charging air conditioning units

[denrayr]

Hey I was wondering what methond you use for charging ac units. I do home appliances and any time we deal with sealed systems we weigh in the charge. I have been reading up on home ac units as I am thinking of branching into that line of work. I havent found a universal method of charging a system. What I have read says to measure evap inlet wet bulb temp and outdoor ambient temp. Then refer to the manufacturers chart to see what your superheat should be. But when you are working on older equipment the chart will most likely be gone. Is their a generic rule of thumb as far as what superheat and subcooling a system should have assuming indoor and outdoor coils are clean?

[dkpd1581]

The only universal method to be dead on is the weighing in of the charge.  There is always super heat, sub cool, and approach methods.  

Depending on the efficiency of the of the unit one can expect approximate condensing temperatures +/- 30 degrees above ambient air temps for standard efficiency and 20 degrees for high efficiency coils.  You can estimate by charging until the temp drop across a coil is +/- 20 degress.

In a worse case scenario you can charge until you are above 32 degrees saturation and below 42-40 degrees in the evaporator.  A sight glass can help to verify that  that there is at least one solid column of liquid to the expansion devise.

If you are dealing with the new ductless mini-splits out, you can all but throw out all of this information if the unit is directly metering from the condensing unit and not the air handler.

I am sure there are other old timer tricks out there; however, in this information age one can always make a call to the manufacturer and have a charging chart emailed.  Each situation is unique in its own right so there are too many vairables to definitively answer your question.

[denrayr]

thanks for the reply. Do you think their is a company that has put together a book that lists the common manufactures and their approach charts? I think that would be a great tool to have for quick reference

[hvacdrd]

There are several slide charts that are available for charging using indoor(return air) wet bulb temps, outdoor air temp and superheat measurements for fixed orfice/restrictors. I've used several (York,Trane, Carrier) and they all cross to very similar numbers.

Any system with a TXV is charged by measuring subcooling somewhat specific by make and manufacture. Most older systems were generally10'F subcooling at full load conditions.

Like dkpd1581 said use the manufacture charts when available.

[denrayr]

thanks for the replies. and the ebay link I really appreciate it.

[applianceman18007260692]

68 PSI on the low side sir and back it up with a clamp on

 make sure amp draw is not exceeded while charging.

nothing has changed