Hot water pipes, joining copper to iron

[georgestair]

I want to replace most of my existing black iron hot water heating pipes to divide the system into zones. Is it OK to use copper pipes to connect to my cast iron radiaters and to iron pipes? Do I need to use dielectric unions?

 

[TBP]

I know a lot of very good contractors who do exactly this kind of work. Do you know what gives them the most trouble with leaks? The dielectric unions. To a man, they absolutely despise dielectric unions, even on water heaters where they are often required by code. I've seen a number of photos of dielectrics that have leaked, and/or choked with rust on water heaters.

The general consensus seems to be that as long as you're not adding fresh water full of oxygen all the time (bad for a number of reasons), the initial shot of dissolved oxygen gets eaten up in a tiny amount of initial corrosion, then it's gone. I don't know of any tight system that suffers from corrosion where copper and iron/steel join. That's not to say that there couldn't be, somewhere.

 

[Koske]

I would like to warn you that the reason for corrosion in the coper-iron pipe union is not the Oxygen but the difference in their electronegativity, therfore I warmly recommend using the dielectric unions.

 

[TBP]

The place to go for a practical viewpoint on this topic is

http://www.heatinghelp.com

and the sub-site is "The Wall". It's a message board like this one, for people who do residential/small commercial hot water and steam heating. Pose the question there, and see what the responses are. I am not sending you into an ambush I believe you'll find the responses will be from a range of locations in North America, and from contractors who actually install and service this type of equipment (and take a lot of pride in their work) They very much dislike dielectric unions as they are the source of many call-backs. And if you're the contractor who has warranted the system, you EAT the call-back cost.

I agree, the textbooks say there should be corrosion in closed loop systems like this, with copper and iron joined together. But in my experience, it doesn't happen, for whatever reason. The cure (dielectric unions) very much seems to be worse than the possible disease (dissimilar metal corrosion). The dielectric unions routinely leak, fresh water is added as make-up, and then the corrosion game is REALLY on in closed loop heating systems.

 

[georgestair]

Thanks for your comments. Maybe I should use black iron pipe and avoid the corrosion issue. The existing 90 year old pipes are totally clean inside.

 

[ajw160]

Well, theory says that everything wants to equalize and be happy. The water moves the electrons. Unless the water itself is non-conductive you will get galvanic corrosion. How bad? How fast? Who knows.

 

[lcleveland]

moved out of a house last year that had cast iron boiler to black iron to copper on a mono-flow system installed in 1955 . boiler had been repllaced but the pipes were ok .no problems on the bi to cop no corrosion no leaks . it was only there 48 years though so time will tell.