Charging to Superheat or Subcooling

[elvis66]

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.

[cool freak]

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

[hands]

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.

[nyrb]

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.

[Andy Schoen]

I always thought you use SH to charge with fixed orifice or distributor and SC for TXV.

This is correct.

But even the Sporlan instructions that come with a TXV talks about charging to SH.

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?

[elvis66]

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...

[JRINJAX]

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.

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.

[Andy Schoen]

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...

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.

[21degrees]

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.

[mechanicalgsxr]

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

[21degrees]

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

Most TEVs are going to fixed positions. Less problems.

[mechanicalgsxr]

so are you supposed to change out the txv because you dont know how to work on it?

[21degrees]

so are you supposed to change out the txv because you dont know how to work on it?

You have to make yourself alittle clearer.