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How to Checkout a TXV

When diagnosing a suspected failure of a TXV a technician often overlooks or does not
understand some important key information which is critical to making the right call,
whether a valve is “BAD” or not.

 

Airflow! Airflow plays a big part in our discussion. It is first on our list of things to
check. We cannot accurately check our charge without knowing what our airflow is.
Superheat! We are all use too checking superheat to determine system charge, but
with a TXV superheat is preset by the valve. All we are doing by checking the
superheat level is verifying that the valve is maintaining that preset level.
Subcooling! Too many times a technician never checks the subcooling level or does
not understand the data he has collected. Subcooling tells us the level of liquid
refrigerant waiting to enter the TXV. This is the number which we need to check to
verify the level of refrigerant.

 

So now that we have our airflow, superheat and subcooling numbers we need to check
these numbers against what the manufacturer requires to see what the diagnosis would
be. The interaction between these three numbers and our pressures is how we will be
able to tell what is happening throughout the whole system, not just one section.

 

We must take a Holistic approach to our system.
So let’s look at a typical 3 ton 13SEER A/C system to see what we should have vs. what
we really have.
  • We should have approx 1200cfm (400cfm/ ton). This is a measured number not
    just a guess, (Just because I have a variable speed air handler that it is set for
    1200cfm doesn’t mean I am getting it.)
  • We should have approx 10°-20° of superheat. This number will vary with the
    “Load” the evap is under.
  • We should have approx 10°-12° of liquid subcooling depending on length & lift of
    our lineset. The slightly higher subcooling level will compensate for lift
Now that we have our “what it should be” numbers lets look at “what we actually have”
numbers.
  • High Suction Superheat, Low Liquid Subcooling = UNDERCHARGE
  • Low Suction Superheat, High Liquid Subcooling = OVERCHARGE
  • High Suction Superheat, High Liquid Subcooling = RESTRICTION
  • Poor airflow will look an UNDERCHARGE, but you will have Low Suction
    Superheat & Low Suction Pressure
  • An OVERCHARGE will have high amp draw & if it is not too grossly overcharged
    have a normal suction pressure & suction superheat, but will have high Liquid
    Pressure & High Liquid Subcooling.
If you suspect a defective TXV then remove the bulb and warm it up. This will show us if
the valve is responding to changes in temperature like when it is attached to the suction
line.