Depending on the type of water heater you may still need to install a di-electric union. This is commonly used when you are joining two dissimilar metals. Most of the water heaters will have a galvanized steel nipple coming out of the top. When connecting a copper supply line to the galvanized pipe you are required to use a di-electric union to keep the materials from being in direct contact. Many water heaters such as Bradford White, have waterways built into the nipples coming off the top. These plastic waterways prevent the need for a di-electric union.
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