A dielectric union is designed to break the electrical contact between two dissimilar metals (to prevent galvanic corrosion). Most of the flexible copper connector hoses on water heaters have a plastic washer that acts as a dielectric union.

Any metal with a negative voltage when coupled with a metal with a more positive voltage potential will sacrifice (corrode) itself to protect the metal with more positive (noble) voltage. By placing a dielectric (insulating) fitting between two dissimilar metals, i.e. iron and copper, there is a break between the two. Therefore, in theory the iron will not corrode to protect the copper. In fact, the circuit is generally completed through the water, and depending on the water conductivity the iron will corrode at various rates.

I’ve attached and old tech bulleting from Rheem on the topic. I’m not sure if it’s still accessible on their site. Also note that the nipples on top of the water heater are supposed to be dielectric, but as far as I know you can’t tell visually once the plumbing is hooked up.

TECHNICAL SERVICE DEPARTMENT Technical Service Bulletin 1-800-432-8373 Dielectric Technical Competence, Product Confidence Page 1 of 1 1221.doc Dielectric – an electricity, especially a substance with electrical conductivity less than a millionth (10 to the –6) of an MHO.

The way I read it is that Dyelectric fittings are not to be used becase the SAME type of pipe is required throughout. Correct?

The corrosive product often fills the pipe diameter restricting the water flow and is often the source of rusty water. Our company does not promote the use of dielectric unions, because they do not effect the corrosion characteristics of the water heater. Due to our cathodic protection system, the juncture of the supply piping and the tank fitting is adequately protected by the cathodic protection system against corrosion without the buildups which occur in the dielectric union.

I did not look at the picture on your post earlier. the nipple on the cold water connection in the photo is shipped by the water heater mfg that are delivered to plumbing suppliers and are used to prevent a galvanic reaction. Because the fittings add about $5.00 to the cost of the heater retail stores such as Home depot buy their heaters with out the nipples. Most of the problems occur when water heaters are installed by homeowners that do not know any better or by the plumber that works out of the back of his truck

Probably better for the TX SOP to call out actual corrosion rather than all of the dielectric / non-dielectric scenarios or we’ll wind up with another SOP standard as silly and confusing to the public as the no AFCI defects in older homes.

This 1/2" vacuum relief valve is suitable for low pressure steam and water service, and is ideal for use in domestic water heaters and supply tanks. This valve automatically allows air to enter into the piping system to prevent vacuum conditions that could siphon water from the system and damage water equipment. It consists of an all brass body construction with NPT male inlet connection, and a protective cap. Maximum working pressure is 200 psi.

The way I read it is that Dyelectric fittings are not to be used becase the SAME type of pipe is required throughout. Correct?

The corroded, leaking dielectric union in the back was installed by a ‘licenced plumber’. I installed the copper tube/hose and fittings. I paid quite a bit more for the brass fittings, I guess that is why people like the galvanized steel ones (cheap).

Visual inspection means just that. If you were unscrewing plumbing to check for missing parts , you, would have worse problems than lack of a proper dielectric union.

IMO Dielectric unions have very limited benefit as the water within the pipe also conducts current, effectively defeating the “isolation” provided by the dielectric device. I see corroded nipples and fittings on and around water heaters all the time, regardless of whether dielectric isolated unions, couplings, connectors, etc. are used. Sometimes it’s caused by the copper flex connector with the integral isolator itself. Brass seems to have more universal compatibility, which is why I prefer to see brass nipples.

I hear ya. All you can do is your best. Kinda like how in Illinois we observe the plumbing which is worded differently from the other systems that we are required to inspect. Everyone says it is because of the plumbing union , but my opinion is it also has to do with not unscrewing and turning shutoff’s. Plumbing is like that as we all know.

The way I read it is that Dyelectric fittings are not to be used becase the SAME type of pipe is required throughout. Correct?

The installation of unions on the inlet and the outlet waterlines and a shut-off valve in at least the cold inlet waterline is recommended, so the water heater may be easily disconnected for servicing.

A dielectric union is designed to break the electrical contact between two dissimilar metals (to prevent galvanic corrosion). Most of the flexible copper connector hoses on water heaters have a plastic washer that acts as a dielectric union.

Ya I thought of that. Seems like the water heater would fail before serious dielectric corrosion. They only last 6 to 10 years here.

You can typically see part of the plastic washer/flange sticking out at the top of the fitting. This plastic fitting is frequently damaged when the fitting is tightened too much, causing a loss of the dielectric properties.

When there is a concern with using dielectrics, a plastic lined nipple have the ends of the plastic fold over the end of the nipple which provided an excellent dielectric. The plastic engages the thread of the fitting a attached, whether it be the tank of the piping, thereby insulating the metal nipple from the water. This prevents corrosion on either part. These nipples have been certified as dielectrics in numerous areas of the United States.

Dielectric unions may have been used many years ago and were installed with the thought to assist in reduction of corrosion that could develop at the piping connections. From time to time, this inquiry is brought to our attention stating the use of dielectric unions may be suggested required or should be, perhaps used.

The required use of dielectric unions on our water heaters is not necessary. With this in mind, uniformity of piping material must be administered by the installer. What is meant by this is “the piping used in the system i.e.: copper, etc. must be the same throughout, to and from the water heater”. When different piping material connectors are used, corrosion can be immediate and will commence deterioration within days of installation Why is this? When two or more different metallic piping materials are used there can be some level of electrical current flow between the materials due to their different levels of electrical voltage potential. By uniformly using the same material to pipe the water heater, there is essentially no difference in electrical potential between the piping and the water heater tank. Therefore, dielectric fittings are not required when following the uniformity of piping material.

The theory that is in practice is described as follows: All metals when immersed in an electrolyte, such as water, have a voltage potential. This voltage potential varies from approximately +3.0 volts to a –3.0 volts. The relative activity of the metal is determined by voltage potential with those metals with negative voltages being most active and more likely to corrode. Magnesium, for instance, has a slightly negative voltage, approximately –2.0 volts, and will corrode very rapidly in certain waters. Iron has a voltage potential of –0.4 volts and rusts (corrodes) very rapidly when exposed to a moist atmosphere. Copper on the other hand has a voltage potential of +0.350