DIELECTRICS IN A HOT WATER BOILER SYSTEM

Weil

I was told a long time ago dielectric unions should not be used on a hot water boiler system since the water should be free from air and therefore electrolysis. Is this correct?

mattmia2

More or less. If you search, you can find numerous multi page threads debating this.

GroundUp

I'm of the mindset that they're not necessary, for the reasons you mentioned, but I wouldn't agree with the statement that they "shouldn't" be used. Certain boiler MFG disallow dielectric unions period, and maybe it's due to their likelihood of leaking, but I've had really good luck with them in cases where I need to go from threads to sweat in one joint. The dielectrics are 10x cheaper than the brass thrd x copper swt ones

rick in Alaska

I absolutely despise dielectric unions in a heating system. It seems that after they are in for a few years, and if somehow the boiler goes cold, they will start leaking like a flat roof. However, if you don't bump the pipe when they are leaking, and can get the pipes hot again, they usually will seal up. But if you do bump the pipe, that gasket that is now like a piece of plastic, will shatter. This is also one of the reasons I still don't trust o-rings as a gasket. And unless you have a 36"pipe wrench with you, you won't be able to get them apart.
I typically just cut the whole thing out and pipe it in direct. I have never seen a problem with a boiler that is piped copper to steel.
fyi: just my observed opinion
Rick

Alan (California Radiant) Forbes

I’m with @rick in Alaska ; they’re a pain. They leak and you can never get them apart again. You pay for what you get.

There is no electrolysis issue with heating boilers. For boiler connection, I like C x male or C x female unions.

Jamie Hall

Just a bit of chemistry here -- electrolytic corrosion has nothing to do with the presence of air or oxygen. Rather it will happen whenever two dissimilar metals -- such as iron and copper -- are electrically bonded and are in the presence of an electrolyte -- such as water.

If the outside of the pipes connected is dry, there will be no corrosion on the outside. On the inside, there will be -- but it will be distributed throughout the length of the less noble pipe (such as in this case, the iron pipe) -- so I doubt that one would notice it in the pipe.

Where oxygen in the water gets into the act is that without oxygen, the iron metal will dissolve in the water as the Iron (II) or ferrous ion -- which is highly soluble. But throw some oxygen in there, and it will promptly and happily turn into Iron (III) or ferric iron -- which will, equally promptly, come out of solution as a brown sludge (or if you're really living wrong, as a black crystal -- magnetite -- which raises havoc with permanent magnet motors). If there is a lot of oxygen and not that much moisture, it will show up as... rust. Such as on the outside of a moist pipe. (but that may be ordinary corrosion as well -- not electrolytic).

HomerJSmith

I have used them before, replacing leaky ones. I always throw away the EPDM gasket and use a fiber gasket that Caleffi sells. I wet them first and never had a leak. I also use silicone grease on the threads of the unions to make tightening and loosing easier.

I see so much copper to cast flanges and steel nipples that when ever I can I take them apart and look for electrolysis. My eyes must be going bad as I never see it. I beginning to think it's a myth that old plumbers talk about sitting around a pot belly stove, haha. Kinda like Bigfoot.

I would think temperature and water quality would increase it.