TXV Hunting Season

     If you didn't call out sick today from Spring Fever, you may be working on some air-conditioners sooner than later!  It looks like it may actually reach 70F in New England and Service Managers will be looking to push the envelope when it comes to Maintenance.  I used to try and fit in some early season cleanings and always had trouble in the mornings.  You see, Thermostatic Expansion Valve (TXV) Hunting Season just opened and it is everyone's best guess if these units will work correctly come July.
     I found out some interesting information about R-410A TXV's a few days back, and as always I am willing to share.  When there was the big switch to 13 SEER and R-410A a few years ago, manufacturers actually adapted their TXV technology to work for the new pressure, rather than redesign their valves.  Danfoss claims, based on independent studies, a TXV will save 15-26% when replacing a fixed metering device.  When this changeover happened, hundreds of thousands of non-optimized valves were installed.  These systems work just swell under a load, but when we are completing maintenance under less than desirable conditions, the TXV's tend to "hunt" and never let the air-conditioner reach a steady state.  This can create some serious issues if liquid makes it back to the compressor, and I am sure we have all heard that wonderful sound on more than one occasion.  Anyhow, if the valve does close entirely under low load, the system operates like a fixed metering device is installed - which we know are very load dependent.  Emerson actually figured this out quite quickly, and in 2008 began to manufacture R-410A optimized valves called their C-Series.  This valve made the unit reach steady state under low loads (little to no hunting), and actually requires less refrigerant. In turn, this lets OEM's create smaller coils.  This valve actually made systems operate .5 SEER higher than ones without optimized TXV's.  The biggest influence is early season maintenance and installations since technicians can actually get the refrigerant charge correct.  When charging under low load, it is very simple for a technician to overcharge a system.  The other day I was on a job site with a valve like this, and it was only 64F outdoors.  At first, the TXV was hunting and we figured it was due to the low load, remember: TXV hunting season!  When I saw the valve, I immediately recommended that the technician adjust and tighten the sensing bulb.  Sure enough, the Superheat pegged at 8F, and now the subcooling was around 22F!  This was actually due to a short line set, he knew better than to charge under such low load - even though he could have with these new valves.
     There was a study completed two years ago by some physical engineers at Purdue that quantified a few points for undercharged units.  Apparently, SEER and COP (for heat pumps) are not decreased until the units reach 70% or less of their recommended charge (Kim, Braun, Purdue University, 2010).  This is the point when the TXV actually shuts down and acts like a fixed orifice.  These units that reach 25% undercharged see a decrease in SEER by 16%, and on average will use $100 per ton, per year more than a correctly charged unit.  But be careful here, don't overcharge a system and create problems come July.  Verify the TXV bulb is tight, to the point you cannot move it by hand.  Also, the bulb needs to be correctly installed and insulated, even if installed at the factory. Just because old TXV's used to hunt under low loads doesn't mean all of them will.  Take the time to correctly diagnose the TXV operation before you add or remove refrigerant!