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View Full Version : HGT = 126 + ambient

mdman
01-11-2005, 07:21 PM
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

tlcartman
01-11-2005, 09:15 PM
a properly charged and operating compressor adds 126° of heat.

01-11-2005, 10:07 PM
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.

texan30
01-11-2005, 10:25 PM
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.

mdman
01-11-2005, 10:49 PM
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

mdman
01-12-2005, 06:23 PM
bump

airworx
01-12-2005, 09:27 PM
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, 09:55 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.

whitepoundog
01-12-2005, 10:59 PM
I have been using that rule of thumb for years as a quick check and it has never let me down!

Keith Lohr
01-12-2005, 11:29 PM
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.

r247yan
01-13-2005, 07:53 PM
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, 08:39 PM
IF ambient + 126º works for heating, why not cooling?

vettech
02-12-2005, 09:41 AM
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.

beenthere
02-12-2005, 10:35 AM
Ambient +126 is just one check, you still need to check your coils, air flow.

Comfortmaker use to use ambient + 127.

It works when all other variables are also corrrect.

No one measurement, or check can verify proper operation of any unit.

acdude32
02-12-2005, 12:07 PM
People,People, ok look Ive been takeing discharge line temps lately and i find that it depends alot on ambient and load conditions and the type and style of compressor the type of unit ect in arizona we work on Goettls mostly (I will post some pics of these and data info for those of you who have no idea what one is) anyway when i take hgt from them they vary so dramatically from home to home in just 10-30 min time ( the time to drive and set up from one unit to another)I can work on a goettl and get 165-185, then go work on a carrier and get 150-177 with no change in temp or from a goettl with a recip and get 155-180 and go to a scroll and get 165-185or 190 so is that accurate i dont know i cant see how althoughi take the reading just to insure how hot my compressor is running knowing that i cant be over 225 that is my concern i have come on some that are running 222-225 with a head pressure of about 420 this makes sence but i did come accross one recently that the compressor disch was 235 and head was 300 we pulled that compressor it was a scroll and had been running like that for awhile after talking to another tech. so as for HGT forget it it is a rule of thumb as far as i can see if a manufacture says to use it it cant be right, he cant possibly test that in every type style and config of unit under those odd and extreem load conditions, not to mention if you carge by that my god , what if you miss a plugged filter or coil and call it low on charge( we have seen this i know , right senior techs?)so please we all know or should know that S/H and S/C tell us right where are liquid levels are in the coil that is the most important tool we could have.We also know it works stick to it till they come out with a more superior way of seeing inside the unit.

beenthere
02-12-2005, 04:40 PM
Acdude, are you taking these readings in heat or cool mode.

125+ambient is for heat pump in heat mode.