Dissimilar Metals - use dielectric unions
You're bound to have dissimilar metals in your hydronic heating system. When two unlike metals, such as copper and iron, come into contact in a plumbing system, they set up an electrical circuit that helps eat away one of the metal surfaces - usually the iron.
In hydronic heating systems, it's often copper piping connected to iron pipes or components like pumps that have an iron body. To reduce this effect, you'd have to buy a bronze pump body, and that gets very expensive.
Some folks call it galvanic action, dissimilar metal corrosion, or two metal corrosion. I've even heard it called electrolysis.
Call it whatever you'd like. The bottom line is that it eats away at metal unless you use a dielectric union.
Dielectric unions are just what the doctor ordered when it comes to unlike metals that come in contact with one another.
A standard iron union mates one smooth metal surface with another to make a water tight seal. The seal requires smooth mating surfaces and a tight connection to withstand high pressures.
In a dielectric union, the smooth metal seal is replaced with a rubber gasket. The rubber gasket prevents one metal from contacting the other, and it makes a good seal against water, even under high pressure.
The dielectric union also uses a polymer sleeve to insulate the steel threaded fitting from the copper pipe and brass coupler that will be fitted tightly against the rubber gasket.
It's the combination of rubber gasket and polymer sleeve that keep the iron pipes and copper pipes from coming into contact with one another.
So, just use a dielectric union, and you're all set to go.
Well, not so fast. There is one enemy of dielectric unions - excessive heat. Most of the unions will fail when they reach about 185 degrees F, so keeping your hydronic heating system around 160 degree F will help you keep a nice margin of safety when working with these type of unions.
Done with Dissimilar Metals, back to Alternative Energy Sources