underground plumbing??

[overdriv]

Hi folks, been awhile since I've been on the board, life, well you know.

This isn't so much about a garage but could be information needed sometime. I just bought a house that is built on a slab. The house set empty for about a year. I don't think it was winterized properly.

All the plumbing is underground. When it was built, they bedded the copper piping in sand, laid a sheet of 6 mil plastic on the sand and then poured 4" of concrete for the slab. The house is 45 years old but in very good condition. I got it for a good price and planning to sell it to my son and his wife, so it's worth putting some money into.

Where the water supply comes into the utility room, there is a 36" rectangular bricked up hole in the floor. All the main runs, hot and cold, outside bibs, etc begin there. Just 30" into the room on the other side of the wall, there was a leak in the hot water line, which made it impossible to pressurize the system. Water would run back into the utility room and fill the piping vault. It looks like there has been a leak on the same line sometime in the past, as the hot water line from the HWH is newer than all the other lines.

So we cut and jack hammer out a 3'X4' section of the slab out of the dining room which is the room on the other side of the wall from the utility room. We find the lines to be in very good condition except the hot water line and where it was leaking, it appeared to have corroded through the copper pipe in a section about 3" long on the bottom about 30" in from the vault.

The vault wall where that line went into the house had been broken out and a lot of the sand had been washed out into the vault.

I cut all the lines back into the house about 3' and have transitioned them to PEX out into the vault and gotten rid of all the leaky 45YO valves.

My questions to the collective here are;

Why is the hot water line the only line that shows any corrosion, and only in that one section?

My solder joints on the copper to PEX fitting, should they be treated with something or wrapped with something or left alone?

Is there some kind of electrolysis going on with the hot water that would cause it to corrode? As far as I know there has never been an electric WH in the house.

Thanks for any comments.

 

[overdriv]

No one has any comments on this. Seems odd. Did I post in the wrong forum? Anyway thanks for reading it.

 

[G_P]

I have heard of cases where the electrical was grounded through the plumbing and a ground rod. If the wire to the ground rod is broken and there is a problem the pipes become the ground and electrolisys can eat away at them.

Could also have been just a defective pipe or the soil conditions that did it.

Sent via carrier pigeon

 

[CTyankee]

No idea personally....but found this...

Thermogalvanic Effects

Driving potentials for the corrosion of copper can be created by temperature differences in under-the-slab, hot and cold domestic water lines that are in metallic contact with each other at the hot-water heater. This phenomenon should be suspected when the external corrosion of copper water lines occurs only to the hot-water tubes.

Thermal effects in the immediate vicinity of underground hot-water tubes can sometimes aggravate the corrosion problem by concentrating salts at these locations through the evaporation of water that might be draining through the soil.

Galvanic action. Although copper is normally cathodic to most commonly used underground metals and alloys such as steel, galvanized steels, and cast iron, two forms of galvanic corrosion can occur to underground copper lines. 14 These are the carbon-copper cells associated with certain cinder fills, and the copper-in-soil-copper-in-concrete cell associated with on-slab construction.

Copper is anodic to carbon, and corrosion can be expected to occur if the cinders used for backfill contain appreciable amounts of carbon. Moisture, however, must be present for this galvanic action to take place.

It is also known that copper embedded in concrete will normally be cathodic to nearby copper that is exposed to soil. This subtle form of galvanic corrosion can cause the deterioration of copper exposed to the soil in the immediate vicinity of the soil-concrete interface. Reportedly, this form of corrosion is the reason that complete concrete encasement of copper water tubes is required in some Southern California areas.

Taken from here -

http://www.copper.org/resources/properties/protection/underground.html#top

 

[Jackfre]

This is a combination deal. Copper erodes with velocity and just over time. That piece of pipe could have been damaged slightly on installation. Water chemistry plays a part. I do not know the lay-out, but I think if you have seen one leak in that old a system with copper directly in contact with the ground, which it is after all this time, I would plan on making a well insulated chase in the attic and re-pipe the whole place. You want to keep it so just get it done.

The electrical current issue is possibly a contributor. The NEC allows water lines to be used as a "secondary" ground. Check all the connections on your panel and ground rod. They can loosen over time. Check it all the way back to the meter head. I had a corroded neutral on the meter bar in my last house that raised all kinds of problems.

 

[overdriv]

Thanks guys, I'll check out and consider all the ideas above. We have considered repiping the entire house, but not now.