Heat causes expansion. And, when water is heated and expands, if it doesn't have an expansion tank to catch the "overflow," it can cause pipes to leak or even burst. Damage to your water heater is also possible. Most areas of the country require that water heaters either have expansion tanks or special valves that release the excess water. Expansion tanks are the best, as they can hold more water.
Determine the size of water heater expansion tank you will need. This is based on how many gallons of water your water heater holds. Purchase the correct size expansion tank, a tee fitting and Teflon tape.
Locate the cold-water supply line, and determine which part of it runs horizontal (side-by-side) to the water heater. This is where you will want your expansion tank to be located, and where you will link it to the water heater itself.
Shut the power off to your water heater while you are installing the expansion tank IF YOUR WATER HEATER IS ELECTRIC. However, if you have a gas water heater, UNLESS YOU KNOW EXACTLY HOW TO TURN THE GAS SUPPLY ON AND OFF, do NOT attempt to do so. Dangerous fumes can build up very rapidly, and the smallest spark or ignition source can ignite them. Also, the fumes can get into the house, and may cause illness or death to family members and pets.
TURN THE WATER SUPPLY TO THE WATER HEATER OFF. Then, hang the expansion tank ABOVE the cold water supply line, where it will be accessible to the horizontal part as well as the water heater. Make sure your expansion tank is securely fastened to the wall or other support from which it is hanging. It will have some weight to it, and could cause damage to the water heater or surrounding areas if it were to fall. The manufacturer's instructions for the expansion tank can tell you how to do this.
Install the tee fitting to the cold water supply line.
Determine which end of the threaded connector on the expansion tank fits the tee fitting.
Wrap the Teflon tape around the proper end; fit the tank connector to the tee fitting.
Tighten the fitting securely, using a pipe wrench.