Non-Bleed TXV & Trapped Nitrogen

The following is based on a true story. No product was harmed in the making of this tech tip, and some facts may have changed to protect the guilty and because I forgot some of them.


We got a job installing a new 1-to-1 split refrigeration case with R448a, and it had a typical thermostatic expansion valve, headmaster, and a suction stop.

We installed it using all the proper procedures we talk about all the time.

Purge with nitrogen, flow nitrogen while brazing, pressurize to the factory mandated low side test pressure, pull a deep vacuum to 250 microns, and then do a decay test. Both sides pulled down nice and deep at the condensing unit and held.

Everything looked beautiful!

That was until we started the unit up.

The head pressure was unusually high, and the sight glass wasn't clearing even though it showed subcooling.

A dead-ringer for non-condensables.

After much consternation and hand-wringing, we recovered the charge, re-evacuated, and recharged with a virgin charge.

Now it ALL WORKED PERFECTLY!

Before I give you the answer, see if you can look at the diagram at the top and figure out what happened.

It's really not that complicated, but it's an easy sort of mistake to make.

When you pressurize a system with a non-bleed (hard shut-off) expansion valve, the valve will go fully shut when the evaporator/suction pressure goes high enough for the external or internal equalizer pressure to overcome the bulb pressure.

You may have noticed this when pressure testing a system where the high side pressure goes up, and the suction goes up with it—then, all of a sudden, the suction stops rising. As you add more pressure to the high side, it will keep going up, but the suction pressure stays the same.

This happens because that valve has shut completely due to the external equalizer force overcoming the bulb pressure.

In this situation, we forgot to force open the suction stop before we vented the nitrogen and performed the vacuum.

This left the pressurized nitrogen trapped between the slammed shut TXV and the suction stop.

Once the system was turned on, the suction stop opened, allowing the nitrogen to mix with the refrigerant that caused the issue.

Preventable issue? Yes.

And hopefully, reading this will prevent you from making a similar mistake.

—Bryan

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