Which Valve Do I Open First?

Testo 557 vacuum gauge and Appion core removal tools shown

I've had a change of heart.

Back in the early 2000s, during the big construction boom, I did many system startups on residential units for a large company I worked for.

When installers ran the line sets prior to startup, they weren't always very careful to keep them clean and dry. Many times, we would end up with a restriction in the piston or TXV.

These new residential systems come with a precharged refrigerant in the condenser. So, after my vacuum was complete, I would “release” the charge by slowly opening the liquid line service and watching to see if my suction pressure would steadily rise.

If there were anything in the liquid line, it would hit the screen or drier before the metering device instead of possibly running the other way and clogging the TXV or orifice.

I often knew that there was a restriction before I even started the system because I got used to watching that suction needle rise. While I did this for a good reason, that reason is in the past.

When we install systems, we take great care to make sure the line set stays clean and dry, and we flow nitrogen while brazing with the line drier installed near the indoor coil.

It's a new day, and I'm giving up my old sins.


So, now I must admit that the better way to do it is to open the SUCTION valve first (slowly!). That prevents oil loss out of the compressor into the discharge line and out of the liquid line.

It's unlikely that you will lose enough compressor oil to cause any damage by opening the liquid line slowly, but any oil that the compressor loses has a long journey before it gets back to the compressor. The other issue is that oil loss in those first few moments in the life of a new system can have long-lasting effects on the operation and longevity of that compressor.

Have you ever taken a liquid line hose off after a new system install and gotten oil all over?

That is often due to opening the liquid line first and the compressor losing oil to the discharge line and then to the liquid line.

When you open the suction side slowly first, oil loss from the compressor will enter the suction line. Once the compressor begins running, it will pull that oil back into the compressor.


When doing it this way, you would attach your micron gauge to the liquid line core remover side port with the Schrader in place in the side port. Once you completed your vacuum and proved you had no leaks or moisture by valving off the VCT's and watching your decay rate, you would then attach your gauge manifold and slowly crack the suction side until you see a few psi on the liquid side. Now, remove the vacuum gauge to ensure the system pressure does not damage it.

Most micron gauges can handle some pressure. For example, the Testo 552 can handle up to 72 PSIG (4.96 bar), and many can handle 400 psi (27.57 bar) or more. It never hurts to remove that expensive and sensitive micron gauge before exposing the sensor to high pressure. Still, it's never a good idea to remove it BEFORE the system is under positive pressure. If you do that, you will lose the entire vacuum.

You would then purge your manifold hoses and fully open the suction valve and then the liquid line valve.

When charging a system with no charge (not running), weigh refrigerant into the liquid line first until both sides equalize in pressure. That way, you can ensure that you are not introducing liquid refrigerant right into the compressor crankcase.

Also, keep in mind that it is good practice to run the crankcase heater once the charge has been released and before the system is started to prevent a flooded start on the compressor.

—Bryan

One response to “Which Valve Do I Open First?”

  1. You mentioned leaving your micron gauge on until there was pressure in the lineset so that you don’t lose vacuum but you attach your manifold while the lineset is under vacuum? Why not pressurize the system and then attach your purged manifold?
    when charging a system that has no charge do you attach your purged manifold to the side port on your liquid line core remover side port before you start the vacuum pump?

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