Dielectric unions and rust?

CBRob

I was looking through some old posts on the subject and the reason for common occurence of rust was still a mystery.
Has this question ever had a definitive answer?
Was it over tightening?
I've only encountered it once on indirect water heater at the dhw inlet and outlet.
Not seen on any of the radiant connections.
The rusted ones in my case were replaced with brass onions and never rusted again.

STEVEusaPA

CBRob said:

I was looking through some old posts on the subject and the reason for common occurence of rust was still a mystery.

Has this question ever had a definitive answer?

Was it over tightening?

I've only encountered it once on indirect water heater at the dhw inlet and outlet.

Not seen on any of the radiant connections.

The rusted ones in my case were replaced with brass onions and never rusted again.

Can I see those onions? Darn auto-correct gotchya.

What kind of rust are you talking about? Electrolysis?

CBRob

Here is the union

Replaced with standard brass and no more rust.

hot_rod

It could be the minerals in the water that have precipitated out. Iron for example will show a red build up like that.

Minerals come out of solution as the water is heated in the tank, if you drain some water from the bottom you should see the same color.

Solid_Fuel_Man

Steel unions, dielectric or not, dont be long in an open system such as potable water. I refuse to use them.

CBRob

These unions were almost full of rust within a couple years.
Owner had rusty water every time hot was used.

The new brass unions have been on for 10 years now, rust free.

mattmia2

I had the same problem with a slug of rusty water whenever my hot water had been sitting a while when my old hot water heater was new and I had installed it with dielectric unions. I switched it to brass nipples and copper unions. I suppose that partially explains why i only got about 18 years out of the tank. People were typically getting 20+ years out of tanks from the 80's around here. I think the city of Ann Arbor started adding something to change the water chemistry when the lead and copper rule went in to effect but I would think if anything that would increase life.

Jamie Hall

It's kind of important to distinguish between "steel" and cast or malleable iron (often lumped as "black iron"). They behave very differently from the standpoint of rusting and corrosion. Not that black iron won't rust. Over a century or so, it will; if it's exposed to fly ash or other conditions which are acidic or have Sulphur present (or certain mortars), it may go more quickly. Most steel alloys, on the other hand, corrode (rust) very rapidly in the presence of oxygen (stainless alloys don't, and Corten type alloys do, but the rust adheres and forms a protective coating).

HomerJSmith

I looked for brass dielectric unions and I can't find any. Is it because regular ones serve a purpose and brass ones wouldn't.

What about dielectric nipples? What kind of rust are you getting on the nipple?

rick in Alaska

Brass unions do the same thing as a dielectric union as far as electrolysis is concerned, except you won't need 36 inch pipe wrenches to get them apart. I can't stand dielectric unions, and in a closed system like a boiler, they are not needed.
Rick

HomerJSmith

"Brass unions do the same thing as a dielectric union as far as electrolysis is concerned..."

How's that. The only time I use a brass union is when I need a disconnect between copper pipes.

I do use dielectric unions on boilers, but theoretically since I always use good water, the possibility of electrolysis is minimal without them. I have seen many connections of steel nipples to copper without serious consequences. It depends, I think, on water quality.

They could manufacture dielectric unions more resistant to oxidation.

rick in Alaska

Well, there isn't any steel in the brass union, which gives it less of a chance to bridge between the two metals. Not sure of the whole scientific process there, but I have never seen a dielectric union on a water heater that wasn't all corroded up and dissolving on the inside. Brass won't do that.
On a closed boiler system, there shouldn't be any oxygen in the water, so electrolysis should not be an issue. I have boilers here that have been in place for over 30years with copper connected right to the steel pipes, and no problems with corrosion.
I can also say with a dielectric union on a boiler, if that boiler cools down, there is a very good chance the rubber in the union will fail and start leaking. However, if you don't physically bump it and crack the gasket, they will usually seal back up when the boiler is heated up again. And like I said above, make sure you have two very big pipe wrenches to get them apart again after they do leak.
Rick

HomerJSmith

Dielectric unions usually use EPDM rubber gaskets, but recently I have come across gaskets that are green rubber in color.

On boilers I like to use the green fibre gaskets that Caleffi uses in its components and discard the black rubber ones. I think the fibre gaskets swell in contact with water. It work good!

There is contact between the steel tank and brass nipples. Hmmm

I'm not sure where stainless steel (indirect tank) is on the noble chart in relationship to brass. It maybe close in which case brass nipples and a brass union would be a good idea.