What is everyone doing or recommending as far as dielectric fittings?

I'm building a manifold system with the main supply and return lines made with 2" black iron. These main lines will both have ball-valves (yellow metal). There will be 6 feet of black iron pipe between the ball-valves and the Garn (as per Garn manual). After these ball-valve there will be a four-zone manifold with yellow-metal isolation flanges and cast iron Grundfos pumps on each branch. Using brass iso-flanges,standard inexpensive readily available pumps and having shut-offs in the main lines is what everyone seems to be doing. But with all of the switching back and forth between dis-similar metals, what is best? Do we rely on the chemicals to handle corrosion or is everyone using dielectrics to isolate every yellow fitting?

The manual that I have shows a shut-off followed by a pump followed by a dielectric. What does this accomplish if the shut off and the pump flanges are brass and the pump and the pipe are iron?

 bump

Good question Rick.  I have over 6 feet of black iron between my GARN and the first brass ball valve on both S&R.  I used no dielectric fittings on any of my plumbing.  I was hoping Steve E or someone from Dectra would respond with more detailed input on the galvanic corrosion issue.  I doubt it is a major problem, and I think you will be fine.  If you are concerned, you only need one dielectric between the GARN and the first non-ferrous component.  After that everything else is isolated.

Jim, 

Thank you.

I understand galvanic corrosion. No, not so, I accept it as an issue, is more like it:)

Anyway, when it happens, what corrodes? The iron fitting that is in contact with the non-ferrous fitting? What are we safeguarding against? The iron fitting rusting out? Or the resulting corrosion spreading throughout the system via the water flow and rusting the tank?

It seems to me that the chemicals are meant to limit corrosion in the entire system. It seems like that keeping the first non-ferrous componant far from the tank must be an attempt to keep any corrosion that does occur, in the pipe, where it can be more easily repaired. The dielectric fittings, IMO, at the moment, subject to change without notice or reason, will only stop galvanic action from occuring between the two componants that they are separating. I really don't see how one dielectric fitting will isolate anything other than the two fittings that it's attached to. If there are other ferrous and non-ferrous componants in contact with eachother, like cast iron pump bodies and brass iso-flanges, elsewhere in the system, there is the possibility of corrosion there as well. Is that what we're looking at?

 Maybe I have it all wrong about dielectrics. Is what's being said here correct?

http://www.radiantdesigninstitute.com/page46.html

Hi Rick

My understanding is that brass (your ball valve) and steel pipe marry very well together.If you are going from steel to copper then use a dielectric coupling, brass valve or brass flange if at the circulator.

Good Luck

 Thanks Paul,

Yes. You are right and I'm glad of it. I was told (lots of misinformation around, in unlikely places) that ANY ferrous metal in contact with any yellow metal was bad. Then I looked at what is pictured in the Garn manual and it didn't make sense to me.

Think I've got it now. Thanks again!!

 I asked Martin to look at this question and he wanted to take some time to do some checking before replying.  This is his response:

The topic of dielectric couplings and their use in GARN® WHS systems has appeared a couple of times on this forum. I have been searching for a way to discuss this in a relatively simple manner as it is a complex corrosion issue. In the April 2009 issue of the ASHRAE journal, author Walter Sperko, PE has penned a reasonably good explanation of this issue. Please take a few minutes to review his article at http://bookstore.ashrae.biz/journal/download.php?file=0409Sperko.pdf

However, please note the following:

• NEVER install oxygen scavengers in a GARN WHS unit, or for that matter any other non-pressurized closed system. Oxygen scavengers will form significant sediment, which will lead to under deposit corrosion. Oxygen scavengers are generally used in steam systems.
• NEVER use galvanized (zinc plated) pipe or fittings as it will increase steel corrosion at water temperatures above 160F.
• The water treatment chemicals provided with your GARN WHS unit have been specifically formulated for the GARN WHS system and include: surfactants, pump lubricants, film forming inhibitors, pH buffer and control chemicals, dispersants, etc.
• Recently, in addition to the water treatment chemicals, a pre-cleaning chemical is being recommended as it removes transportation "debris and dirt" and passivates the steel surface thus enhancing the effectiveness of the chemicals.

As stated in the GARN® Manuals we recommend the installation of a dielectic coupling wherever steel piping is connected directly to copper piping. There is no need to install dielectric couplings between: steel pipe and PEX tubing; steel pipe and radiant floor manifolds; or copper piping and a cast iron pump. Bronzed based valves can be installed between steel pipe and copper pipe when good water quality and low water conductivity are combined with proper water treatment chemicals.

Thanks to Martin for the response.  Please let me know if you have any thoughts or questions.