How do You Test a TXV?

  Thermostatic Expansion Valves (TEV or TXV), one of the most popular metering devices for residential, high-efficient air conditioning and heat pumps, have performed almost impeccably for decades.  Unfortunately, some manufacturers in the past few years have identified batches of valves that have had high superheat issues.  Have we just been blind for so long, not realizing these problems existed?  Or have these valves really been so reliable for such a long time?
  If you think you may have a TXV issue, there is a very simple test that can be completed using just crushed ice and a set of accurate gauges:

1. With the system operating, attach your accurate/calibrated manifold.
2. Detach your TXV's sensing bulb and submerse completely into crushed ice.  (Caution: not just ice water, must be 32F).  I recommend using an insulated cup!
3. Your Saturation Temperature of the Evaporator should be (32F - TXV Superheat).  Example:  Your R-410A TXV has a desired Superheat of 8F.  32F - 8F = 24F Saturation Temp
4. Using your Pressure/Temperature Saturation Chart, convert Temperature to Pressure for the refrigerant used (Fig. 1).  The Suction pressure should be relatively close to this value.  

Saturation Chart

Sporlan Recommendations, Bulletin 10-9

If outside of the acceptable range (Fig.2), adjustment or replacement is recommended. 
When adjusting TXV Superheat, remember that you make a single turn at a time.  Changes to the TXV Superheat can take as much as 30 minutes of system operation to be measured.



To Reduce Superheat: Turn valve stem COUNTER-CLOCKWISE.
To Increase Superheat: Turn valve stem CLOCKWISE.

  There are many possibilities that could cause high superheat, besides a faulty TXV.  By using this method, you could save yourself some serious service time needlessly replacing a TXV during the busiest time of year!

Please, Make the Transition!

  For as long as I can remember, the agreed epidemic with ductwork was always undersized return ducts.  Although we are far from perfect with duct sizing in New England, I have frequently seen great strides in fixing this issue - particularly with replacement systems.  Lets face it, you should just be properly designing the duct system on new installations.  Undersized ducts cause great restrictions in airlfow, raising static pressure and lowering the cubic feet per minute (CFM).  Or, with ECM motors, raising the amperage draw above full load.  More recently, I continue to find efforts with regards to sizing, but other rules of duct design being ignored.  I am going to concentrate on one particular rule that can have the same affect as undersized ducts:

• On supply and return, when the trunk is wider than the plenum, a transition fitting must be used!

Fig.1  No Transition: Filter Box

   Lack of transitions create turbulence and restrictions in your duct system.  Even if the Return Duct is large enough for your desired CFM, abrupt changes in sizes without a tapered transition raises static pressure drastically above design.  Take Figure 1 for example.  Imagine the force needed to pull the same volume of air through the nice IAQ Filter installed.  At least the entire filter area is being used! I frequently find larger filter boxes than the air handler opening, a waste of filter area - but at least less of a restriction.

Fig. 2 Return Drop

  Figure 2 is an all too common mistake on replacement system, when installing a high performance filter in basement systems.  The new filter box pushes the return drop out of the range of connecting to the trunk, without an offset transition.  Most tin-knockers will do what they can to get the furnace operating.  Following this up by cutting in a grille in the return drop to either fix the undersized ducts, or lack of a transition, is not going to work when it comes to air-conditioning!  The air must come from the conditioned space in order to remove the latent heat, not from a moisture laden basement...

  Can anyone tell me what is going on in Figure 3???  I hope this wasn't you!

Fig. 3 Supply Transition(s)?