Eliminating Dielectric Unions between Steel and Copper??

[xjken]

Hello All.

I'm working on an existing 6 story building hot water heating system. The client wants to eliminate all dielectric unions in the system because there are many that leak. The system consists of sch.40 steel pipe for the mains and risers and copper for the branches to the fintube. The steel and copper are separated by dielectric unions.

My question is can the dielectric union be replaced by a brass fitting? Does the brass act like a dielectric? Is this acceptable practice, or poor engineering? I don't feel comfortable with this setup and am looking for solid proof that this is a good or bad setup.

Any thoughts? Any links to hard facts about this?
Thanks in advance!

Ken

 

[JStephen]

No, the brass wouldn't act like a dielectric union.

In other applications, I've run across the issue of corrosion from dissimilar metals. The problem is, everyone knows metal A will make metal B corrode, but no one seems to have a clue as to how fast, or if it's really a problem or not. So we find ourselves using insulators between epoxy coated steel and galvanized steel. Or one of my favorites, a real Dilbert candidate, is having two pieces of metal welded together at one end, then putting insulators between them at the other end.

You might check if the plumbing codes specify the insulation. Or see if you can get a higher quality fitting that doesn't leak.

 

[xjken]

Thanks JSteven.

So there definitely needs to be a dielectric fitting between the two?

 

[StoneCold]

xjken
The answers you are going to get are going to be "well it depends" If you think about it the cheapest thing to do is buy better dieletric unions and replace the old ones and be done. If you decide that you don't need them and eliminate them and then five years from now you have a ton of leaks that will be a lot more expensive then going the conservative route.
That being said we have copper pipes connected to carbon steel with just teflon tape on the pipes in a cooling water system. I have ohmed them out and they conduct across the transition. I have not seen any pipe failures close to these junctions and some of them are 15 years old or more. But if I was putting in new, I would go with the dielectric the cost is all in labor when you work it out anyway.

Good luck
Stone cold.

 

[TBP]

I've seen a large number of closed-loop field installations where copper piping was tied directly into steel, and I can't remember actually seeing any problems as a result. Some of these systems are 40 or 50 years old.

As far as I can tell, dielectric unions come in one grade - "double extra crappy" (XXC?). They are routinely chronic leakers, and the dissolved oxygen filled make up water, that is fed into the system to replace the leakage, is the source of more corrosion that the dissimilar metal issue ever caused. They seem to be a prime example of the cure being worse than the disease.

 

[jte]

I bought a house a few years ago which needed some immediate plumbing attention. I "temporarily" connected the copper flex-hose (corrugated) serving the hot water heater inlet to a (new) 3/4" galvanized street elbow. As "temporary" things go... I got around to installing a more permanent solution (with dielectric union) about 18 months later. By that time the 3/4" ell had built up so much corrosion product that the actual flow opening in it was around 1/4". I didn't think that I'd corroded the ell much in terms of wall thickness losses, but the products sure choked the flow.

A probably more expensive, but reliable, alternate for your system might be to find some insulating flanges with sleeves for the bolts etc. Properly fitted the flanges should be much more reliable than unions, and will provide the electrical insulation you need. Same concept as the flanges we use at the ends of our cathodically protected pipelines.

jt

 

[TBP]

There's a difference in application, though. Domestic hot water systems see a constant flow of fresh water that is just full of dissolved oxygen. My experiece with galvanized pipe and fittings in domestic water service (both hot & cold) is that it'll be full of rusty barnacles inside a year regardless of other factors. I've seen a number of pictures posted by plumbers on other sites that show the galvanized side of dielectic unions corroded almost shut (like your street el) and that's oxygen corrosion. Unless a hot water heating system has just been filled/refilled, or there's an ongoing requirement for make-up water due to leaks or a relief valve that lifts regularly, the water will be oxygen free.