di-electric unions good or bad?
di-electric unions good or bad?
(OP)
I've searched multiple threads in these forum and still don't know if i should use di-electric unions. I understand the issue of galvanic corrosion, and also understand that if I have an open or domestic water system I introduce new oxygen all the time (I know galvanic corrosion doesn't need that oxygen), and also that it depends on pH value. i know this has been discussed before, but there still is no conlcusion beyond that the theory says they are needed, and the fieldstaff says they are evil 
but I'm more looking for advice on closed loop HVAC piping with copper/steel pipes. I know theory (and code) prescribe di-electric unions. but I talked to many contractors and also to our maintenance crew and all said:
Should I look for adding di-electric unions/ and if so, what brands would be good? I believe the issue with leaking di-electric unions also is related to the cheapness.
thanks for any insight.

but I'm more looking for advice on closed loop HVAC piping with copper/steel pipes. I know theory (and code) prescribe di-electric unions. but I talked to many contractors and also to our maintenance crew and all said:
- they never saw a corroded pipe where copper/steel met (at least not to the point of failure beyond some light surface corrosion)
- they have seen many di-electric unions leak
- many devices with internal copper / steel transition don't have di-electric unions from the factory
Should I look for adding di-electric unions/ and if so, what brands would be good? I believe the issue with leaking di-electric unions also is related to the cheapness.
thanks for any insight.





RE: di-electric unions good or bad?
RE: di-electric unions good or bad?
While your crew may have never witnessed the slow corrosion beween carbon steel and copper ( TIME is a factor in corrosion), the galvanic coupling between steel and copper if far, far weaker than the couple that exists between zinc and copper.
I have seen galvanized/copper joints degrade in a few weeks.....its a better battery !
My opinion only.....
RE: di-electric unions good or bad?
what are good brands for di-electric unions? I see there are 2 major types. For example Hart has them threaded with a seal. And many other manufacturers have threaded/soldered ends and have them in one piece. I would think the latter is less prone to leaks. which one is better? and what would be good brands?
RE: di-electric unions good or bad?
Di-electric unions are generic items (like pipe) that are sourced from a piping distributor and are not something that you shop around for. If di-electric unions are leaking, it means that they were not properly installed.
Corrosion at the connection between copper and steel is different than the general corrosion that you may see in a closed loop HVAC system.
I agree with MJCronin, installl di-electric union and be done with it.
RE: di-electric unions good or bad?
The problem is that when I just specify di-electric union, I will get the cheapest crap and that is more likely to leak. Sure installation plays a role. But the initial quality makes a difference as well.
typically for valves etc. I specify a few brands that are good (based on what maintenance tell me etc.). The same way I'm looking for a few "good" brands for di-electric unions.
I guess I go with the manufacturers that make the vales and other items we like, like Watts etc.
RE: di-electric unions good or bad?
Your system is closed loop with limited to no make-up water, so no problem with galvanic corrosion. Same with treated cooling water loaded with corrosion inhibitors and antioxidants- to the extent that the water is properly maintained.
Potable or fresh/river water? External water with high ionic strength? BIG PROBLEM!
My 80-90 yr old house's hydronic heating system has plenty of copper- and brass- to carbon steel joints and all of them are 100% fine. No oxygen and no food for the bugs of rust = no problem.
RE: di-electric unions good or bad?
I'm sure not having oxygen, and having steel/copper which are not too far apart help. Also the chemicals we use keep pH up and the teflon tape and paste also may help a bit to be an insulator.
But in the end I may go over to the dark side and prescribe di-electric unions just to be safe.
RE: di-electric unions good or bad?
RE: di-electric unions good or bad?
Still better safe than sorry, even in closed systems.
RE: di-electric unions good or bad?
Galvanic unions are NECESSARY in open systems where there is a continuously refreshed source of either oxygen or food for MIC organisms (organic matter). Such systems include potable water, river water, external corrosion due to weather/seawater exposure etc. Galvanic unions are unnecessary in closed loop water systems unless you're concerned more about water purity than about corrosion of the metallic parts.
As you noted, the galvanic unions are also not without problems. And their intent is easy to defeat, say, by means of misplaced electrical grounding wires, poorly installed pipe supports etc.
RE: di-electric unions good or bad?
OM, in closed loop systems with steel-copper i install good de-aerators, and use annual chemical treatment and call it a day.
In open systems, or domestic water (or system with zinc etc.), I use di-electric unions.
does that sound like a good policy?
RE: di-electric unions good or bad?
Dielectric coupling with non-conductive polymer liner, Victaulic Style 47, Gruvlok “Di-Lok” and Lochinvar Corp. “V-Line” Dielectric fitting on services 200 degrees and less, and pressures less than 300 psi.
RE: di-electric unions good or bad?
The exterior of the pipes can certainly galvanically corrode if you have moisture present. Once the "necessary" portions of our corrosion cell are satisfied (oxygen, metallic pathway) and ignoring for the moment confounding mechanisms such as crevice corrosion and surface passivation/depassivation, the major factors which influence speed of galvanic corrosion are:
1) Time-of-wetness. How often and how long the surface is wet.
2) Voltage difference between the metals (more difference = faster corrosion)
3) Salts, particularly chlorides and to a lesser extent sulfates. The more corrosion-accelerating salts are present, the faster the corrosion goes. Some salts inhibit corrosion - Pb3O4 (also known as "red lead" or lead tetroxide) is the classic example of a corrosion inhibitor. Different salts also absorb moisture (hydrate) at different relative humidities. The effect of salts on corrosion is pretty darn complex.
4) Anode to cathode ratio. Is it galvanized bolts on brass, or brass bolts on galvanized?