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