Charging Air Conditioning Systems

If you're an a/c technician looking for information on charging air conditioning systems,

I'm glad to meet you, and I hope I can provide some guidance that will make your job easier.

If you're installing a new unit, there should be a charging chart in the installation instructions, or somewhere on the condensing unit itself.

The best thing you can do with a new unit or any unit is follow the manufacturer's recommendations.

You'll need a wet bulb thermometer or sling psychrometer.

Get one.

It will help out when you're charging air conditioning systems, and when you're checking cooling capacity on other service calls.

If you're working on a unit with no factory charging chart, use a superheat and subcooling calculator from Carrier or Trane to determine the proper charge for the unit.

With regards to subcooling, most DX a/c systems with TXV's will run with 10 to 15° of subcooling.

If there's no way you'll be able to get your hands on a superheat and subcooling calculator, our System Evaluation Manual has a troubleshooting diagram, and guidelines on the operating characteristics to look for when charging air conditioning systems.

Before charging air conditioning systems, you must be absolutely sure the unit is low on charge.

A bubbling sight glass, frosting coil, frosting metering device, or poor cooling capacity don't necessarily mean that a unit is undercharged.

Check your coils, blowers, and filters for cleanliness and obstructions, and be sure all your components match.

Does your condensing unit match your evaporator?
Is your compressor the right size?
Are your blowers the right horsepower, speed, and rotation?
And are your blower wheels and fan blades the right size and rotation?

If there was a leak, have you found it, repaired it, replaced the filter drier, and vacuumed the system to 500 microns?

If you've made sure the system is ready to be re-charged, the following is what I'd do if I was charging air conditioning systems without a superheat and subcooling calculator, and without a wet bulb thermometer.

If you're working on a window or package unit, weigh in the charge to factory specifications if you can find them, and you're done.

Check the operating characterisitics to be sure they're normal, but as far as charging the system, weighing in the exact charge is quick and easy.

If the unit is in a vacuum, charge liquid into the high side at the receiver king valve, or the high side access valve.

For window type units, and other small appliances, charge in vapor only.

Watch the low side gauge.

If it doesn't rise, stop charging liquid, and charge vapor into the low side until pressure comes up to about 5 psi.

On small R-22 a/c units, 5 tons or below, at this point I'd charge vapor into both sides until pressures equalized between the can and the system, and then run the unit to finish charging.

On R-22 units over 5 tons, at this point with the unit not running, I'd charge in 1/4 pound of liquid, per ton of rated capacity, into the high side of the system.

So if the unit was a 10 ton unit, I'd charge 2 1/2 pounds of liquid into the high side, or less if pressures stabilized and refrigerant stopped going in.

Then I'd run the unit and finish charging.

If the system components all match, and there are no air flow or piping problems, I'd start by looking for the following running conditions:

As long as the compressor amp draw stays normal, and head pressure stays normal, charge in vapor until you get 2° of subcooling.

Let the unit run until the space is within 5° of design temperature, then finish charging.

Charge TXV systems to 10° to 15° of subcooling, then check to make sure superheat is in the 20° to 30° range.

Charge cap tube or orifice systems to 20° to 30° of superheat, then make sure subcooling is in the range of 10° to 15°.

Once again, you might want to take a look at our System Evaluation Manual for more detailed information about the final ranges of pressures and temperatures to look for when troubleshooting and charging air conditioning systems and refrigeration equipment.

I hope this page has helped, and please, feel free to contact us with any specific HVAC questions you might have, including questions about air conditioning on Guam, and refrigeration on Guam.