Is it too cold to set the charge?

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I’ve heard the phase “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 other thing.

Granted, there are cases where you do actually need to come back, but it 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’s 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 manufacturers specs on how to properly charge? They will have low ambient charging info and much more. Look it up.

#2 – Have to 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 a ton of past articles on charging.

#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 setup 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.

— Bryan

3 comments

  1. Avatar Cliff krug says:

    Sound advice.

  2. Avatar Mark Maloney says:

    On new system installations. The manufacturer usually sets the condenser with enough refrigerant for 15′ if lineset ( check your installation manual to be sure). Then use the described calculations. (.6 oz. per linear foot of line set). Weigh this in. I usually add the additional refrigerant to break the vacuum before opening the isolation valves. If I can’t get it all in beforehand I add with system running. I have used this practice for several years and in many cases the subcool and superheat were spot on. Of course the duct and system were properly sized. Delta T and static PSI always came up within spec.

  3. Avatar byenzer says:

    Here’s an additional charge level validation test. In cool weather, for a minimum duration of 1hours after setting the charge, set tstat to highest heat temperature, then COOK the house HOT. Then activate A/C only. After ONLY 5 MIN MAX A/C run time, quickly check superheat too.

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