I’m installing a Noritz tankless water heater on an exterior wall and want to tap into existing galvanized pipe for the cold water supply. The pressure to the house is 85psi, so I ain’t ripping out the galvanized, just connecting to it with new 3/4 inch copper.
When it is necessary to connect galvanized water supply pipes to copper I’ve seen plumbers connect the galvanized pipe to a 6 inch brass pipe then to the copper pipe. This reduces the amount of voltage difference between the galvanized and copper pipes which reduces the galvanic corrosion and makes the pipe joint last longer.
So, I’ll do this, but what happens if I want to mount a 3/4 copper pipe to an exterior wall using zinc-plated strut and a zinc plated strut-mount clamp ? Will I get galvanic corrosion between the copper pipe carrying water and the strut and the strut-mount clamp assembly ? Is the easiest solution to isolate the copper pipe from the strut by using a clamp with a rubber insert ?
if you don’t know what a strut or strut clamp is, look at page 1400 at http://www.mcmaster.com/
thanks
Replies
The right way to connect copper to galvanized is to use a galvanic union. Yeah, some of them are cheaply made and can be a PITA, but if you buy good stuff and install it properly it completely removes the problem rather than just staving off the corrosion for a while longer. Provided there's nothing grounded to that old galvanized pipe, that is! Putting a ground jumper across a galvanic union defeats its purpose.
As you've seen, you can fudge the connection using a long brass nipple, and apparently that works too. The galvanized will still be attacked, though, just slower- there's still a large galvanic difference between copper/zinc alloy (brass) and pure zinc (coated on steel). I've got lots of examples from my own house of places where brass connections to galvanized pipe caused severe corrosion of the galvanized.
If the galvanic union isn't to your liking, a long stainless nipple would be a better choice. Though there are significant galvanic differences between copper and stainless and between stainless and zinc, in practice the greatly lower electrical conductivity of stainless steel limits the galvanic currents and reduces the rate of attack very significantly. It's still conductive enough to work as a service ground, so no ground jumper is required.
As to the connection to your strut, it's the strut which will be attacked rather than the copper. If the location is otherwise dry, it may not be such a big deal. But yes, you can go one size up on the strut clamp and buy that rubber sleeve material sold by UniStrut to provide electrical insulation around your pipe. We buy the rubber sleeve material that comes on a roll rather than trying to stock a million different sizes of plastic split sleeves for this job. The roll stuff is expensive for what you get, but it does work.
WOW!,
That is intristing. I always thought a small brass fitting would work to go from copper to steel pipe.
TRIGGER
Brass is great. You can also use a dielectric union with threads on one side and copper on the other.
Dielectric unions do serve a good purpose. You do have to keep an eye on the rubber washer they use however. Domestic water might be fine but anything heat related, beware...
You are right about the rubber washer. I have used them at the top of hot water heaters for years without any problem though.
That's what I was alluding to: the cheap dielectric unions have sub-standard parts and have given the component an undeserved bad name. Assuming you use good parts, it's still the best way to connect copper to galvanized pipe.
There's a lot you can get away with in a closed-loop cooling or heating system that you can't get away with on supply water. Lots of industrial users mate brass valves to galvanized pipe and fittings for all their cooling water systems and the pipe lasts for a long time- because they load their cooling water loops with corrosion inhibitors etc. But with fresh water, there's a continuous source of fresh oxygen and a continuous discharge of corrosion products, and no corrosion inhibitors- galvanic corrosion runs rampant and the pipe fails quickly.
the last noritz i put in cost me alot of money. there wasn't an automatic restart if the power went out. might want to check it .