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Is it too cold to set the charge?
I've heard the phrase, “It's too cold to set the charge!” for as long as I've been in the trade.
“We need to come back and set the charge,” or we need to come back to do XYZ [some other thing].
Granted, there are cases where you do actually need to come back, but in my experience, a lot of this is just punting the ball to the next tech. Admittedly, I'm in Florida, so if you live in the great white north, you will likely be doing your A/C startups in the spring. Understandable.
So, here are the next questions you need to be able to answer if you are going to say you “can't set the charge:”
#1 – Have you read the manufacturer's specs on how to charge properly? They will have low ambient charging info and much more. Look it up.
#2 – Have you taken suction, head, subcool, superheat, delta T, and static? If not, you haven't done your full due diligence.
#3 – For systems with no data, do you have a good feel for the common rules of thumb related to charging? If not, you are in the right place; we have several articles on charging. (You can read an article on refrigerant charging basics here, one on checking the charge on a heat pump in winter here, and one on charging a sealed system here. Those are just the tip of the iceberg.)
#4 – Drive up the condensing temperature (on TXV and EEV systems) and check the subcool.
One of the best ways to drive up head pressure in a controlled manner so that it can stabilize is the Fieldpiece charging jacket. You can control the top opening size to drive up the liquid pressure until you get to a pressure higher than the minimum pressure difference across the valve. I will often use a 100° condensing temperature as a rule of thumb if the manufacturer doesn't give a guideline, though many will use 110°.
Return trips leave the system running improperly, waste money, and annoy your co-workers.
Sometimes you have no choice but to set up a return trip for a warmer day, but any job you can finish the first time is time and money saved for you and the customer.