Regular Guest
Charging to Superheat or Subcooling
What is the proper measurement to charge with?
I always thought you use SH to charge with fixed orifice or distributor and SC for TXV. But even the Sporlan instructions that come with a TXV talks about charging to SH.
One of the guys I work with went on an interview yesterday and the guy asked him the proper way to charge a system. He said SH and the guy said he was wrong. He said you charge to SC. The guy didn't specify the metering device. I told the guy I work with what I said in the first paragraph and that the guy he interviewed with didn't know what he was talking about.
Regular Guest
alot depends upon application........is it sized right.........is it an old system that isnt really efficient....sometimes when trying to get to your 10 deg of subcooling on a older system you may be raising your head pressure out of sight trying to get there....compressor is most important part and you dont want your sh to be to low or to high. but check both anyways
Last edited by cool freak; 10-31-2008 at 05:20 PM.
Professional Member
You are right, if the system has a TXV, you charge by subcooling because the TXV will maintain whatever Superheat it is set for. It is always a good idea to check both SC and SH, but charge by SC. You can also go by discharge superheat, this works well with either fixed orface or TXV systems.
Professional Member
Charging by superheat will only guarantee you an adequate charge so long as the load doesn't change, if you have just enough refrigerant to achieve a desired superheat under a moderate load you will not have enough refrigerant to maintain that superheat when the load increases. Superheat also does not guarantee you that a full column of liquid is getting to your metering device, flashgas can occur in the liquid line before it gets to the evaporator. If there's no charging chart with a condending unit for reference, I generally charge to provide 10 deg SC and then check my suction and discharge temperatures to ensure they are in a good range.
Professional Member
This is correct.Originally Posted by elvis66
The TEV service installation instructions can be downloaded from here: http://sporlan.jandrewschoen.com/10-11.pdf
Where does it state systems with TEVs are to be charged to superheat?
Regular Guest
Andy, it may not say to "charge" to SH, and I didn't read the whole thing word for word, but it doesn't say anything about SC, it only talks about SH, and that is the point I was trying to make...
Professional Member
I agree, charge with suction superheat on straight cool [only] and then verify the subcooling with a clean-as-new condenser coil. The subcooling changes as the coil ages too.
Verify with checking Discharge superheat [apx 100 degrees].
Charging with subcooling only works to get the highest efficiency on High SEER matched system. High SEERS must be charged to the Mfgs specs to get the highest SEER.
Professional Member
Most residential hvac OEMs will recommend their systems be charged by weight. As a result, it behooves us not to suggest something different. But I suppose we should note charging a system with a TEV and less a receiver should be done by subcooling given the absence of an OEM's charging recommendation.
Professional Member
When charging a system with TEV, your super heat is basically fixed between 8 to 12 degree SH. Charge by SC and check SH to make sure evrything else is good.
Do it right the first time.
Regular Guest
you cant charge to the proper subcooling unless the txv is metering the desired superheat so you have to charge to both and adjust the txv as necessary
Professional Member
Most TEVs are going to fixed positions. Less problems.
Do it right the first time.
Regular Guest
so are you supposed to change out the txv because you dont know how to work on it?
Professional Member
You have to make yourself alittle clearer.
Do it right the first time.
Regular Guest
you said its not necessary to know how to adjust the txv because newer models are coming out fixed i was asking if a tech should just change the device out to one of these models rather than learning how to properly adjust it
if you just keep adding refrigerant to try and get the subcooling up without analyzing the load you can run the superheat into the low single digits and release that puff of smoke they put in the compressor
Professional Member
But I no i should have made myself cleared to you. Didn't you already talk about this in another post.
Do it right the first time.
Professional Member
How many puffs of smoke are in each new compressor?
Professional Member
Reconditioned compressors have the smoke reinstalled.
Regular Guest
only one
Regular Guest
i dunno i probably did but sorry if i misunderstood what you were saying
Professional Member
It's cold and I don't know the best way to get my charge correct, I mean I understand Superheat and subcooling should both be checked but is this going to be that acurate, In what heat mode or cooling mode ? I need to know the chart on placement best for a heat mode txv hp with a substantial vertical rise ? any help, thanks.. ...
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