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-   -   Checking charge using Subcooling (http://www.diychatroom.com/f17/checking-charge-using-subcooling-104851/)

civiltoatee 05-18-2011 01:25 PM

Checking charge using Subcooling
 
I installed my own furnace and A/C last fall and hired a tech to braze and startup the A/C side of the system. He used a seat-of-the-pants method to add refrigerant to the system at startup. I suspect it is overcharged, so I was about to hire a tech to come out and check the charge using subcooling.

My first question is about how far off could the SC number be and still be considered acceptable? I do not know for sure the manufacturer's SC listed on the rating plate, but it is likely in the 10- to 12-degree F range. Should I expect the SC be withing +/- 2- or 3-degrees F or tighter?

Second, my only fear is that the system has been overcharged. My 2.5-ton unit, according to the product data sheet, comes with 11-lbs of 410, which includes charge for a 15-ft lineset. It recommends adding 0.6-oz for each additional foot of lineset. My lineset is 25-feet, which means the tech should have added about 6-oz. So, apparently the right amount of total refrigerant for my system would be 11 lb - 6 oz. If my installing tech, in fact, added 1-lb of charge (not 6-oz), would a tech I am about to hire, with average technical skill running a SC test, find the SC to be out of tolerance? In other words, about how much would the system have to be overcharged, relative to the recommended 11 lb - 6 oz, for the SC number to be out of tolerance? Would it even have been a problem if the original installing tech had not added the 6-oz needed?

Ultimately, I am concerned that whatever tech I hire to check the SC is sufficiently knowledgeable to do the work properly and adequately adjust the charge. Will the average tech be very capable of doing what I need done? If not, how does one "delicately" ask about a tech's understanding of the work prior to having them do the work?

Thanks.

hvac5646 05-18-2011 04:47 PM

Your system comes with 11lbs of 410A.
You should post model and serial of condenser.
lbs sound WWWAAAAY too much 410A in a 2 1/2 ton system.

If you are trying too have a little fun at the expense of this board, I'd advise against it. you won't get any usable advice if you are.

civiltoatee 05-18-2011 05:03 PM

Quote:

Originally Posted by hvac5646 (Post 650489)
Your system comes with 11lbs of 410A.
You should post model and serial of condenser.
lbs sound WWWAAAAY too much 410A in a 2 1/2 ton system.

If you are trying too have a little fun at the expense of this board, I'd advise against it. you won't get any usable advice if you are.

I am absolutely not looking to yank anyone's chain. I got the info from page 3 of this document:

http://www.docs.hvacpartners.com/idc...s165a.18.1.pdf

It appears to show the charge for a 2.5 ton unit as 11-lb. In the installation manual for the same 165A condenser, it lists the standard length at 15 and 0.6 oz for each foot greater than 15.

On all of my previous post I have been most appreciative of the help I have received here, so I am confused as to how my straightforward post would have been interpreted as satire, except for the fact that I may have misread the table.

It sounds like I have misinterpreted this table. Please help me understand what the actual charge is for this condenser. Thanks.

JJboy 05-18-2011 05:38 PM

Why you suspect it is overcharged?
Did the technician used a digital scale to charge system?

Basically you add the refrigerant according to the manual and then add more according to the size line set length. The subcooling is checked after the system is stabilized for fine adjustments.

beenthere 05-18-2011 05:39 PM

According to that doc. You are correct, it has a factory charge of 11 pounds. Not uncommon for high SEER equipment to hold a large amount of refrigerant.

Is your 16 SEER or more?

civiltoatee 05-18-2011 05:47 PM

Quote:

Originally Posted by beenthere (Post 650522)
According to that doc. You are correct, it has a factory charge of 11 pounds. Not uncommon for high SEER equipment to hold a large amount of refrigerant.

Is your 16 SEER or more?


Thank you "been", so I am not nuts! Yes, with my coil combination, the ARI rating is 16-seer.

brandonmcginnis 05-18-2011 05:49 PM

If it's 10oz overcharged it is likely that it will still be within 2 degrees of the required subcooling. You're probably spinning your wheels.

brandonmcginnis 05-18-2011 05:52 PM

It's also possible that he charged it to manufacturer's superheat specs, but didn't use a full pound. A lot of companies won't bill you in increments of a lb, in other words the required 6oz may have been added but the bill reflected a full pound.

beenthere 05-18-2011 05:53 PM

Quote:

Originally Posted by civiltoatee (Post 650531)
Thank you "been", so I am not nuts! Yes, with my coil combination, the ARI rating is 16-seer.

You install or work on a few "high" SEER units. You get use to seeing large amounts of factory charge.

If you don't work on a lot of "high" SEER, it sounds like too much of a charge.

Hubcap626 05-18-2011 06:03 PM

6oz to 1 pound I doubt you will see any difference. Hell if you hook up a set of gauges you will basically take that amount out with your hoses.

Marty S. 05-18-2011 06:05 PM

A few extra ounces might show up as a +1 degree of SC and the charts usually state +/-1 degree so it should be good to go. High SEER units run way less SC then the old 10-12 SEER did. I helped the installers set up a 21 SEER and the charging chart called for a 2 degree SC.

civiltoatee 05-18-2011 06:06 PM

Quote:

Originally Posted by JJboy (Post 650521)
Why you suspect it is overcharged?
Did the technician used a digital scale to charge system?

Basically you add the refrigerant according to the manual and then add more according to the size line set length. The subcooling is checked after the system is stabilized for fine adjustments.

When I said seat-of-your-pants, I mean he hooked up the tank, opened the valve for a little bit, let some refrigerant run in, and then closed the valve. No weighing, no SH or SC before or after. Therefore, since it only appears to have needed 6-oz more than the base charge (11-lb), I suspect that it is overcharged, but do not know for sure.

brandonmcginnis 05-18-2011 06:14 PM

Then there's no way to tell unless you have it checked. I'd still bet it's not badly overcharged. Did you base the 1 lb estimate off of what he told you?

Doc Holliday 05-18-2011 06:28 PM

I hear furnace and a/c but does the evaporator coil have a txv or a fixed orifice metering device such as the normal piston? If piston (or old school cap tube) then charge superheat, not subcool.

And make damn skippy both your inside and outside coils, evaporator and condenser coils, are clean and that your static pressures are correct or there is no reason in checking your charge in the first place.

Industry standard and manufacture design, if you want it done right.

yuri 05-18-2011 08:35 PM

Quote:

Originally Posted by beenthere (Post 650536)
You install or work on a few "high" SEER units. You get use to seeing large amounts of factory charge.

If you don't work on a lot of "high" SEER, it sounds like too much of a charge.

So True, Just look at the literal physical size of a larger SEER unit. The condensor coils are huge/have more feet of tubing and the evap coils are a lot bigger too. Therefore they hold more refrigerant. And as every good tech knows the bottom coil or 2 of the condensor holds liquid refrigerant so the quantity needs to be there to fill them up.:thumbsup:


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