I'm new to the forum and it has made me a better tech. I really appreciate the knowledge base. I installed a residential Fedders split system AC with a TXV this week, and found something interesting in the manual when looking for the recommended subcooling value. It read as follows:
"If the measured superheat is less than 8 degrees or greater than 12 degrees an adjustment of the TXV is required. To increase subcooling tighten the adjustment stem and to decrease subcooling back out the adjustment stem".
Firstly, is the use of superheat a misprint and is meant to be subcooling? If so, how does one know to add/remove refrigerant or adjust the TXV to get a proper subcooling reading?
I have replaced but never adjusted a TXV and I want to be as good as I can be. Any insights you all can give will be greatly appreciated.
My vote is for a misprint replacing the "SH" with "SC", but the end result would be the same, subcooling would do exactly what the instructions say....
Originally Posted by genther
Kinda a weird if you think about it..
If a day goes by and you have learned nothing, I hope you got a lot of sleep.
it sounds like that is worded correctly. what they are saying is, with youre temp probe on the suction line, as colse to the outlet of the evap. coil as you can get it what is the temp? if it is between 8-12 degrees of superheat, you are good to go BUT if it is less than 8 or greater than 12 then you need to either open or close the valve a bit. this is done by removeing the nut on the end of the txv WITH 2 WRENCHES (sporlan is 3/4 usually) and adjusting the stem in or out. the rule of thumb that someone told me to get in the ballpark is one complete turn is= 3deg so if you are at 6 deg of sh. then close, or in by 2/3 turn and go from there. hopes this helps? oh, and welecome to the forum
Thanks for your thoughts. So what this means is first set the subcooling by adjusting the refrigerant level, then check the superheat and adjust the TXV? Also, for super heat I have always checked the suction line temp at the condenser, you mention to do it at the evaporator. Have I been doing it wrong all these years?
Originally Posted by jricer2001
Last edited by genther; 05-01-2010 at 09:41 PM.
for the commercial that I do we check superheat at the outlet of the evap coil. you are looking to find out how much extra heat the refrigerant picked up in the evap above and beyond its boiling point. you want to take that measurement as close to the outlet of the evap as possible, but dont clamp youre probe around a braze joint because it takes a little longer to heat and cool the joint than it does the pipe itsself-or at least that is how it was explained to me when I started. subcooling is nice but as long as the charge is correct it is not detremiential to operation it is just "extra" work that the valve can do(ie: instead of haveing a 1 1/4ton txv you may be able to get away with a 1ton if you have good subcooling) superheat is more cruetial because with low SH you may send liquid back to the compressor and slug it, and with to much SH you arnt cooling the compressor enough and could burn up the windings. not to even speak of the loss of potential cooling you could of had if you have a low SH.
it does occure to me that you need to take the SH temp reading as close to the charging port as possible to aviod picking up any heat between the temp probe and the gauges. on commercial equipment we put service taps where we want them but with you you HAVE them at the cond. so that is probbly fine. unless you really want to do it 100% and install one right at the evap.
Using readings at the outdoor unit to adjust the TXV can be problematic. If there is no port at the evaporator then you should compare the actual temperature at the evaporator against the actual temperature at the outdoor unit to see if you're gaining any heat that would skew the calculation. There could also be some pressure drop, and that's more trouble because without a port at the evaporator you can't see how much pressure drop there is. Ideally the pressure drop should equal no more than 1 degree, but 2 degrees is allowable. Without being able to measure the actual drop I would assume a 2 degree drop to be safe. So, if there is no increase from actual heat gain, and assuming a possible 2 degree pressure drop, 10-14 degrees superheat at the outdoor unit would equal 8-12 degrees superheat at the evaporator.
C.C.W opens txv valve -decreases superheat
C.W. close Txv valve - increases superheat
But what angle are you looking at the txv
do you back the adjuster srew out to get C.W. ??????
I think the OP is wondering about the use of the word "subcooling" when it comes to adjusting a TXV. I've always adjusted (the few times I've adjusted one, mind you) a TXV via evaporator suction outlet temp, where each time I was lucky enough to have a schrader port right at the evap.
In general, I would make adjusting the TXV the very LAST thing to do regarding obtaining proper superheat. Most of the time it's a charge issue or other system effect factor that is skewing the superheat off from expected range. Look for those first. Leave the valve alone unless you're reduced to no other factors.
- Electricity makes refrigeration happen.
- Refrigeration makes the HVAC psychrometric process happen.
- HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.
A building is NOT beautiful unless it is also comfortable.
I agree with the above quote.
Originally Posted by shophound
I have only adjusted a few TXVs in 20 years of working in the field.
Waddya mean don't thaw out the frig with a knive?
Looking at the end of the stem. Turn left to decrease and right to increase.
Originally Posted by summertime1004
[QUOTE=VTP99;7904741]Looking at the end of the stem. Turn left to decrease and right to increase.
lossin adjustment stem - opens txv decreases superhear
tighting adjustment stem - closes txv increases superheat