Is there a way to find out if a split system has a TXV or piston metering device without having to crawl in the attic and take the evaporator apart just to find out? It's such a pain having to take apart the panels just to look. The panels never come back together the right way.
Finding out if a unit is a TXV or piston without having to crawl in the attic
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Pcmaker
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Is there a way to find out if a split system has a TXV or piston metering device without having to crawl in the attic and take the evaporator apart just to find out? It's such a pain having to take apart the panels just to look. The panels never come back together the right way.
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Model number and brand of Air Handler if it’s available typically will say. If you have it look it up, if it’s a Goodman such as ARUF those are pistons, but a field installed TEV can be mounted just outside of the Air Handler or coil.
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3,454 Posts
Maybe, run the unit for a few minutes, shut down and watch the hi/lo press.
I can generally tell whether it’s a txv or piston just by watching gauges and sh/sc for about 10 minutes. Helps if you have real time indoor temperature and humidity so you know what the target superheat is.
Fixed orifice usually gets to its target sh relatively quickly, txv will hunt for a good 5-10 minutes and you can watch sh go up, sc go down then sh go down and sc go down until the bulb temperature and valve stabilizes.
With that said it’s best to confirm, especially if it’s a suspected under/over charged system.
Fixed orifice usually gets to its target sh relatively quickly, txv will hunt for a good 5-10 minutes and you can watch sh go up, sc go down then sh go down and sc go down until the bulb temperature and valve stabilizes.
With that said it’s best to confirm, especially if it’s a suspected under/over charged system.
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on pistons ....when you get your desired SH , do you ever look to see what the SC is just out of curiosity ? or do you rather just get your SH and call it good...
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Should always cross reference readings.
Block the condenser off, sh will drop if fixed orifice .
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That works for me.
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since a piston is wide open .... i assume it equalizes quickly
Yes I always look at sh and sc no matter what type of MD
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I assume a piston is still around 10 SC ?
No, usually anywhere from ~5-25 on a piston depending on indoor/outdoor conditions/system type and brand, assuming it’s the correct piston and correct refrigerant charge.
Measurequick says 3-42°F allowable range but usually, in my experience, if it’s over 20-25 then you’re looking at overcharge or restriction.
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33,565 Posts
Same here.
I rarely see over 25° subcooling unless there is an issue.
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I thought you sent the helper in the attic to verify stuff like that !:cheers:
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I usually can tell simply by watching the low side pressure at start up. Turn a/c off (at disconnect) and wait about 5 minutes, connect low side gauge, plug disconnect back in and watch the low side PRESSURE. Normally on a system with a piston, the low side pressure will drop like a rock below "normal stable psi", and then slowly climb back up to a stable point. Whereas a txv system, the low side pressure will start to drop slower and drop gradually until it gets to a "normal stable psi". Because the sensing bulb of txv is warm at start up and txv is wide open, you can see the low side pressure gradually start to drop and as the bulb starts getting cold, it starts to close slowly which affects the low side pressure slowly.
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12,358 Posts
The piston is at operating opening. Adding refrigerant to an orfice lowers superheat , while adding refrigerant to an expansion valve increases sub cooling.
Been twenty years since I had a set of gauges in my hands.
Take my comments with a grain of salt.
Regards Teddy Bear
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This thread made me think of the system I've worked on.
I've got a small self-contained Refrigeration unit I work on periodically that is fascinating to me. I know for a fact without a doubt it has a capillary tube metering device and that thing hunts like a TXV. I put my probes on graphing and the superheat makes a perfect sine wave and it's always done this since it was brand new.
I've got a small self-contained Refrigeration unit I work on periodically that is fascinating to me. I know for a fact without a doubt it has a capillary tube metering device and that thing hunts like a TXV. I put my probes on graphing and the superheat makes a perfect sine wave and it's always done this since it was brand new.
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16,278 Posts
OP off grid..?
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