter. Having a can of the stuff explode in your lap is no joke either. The least damage you can do is introduce air into the system through mishandling of refrigeration gauges, as that will cause mayhem for someone coming in to clear up the mess.

8. Do not get bamboozled by a technician with fancy digital gauges.

refrigerant gauge PC44595 lgHave you seen these things? They look like a gaming console or something out of a sci-fi movie. I guess that amongst all those digits there is something relevant to our itsy-bitsy refrigeration systems, but a simple two-dial gauge-set is really all that’s normally required, and then more for troubleshooting than for charging. They are rather impressive though…

9. Do not neglect to use your senses.

One of the intriguing things about refrigeration is that you can’t actually see what’s going on inside the system, but you can see, feel, and hear the effects. You should be able to determine the refrigerant charge level in a working system by seeing the extent of the frost coverage of the evaporator, feeling the temperature of the various components, and listening to the sound of the refrigerant boiling away in the evaporator.

10. Do, Do, Do use the Frost-Line method to determine the correct refrigerant charge level.

Yes, yes, yes. This is the only method you should use to charge a capillary tube (cap tube) system. Watch our YouTube  demonstration. There is a finite amount of refrigerant required for a perfect charge, and anything else will result in an over-charged or under-charged system. If there is enough liquid refrigerant in the system so that it boils away merrily all through the evaporator but has all evaporated back to gas right at the exit tube, you can’t get better than that no matter what the pressures. Where the last drop of liquid refrigerant boils away back to gas will be marked by a distinct demarcation line between frost and no frost, and that’s the magical frost-line that is our friend and guide.

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Phone: (301) 352-5738
Office | Warehouse:
4831 Tesla Dr., Suite H
Bowie, Maryland 20715
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Coastal Climate Control
Cooling, Monitoring & Solar Solutions


Office | Warehouse:
Coastal Climate Control
4831 Tesla Drive
Suite H
Bowie, Maryland 20715
Phone: (301) 352-5738

Request Information
Click here for directions.