# HGT = 126 + ambient

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• 01-11-2005, 08:21 PM
mdman
I'm not an easy sell for rules-of-thumb, left unqualified they can be conditionally true or an oversimplification of a concept lost or watered down. Anyway let me run this one up the flag pole.

*** HGT = 126 + ambient ***

(1) This a good one?
(2) Can you also say HGT = 126 + RA DB for a Heat Pump in Heat Mode?
(3) I've read of charging by HGT (scary), but even if true it would have to be under some base line, induced ARI, or peek demand conditions. I'm not willing to set or adjust change by HGT alone and then walk w/o looking and SH and SC to complete the picture.
(4) Most importantly, can HGT = 126F + {ambient/RA}DB be used as secondary information to confirm other indications of charge or heat exchange problems?
(5)Anybody know the basis for this rule-o-thumb? Lift? CR? Specific volume? PFM??

TIA,

md
• 01-11-2005, 10:15 PM
tlcartman
a properly charged and operating compressor adds 126° of heat.
• 01-11-2005, 11:07 PM
Quote:

Originally posted by tlcartman
a properly charged and operating compressor adds 126° of heat.
If a properly charged & operating compressor adds 126º of heat, should we not be measuring the temperature of the compressor suction line & adding that figure to the 126 instead of adding ambient? Not trying to be argumentative, just trying to learn more about heat pumps since I seem to be stuck working on the @#\$%^&* things lately.
• 01-11-2005, 11:25 PM
texan30
I think one of the real experts needs to jump in here and discuss the terms 'heat' and 'temperature'. I think some confusion is brewing over the use of these terms.
• 01-11-2005, 11:49 PM
mdman
gona catch hell for this
Thank you, that would work for me.
and don't water it down to were it makes no sense. LOL

Is it to be found in (PV/T)=(PV/T), the combined gas law.

If so,

T2 = T1(P2/P1)(V2/V1), but V.E. is never 100% so,

T2 = T1(P2/P1)(V2/V1)(V.E.) if compression is adiabatic.

Oh, yeah...
T2 - 75F is what you would measure about 6" out says Copeland.

I'll pay around with that and look up some Copeland V.E. charts and see if I can find any loose agreement with HGT = 126+ ambient.

md
• 01-12-2005, 07:23 PM
mdman
bump
• 01-12-2005, 10:27 PM
airworx
if subcooling and superheat are correct then hgt will always be correct. remember subcooling tells you if your system has the proper amount of refrigerant and superheat tells you if there is any problems with your system and how much superheated vapor is returning from evap coil so trying to charge refrigeration systems with your guages only, rules of thumb are basically out of the question with all the differant efficient systems now or any other method is usually not a safe or viable practice.
• 01-12-2005, 10:55 PM
We are not talking about a Rule of Thumb. The hot gas method is the specified method by at least one manufacturer. If other parameters do not fall in line, there is some other problem as well.
• 01-12-2005, 11:59 PM
whitepoundog
I have been using that rule of thumb for years as a quick check and it has never let me down!
• 01-13-2005, 12:29 AM
Keith Lohr
How about suction pressure in the heat mode equal to outdoor temp. for a close charge. Anyone heard of that one before? How about amp draw? Anyone heard of that either ? i have always heard that superheat method is best and proper.
• 01-13-2005, 08:53 PM
r247yan
charging by outside air temp
Quote:

Originally posted by Keith Lohr
How about suction pressure in the heat mode equal to outdoor temp. for a close charge. Anyone heard of that one before? How about amp draw? Anyone heard of that either ? i have always heard that superheat method is best and proper.
I have done that a few times, it was pretty cold and had to use a bucket of hot water to charge unit... Seems to be ok with me, havent heard of any call backs or so... ill have to write down some pressures on my next changeout.
• 01-13-2005, 09:39 PM
More food for thought or fuel for the fire
IF ambient + 126º works for heating, why not cooling?
• 02-12-2005, 10:41 AM
vettech
HGT
Quote:

Originally posted by whitepoundog
I have been using that rule of thumb for years as a quick check and it has never let me down!

Just trying to gather info for using HGT as a diagnostic tool. First off why and how do you use discharge gas temp and what does it tell you? Do you take this reading first or do you measure your subcooling or superheat first? Do you add the OAT or the R/A.
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