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OtherHand 04-25-2011 03:17 PM

Questions on replacing lineset
I have a good sized place in the Los Angeles area, built in 1979, and which has a 5 ton central air unit. It only gets run for cooling about 3-4 weeks per year as the climate is mild.

Two years ago the original 1979 AC unit died, and I replaced it with a new 13 SEER 5 ton R-22 Rheem outside compressor, indoor cased coil and gas furnace. I did the install myself. I had done some AC work way back while in college and had the necessary tools (gauges, vacuum pump, recovery system, etc). It was a PITA, but worked pretty good. I made sure to install a TXV also and very carefully charged the system.

A couple of weeks ago I did a routine check prior to Summer, and discovered I had no cooling. Checking with the gauges showed essentially no pressure. I pressurized it with gas to about 50 psi, and it leaked down about 10 psi per day. I went over all inside and outside components with a leak detector and found nothing. I double checked any of my brazing I thought could be questionable and it doesn't appear to be leaking.

The original 7/8" x 3/8" 1979 lineset runs under the at-grade slab for about 28', connecting the condenser and evaporator. At this point I'm thinking that must be where the leak is, but don't know of a way to check it, other than by process of elimination. The linset is run far enough in from the edges of the house slab that I can't pick up any trace of leaks at the slab edge.

I'm looking at replacing the below slab lineset with 50' of 7/8" x 3/8" run through the walls and ceilings. It's gonna be an ugly job, so I want to make sure I have everything I need before knocking holes in the walls.

Is there anything special I should do as part of the lineset replacement? I plan on including a new dryer/filter in the liquid line. Do I need to add any oil to the system to replace whatever might remain in the abandoned below-grade lineset? Do I need to be concerned that the system might have sucked in any outside contaminates through the underground leak? While it might be desirable to to increase the suction line size to 1 1/8", the fittings on each end are 7/8", so I'm inclined to stay at 7/8".

Thanks for any advice you might have.

COLDIRON 04-25-2011 04:04 PM

Cut and cap the line sets and install schraders and pull a vacuum on just the line sets see what happens. Watch your micron gage. I hope the old line sets are not touching the concrete I think it deteriorates copper over time.

newtech 04-25-2011 04:20 PM


Originally Posted by COLDIRON (Post 636262)
Cut and cap the line sets and install schraders and pull a vacuum on just the line sets see what happens. Watch your micron gage. I hope the old line sets are not touching the concrete I think it deteriorates copper over time.

Might also want to cap the evap and condenser as well ( with schraders )

This way you will have the whole system isolated. Charge each component and see what happens.

I have had some evap coils that leaked and couldn't find them with an electronic leak detector.

Instead of a micron gage for leak checking, I would use nitrogen, but that is just my preference.

OtherHand 04-25-2011 07:10 PM

From watching the pressure drop, it looks like a fairly considerable leak. I've used soap solution and a sensitive electronic detector and found nothing. I've also looked around for traces of oil or dirt residue in the coils but everything's very clean. Capping the lineset and pulling a vacuum sounds like something I should try before whacking holes in the wall. Not as much fun though....

Marty S. 04-25-2011 07:16 PM

Nitrogen would be better then a vacuum. No sence pulling crap into the system.

COLDIRON 04-26-2011 05:50 AM

Don't forget to stick your leak detector in the evap drain line at the A/H I have found many evap leaks by doing that.

OtherHand 04-26-2011 08:37 AM

Thanks for the good ideas. From the feedback I got here, this is what I was thinking of doing to nail down the diagnosis:

1. Attach gauges to service ports, pressurize system, and close service valves to condenser unit. If leak goes away, problem is in condenser. If leak remains, problem is in lineset or evaporator.

2. Cut and cap lineset at evaporator, then pressurize with the condenser service valves still closed. If one of the two lines loses pressure, there's my problem. If neither do, the leak is in the evaporator.

If I find the leak is in the liquid line, anyone see any problem with just replacing it, running it through walls, and leaving the suction line under the slab? I figure because of the smaller size it would be pretty easy to run compared to the suction line. But if I have to go to the trouble of replacing the suction line, then I might as well do the liquid one too. Or is replacing only one of the lines bad form?

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