GrumpyPlumber said:
After thought.
I was so caught up in the interest of reading the galvanic table and "looking at the tree instead of the forest", I forgot to mention...IF you have the pipes open & cut, you might actually think about simply converting to copper, or pex.
Steel pipe is awful with water, rusts from the inside out, rust particles wind up at the bottom of your boiler as well, which isn't good for it's life expectancy.
Most heat lines are run with copper for this reason.
If you can't remove it all, then consider removing as much as you can & use brass connections, unless of course you only have a few feet of the pipe exposed.
Grumpy,
Your big-picture advice is good, but doesn't fit. We don't have much pipe visible.
Here are the facts that prevent an ideal solution:
1. This is a forced hot water system and the radiator is steel tapped 1/2" FPT
2. We have only 5 feet of original steel pipe exposed.
3. The clearance between the wall and the wall-mounted radiator is 2 inches.
4. The only metal PEX-to-NPT adapters are made from is brass (that's all I have been able to find, anyway).
#1. I have not been able to find dielectric unions rated for FHW. 180 degrees F. When I phoned mcmaster to ask about them, they stressed "Not for FHW".
#2. If, under the tile floor, we replace the last 5 feet of old steel with something else, galvy or copper, we still have dissimilar metals in the transition adapter. See #1 above. I do not like the idea of a brass-to-steel adapter being invisible under a tile floor; if it were to fail, would be expensive to repair.
#3. If we put a steel zero-length nipple [not sure of their proper name -- nothing but facing tapered threads: this <> rather than this <|||> ] into the radiator and also into the union, we cannot fit a 1/2" NPT union in the two inches, and would then have to run supply and return to the wall-mounted towel-radiator from the floor, or transition to a brass-adapter/copper-sweat connection.
#4 If we use a short piece of PEX to solve the 2" clearance problem, with 1/2" MPT-to-1/2" barb adapters and a small 2" piece of PEX, we have dissimilar brass into the steel of the radiator and into the old steel supply and return lines. [These connections are both visible, so this is probably a decent solution.]
#4a For FHW temperatures, Zurn engineering support said I should NOT use their potable water PEX because it is only rated for 140 degrees, but to use instead their oxygen-barrier PEX. EASIER SAID THAN DONE. Zurn is in the process of beefing up its retail supply, and I have not been able to find oxygen barrier PEX from the big retail stores or even from the local plumbing supply houses that all the pros swear by. On the internet, I found a source who sells in 100' coils. All I need is four 2-inch pieces.
#4b Local code, I believe, requires the PEX crimps to be visible.
Thanks for any suggestions. I think brass PEX adapters into the steel is probably the solution; at least I can see the connections and examine them for corrosion periodically. Using brass will probably void the radiator warranty
T.
