I am often asked how much refrigerant should be added to a split system? We preach that the best way to charge a new install is to always WEIGH the charge into the unit.
Well, that varies from manufacturer to manufacturer. There is no standard answer. You need to see what the “operating charge” for the unit is from the manufacturer’s data on the specific piece of equipment. Yes, it means that you actually have to read the instructions.
In those instructions that you are reading, you need to find out what this charge includes. Does it include just the outdoor condensing unit? Does it include the outdoor unit and a matched indoor coil/unit? Does it include the outdoor unit, matched indoor unit and so many feet of line set?
This data is published in every manufacturer’s instruction packages or technical guides.
The only constant is the possible additional charge required for lines over what the manufacturer is providing charge information on. As an example, if the manufacturer’s data says a unit holds 14 pounds 4 ounces of R-410A and it includes a matched indoor unit and say, 25 feet of line set — but you have 63 feet of line set — so what do you do? How much additional refrigerant do I need to weigh into the system to be accurately charged? You use this handy-dandy chart below to calculate the additional refrigerant needed to fill the system by weighing in the proper amount.
You need to find your line sizes, both suction and liquid (and remember — air conditioners are outside the building so all these pipe sizes are OD). You need to find your refrigerant type (and in the example we said R-410A). You need to get your calculator out, subtract 25 feet of line that the manufacturer already allows for from the 63 feet total line set ( 63 -25 = 38 feet). Now you need to know that pipe size so let’s say 1-1/8″ suction line and a 5/8″ liquid line (commercial unit).
Now we look at the chart and see that 1-1/8″ SUCTION lines take 0.20 ounces for R-410A and 5/8 liquid takes 1.64 ounces of R-410A. (Suction lines contain vapor so they never require a lot of additional refrigerant but liquid lines should always be full of liquid so the majority of the additional charge is for the liquid line). Add these two together and multiply the total by 38 feet — the additional line set we need to add for — and you have:
0.20 + 1.64 = 1.84 ounces / ft X 38 feet additional = 69.92 ounces
Then take the 69.92 and divide that by 16 ounces per pound and you need to weigh in 4.37 pounds (or 4 # 5.92 ounces) of additional refrigerant to complete the system charge.
So, to sum it up — weighing in the charge is always the best way to charge any new piece of equipment. To do it right the first time, you need to read the manufacturer’s data on what they provide in the unit and what that charge includes. Then you can used the chart above to weigh in any additional charge that might be needed.
One other thing — if your line set is SHORTER that what the manufacturer accounts for then you can use the chart to REMOVE charge accurately by weight also based on the line sizes you have at the site.