Repeated pin hole leaks

[Bad Wolf]

I have been having pin hole leaks in my DHW coil for the last couple of years. This is a non pressurized system with a 150’ coil of ¾ refrigeration tubing for my HX. My water was on the acid side around 5.2 pH, but a year ago October I installed a neutralization system and my water is now 7.1.
I had another leak last August which I thought might just be a residual weak spot, fixed it, and it was fine.
The leaks are always in the bottom third of the coil (water enters at the bottom). In the past I’ve had leaks throughout the house always in the cold water side but nothing since I installed the neutralizer.
The heat exchanger from the furnace (two 150’ coils) is made of the same material and there have been no problems in 6 years.
Last night I noticed the tank was getting high again. Is there another factor that might be causing this corrosion? I have heard that bad grounding can cause problems.
There is no real rush; I can redirect the DHW to an electric heater for the rest of the heating season. It’s just a pain to drain the tank, climb in, find the hole, cut the tubing, solder a union in, refill it and heat it back up again.

 

[BoilerMan]

What are you using to treat your water in the tank? Also what chemical is the neuteralizer using?

A grounding issue would be revealed by using a digital voltage meter on the DHW coil and another copper/metal element in the tank. You should not have any voltage DC. If you do then the tank water is acting as an electrolyte.

TS

 

[tom in maine]

If your coils are tied together with copper wire, there can be some potential developing between the coils and wraps.
Have you checked the pH of the tank water?
If you have EPDM, it can sometimes break down and the byproducts can affect the copper.
Since you have had copper plumbing in the house leak, even an occasional drift of the pH into acidic range will cause corrosion.
You might be dealing with some legacy corrosion from the original problem.
It can be insidious. An educated guess is the best course is to install a new coil and keep the pH basic in the tank and the DHW.

 

[Boil&Toil]

Depending on what your tank situation is, it might also be worth considering a move to plastic (PEX.) While the heat transfer is poor compared to copper, the chemical resistance is considerably better, and if you have room to use more, it should balance out. ie, rather than use 150 ft of 3/4 copper, use 2 or 3 (in parallel) X 300 ft of 1/2" PEX...

 

[maple1]

Depending on what your tank situation is, it might also be worth considering a move to plastic (PEX.) While the heat transfer is poor compared to copper, the chemical resistance is considerably better, and if you have room to use more, it should balance out. ie, rather than use 150 ft of 3/4 copper, use 2 or 3 (in parallel) X 300 ft of 1/2" PEX...

Pex might work - but I wouldn't go smaller than 3/4". I've got around 200' stuffed inside my storage enclosure, for before it gets to my sidearm. At the tap, there is a noticeable difference in water pressure between the hot side & cold side - that much Pex really scrubs the pressure off by the time it comes out the tap.

 

[Boil&Toil]

Which is why I'm suggesting going in parallel, and possibly replacing one 3/4 with 3X 1/2 rather than the usual 2X 1/2"

Of course, if you have room, 3/4 will be less resistance/dynamic head. My radiant floor tubing is 3/4 (or 3/4 purporting to be 7/8 - I'll have to compare it to some standard 3/4 now that I have that in for the plumbing, but I think I misplaced my 6" cutoff of the floor tubing in the meantime) specifically because I did not want high head and a lot of pump power for the radiant floor (thought it might have to be off-grid when I put the tubing in)

 

[maple1]

True - that would help.

 

[BoilerMan]

REF tube is the thinnest wall of copper also. Type K would give you more wall thickness, but you should test Ph in the tank and what chemicals are used in the water/tank.

TS

 

[Bad Wolf]

pH of the water in the tank is 7.1 The neutralizer uses Calcium Carbonate 90% and Corosex (mangisum oxide) 10%. The tubing is corroding from the inside out in the same fashion as the houehold pipes did before. The HX for the furnace is the same material I used ref tubing because of the cost and the ease of coiling it.
I'm hoping this is legacy damage and that its the last of it. I'll try ground the lines directly, I don't think it woiuld hurt anything.
Then I'll fix it one more time and if it comes back I'll either replace the lower third of the coil or maybe just cut it out and shorten the coil. 150' might be more than I needed in the first place.