Diagnosing Improper Fixed Orifice Sizes

   I have found that this topic often goes unnoticed, or can be a guessing game out there in the field.  Diagnosing improper fixed orifice sizes is actually a fairly simple, cut and dry procedure.  First, I would argue for efficiency reasons, as well as ease of proper charging, you should just field install a TXV.  Of course, when you are on the job site you don't want to spend precious time attempting to adjust refrigerant charge with the incorrect orifice. You would never be able to get the Superheat and Subcooling within proper parameters.  The next best thing to field installing the TXV is actually installing the correct orifice to match the condensing unit - which is why they are shipped accordingly.

  So, what happens if you don't have the correct orifice installed?  Sure, you could attempt to adjust the Superheat to the target, but it will never reach the desired goal.  If you did not know what the target should be, use this simple formula that most to all Superheat Charts are based on, using the Condenser Ambient Temperature and Return Air Wet Bulb (taken at the unit, not in the space).

Target Superheat Formula

Low Superheat & Low Subcooling

If you have an over-sized orifice, you will be overfeeding or flooding the evaporator.  If you did not verify that there was some sort of reasonable subcooling value, than you would expect that the unit was overcharged based on the low superheat.  Taking a look at the first picture of the digital gauges, you can see that the Superheat and Subcooling are both low, indicating that adjusting the charge will not only be impossible, but could cause the refrigerant to flash off prior to the metering device since the subcooling will also go down as refrigerant is removed.  When charging to the proper Superheat, you should have at least 5F, but not more than 25F subcooling with a fixed metering device. This indicates the proper size orifice and that the refrigerant will remain a liquid until meeting the orifice.

High Superheat & High Subcooling

  If you have an undersized orifice, you will be starving the evaporator.  Without looking at the subcooling value, you would believe the unit is undercharged due to the higher than target superheat.  If you can see on the second picture, we have high superheat and high subcooling, noting that the refrigerant charge is not the problem.  This is generally either a restriction, or an undersized orifice which in it's own sense is in fact a restriction.  If you were to start adding refrigerant, the subcooling would continue to rise, with little to no change in the Superheat.  Once the head pressure reaches a point that  the scroll compressor cannot handle, somewhere in the range of 20-30F of subcooling, the head pressure will start fluctuating drastically and you will see 75 - 100# swings in some cases.  Do not mistake this for non-condensables!  This will happen with significantly overcharged TXV's too.

  I guess the moral of the story here is to install the correct orifice for the condenser being installed, this way you can avoid this diagnosis from the start.  As a technician, if it came time to verify the orifice size, I would take no chances and just install a TXV.  I would hate to go through the process of pumping down the system, changing the orifice, vacuuming the system, and trying to adjust the charge just to find out that the new orifice was still an issue.  Plus, by installing a TXV you will for the most part add an additional 1 SEER to the system!

  http://excessair.blogspot.com/2012/05/diagnosing-improper-fixed-ori...

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Comment by Bob Blanchette on June 2, 2012 at 4:41am

To add insult to injury homeowners sometimes don't realize something is wrong until the system has been in for years. Oversizing 50% covers up things like this. Telling a customer who system has been "working fine for years" that it was installed wrong can sometimes be a tough sell to correct a problem like this. Undersized orifices happen when condensers get upsized @ replacement while the original A coil remains. Oversized orifices happen when the A coil has a higher rated capacity than the condenser and the orifice was never changed to the one that came with the condenser...

Comment by David Meiland on June 1, 2012 at 7:45pm

Please keep these posts coming--they are very well written and informative.

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