Charging system by superheat/subcooling - http://www.hvacmechanic.com/ Forums

Original message

midwayice

67.85.135.23

"Charging system by superheat/subcooling" , posted Mon 1 Sep 23:16

I am residential oil heat tech learning a/c. I'm certified and insured and I'm practicing on friends, neighbors, and relatives. I want to learn how to charge units properly. I'm reading and trying to put it together.

Lets use my unit at my office:

Weather King (Rheem) Model 10AJA3601 with a TXV

suction pressure - 67
suction temp. - 51

discharge pressure - 195
discharge temp. - 94

outside dry bulb - 82
inside wet bulb - ?

temp. split indoor coil - 18 deg.

Is this enough info to get superheat and subcooling?
According to this ARCO manual I have it is. It says this word for word:

Check superheat: Measure suction-line temperature and pressure. Convert pressure to temperature and subtract from actual suction line temperature. The result is superheat.

Check subcooling: Measure high-side pressure and convert it to temperature. Take the temperature of the liquid-line, and then find the difference between the two temperatures. The result is subcooling.

So I use temp/press. chart to do conversion, (which seems to be built in to my gauge manifold), any way, I come up with:

Superheat - 12 Suc.temp 51 - 39(suc.press 67 convert)=12
Subcooling - 6 (Highside 195 conv.)100 - 94(highside temp)=6

Questions:
Did I do that right?
Is my system charged correctly? What should it be?
Is it true that if unit has TXV, you should charge by subcooling?
Whats the indoor wet bulb for? The hvac guys I talk to don't go inside to take it.
Where can I get more reading info?

Found this site about a month ago, you guys are excellent.

Replies:

TnNATE1412

24.158.140.10

"Re(1):Charging system by superheat/subcooling" , posted Tue 2 Sep 06:31

Yes, you did it right. Keep up the good work and do it RIGHT every time! The indoor WB is used to figure the proper Superheat required. In order to figure required superheat, you need the Outdoor Ambient entering the coil, Return air ambient and Return air Wet Bulb Temp. Normally, if you have a TXV coil, you would charge by subcooling. Some systems, Rheem, Ruud, etc., still use Superheat for their charging whether you have a TXV or not. You have to look at each individual system and follow their directions for charging. Not all systems use the same subcooling values either.
Check out the HVAC News.com for more info.