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.

—Bryan

Related Tech Tips

What does "Saturated State" mean for Techs?
As we often do in these tech tips, we will start with the common and more practical explanation of saturation and then move to the more technical and nerdy explanation later. When we say “at saturation” or “saturated” in the HVAC/R trade, we are generally referring to the refrigerant that is in the process of […]
Read more
4-20ma Control Basics
Back in the “good old days,” controls were all analog and mechanical, which simply means that they acted in a directly connected and variable manner based on a change in force. Both pneumatic (air pressure) or hydraulic (fluid pressure) systems are examples of mechanical or analog controls. When the pressure increased or decreased on a […]
Read more
Start Flowing Nitrogen Sooner
My technician (and brother-in-law), Bert, made a good point today. (It's hard for me to admit it, but it's true.) When he needs to open the refrigerant circuit to make a repair, regardless of whether he is recovering or pumping down, he pulls out his nitrogen tank and his regulator. (We like the VN500 shown […]
Read more

3 responses to “Is it too cold to set the charge?”

  1. 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.

  2. 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.

Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.

loading

To continue you need to agree to our terms.

The HVAC School site, podcast and daily tech tips
Made possible by Generous support from