Steel-to-Brass: do I really need a dielectric union?

[rbattelle]

I'm installing a thermal expansion tank made of steel into a cast brass tee fitting (3/4 NPT threaded). I'd like to avoid a dielectric union if possible (I hear many stories of them leaking).



My house is grounded through the water pipes. I would think that would bleed off any currents generated between the dissimilar metals.



What do you guys think?



Ryan

 

[klenger]

You coud always avoid the dissimilar metals by using plastic. :-laf

 

[sstefanini]

Yes, do not install copper/bronze to carbon. The dissimilar metals will eat each other. Go to a large plumbing supply house to get one (I don't think the home stores even carry them). It will be plated carbon NPT with a plastic isolator on one side and usually has a soft sealing rubber gasket on the face. I don't know if you can get NPT on the copper side as I've only seen them in a sweat joint configuration. I' ve seen some really old ones in service and can't see that they're unreliable.

 

[DJW]

Ask at a Plumbing shop, I have seen a special fitting for such use, it's claimed that in time the improper connection will crumble.

 

[bighammer]

My understanding is that it's not the electrical grounding, but the different metals acting (chemically) like a weak battery. (made worse by the heat)

 

[rbattelle]

All right, I'll get the dielectric union.



Of course, solder and copper are different metals but no one seems to consider that a problem.



Ryan

 

[rbattelle]

What about stainless steel to brass?



Ryan

 

[rbattelle]

And it just occurred to me: my water heater is steel, but the T&P relief valve and the drain valve are both brass and both threaded directly to the steel without any kind of electrical isolation.



What gives?

 

[nickleinonen]

at work we use brass fittings with steel and cast iron with nothing more than pipe dope sealing. the fittings usually fail from erosion from the flow of the water inside [engine cooling water]



you could always drive in an extra/external ground rod and tie that into the panel. .



does it need to be a cast brass fitting? no steel or iron fitting avalable?

 

[jtisdale]

How long (time) are we talking for failure between brass/copper and carbon steel? Years, decades, what? Is brass to copper ok, but not either to carbon steel?

 

[rbattelle]

I'd rather not put iron or steel into the system because it's a house and I don't want rusty water (yes, the expansion tank is steel but apparently it won't put rust in the system).



I think I'm going to just put 'em together and see what happens. Brass and copper are compatible, but not brass/steel or copper/steel.



Ryan

 

[aphat426]

stainless to brass ok, carbon steel or stainless to copper bad ( need phenolic washers and special valves) brass is one of those weird materials that is for the most part inert. copper on the other hand is not.