Sizing expansion tanks for water heaters can be a complex engineering challenge, but residential expansion tank sizing has been simplified through the use of calculators. New or larger expansion tanks may be needed due to pressure reducing valves, check valves and backflow preventers added to the community's water system. These valves isolate the home's plumbing. Water expands as it is heated, and in an isolated system this expansion increases the pressure in the pipes. The increased pressure can create leaks and damage plumbing or fixtures.
Measure the water pressure. Because water pressures vary within the same city, this step is very important. Hardware stores sell inexpensive water pressure gauges with a hose fitting. Attach the pressure gauge to the hose bib or outdoor faucet, and open the handle. Make sure the water is not running elsewhere in the house to measure the static water pressure. Write down the static pressure reading.
Determine your water heater size and hot water temperature. Typical residential tank-type water heaters are 30, 40 or 60 gallons. The label on the water heater will list the capacity in gallons. Write down the capacity.
Look for the temperature setting on the water heater. On natural or propane gas water heaters, the temperature setting is indicated on a dial near the gas valve. Typical settings range from 125 to 140 degrees Fahrenheit. The dial may have only a few markings, or labels such as Low, Medium and High. If the water heater is set to High, use 140 degrees. For Low, use 125 degrees and for Medium use 130 degrees Fahrenheit. Electric water heaters will have similar dials. Or, use a cooking thermometer to test the hot water temperature at the nearest faucet. This method requires running the water long enough to get the maximum temperature and making sure the faucet is not mixing cold water with the hot water. Write down the temperature.
The water heater will have a temperature and pressure relief valve (T&P valve) on the top or side of the tank. A circular tag under the handle will list the pressure rating of the valve. Add this pressure rating to your list of information. Some calculators require the T&P valve pressure rating.
Calculate the minimum expansion tank size. Armed with your information, proceed to one of the calculators provided by municipalities or expansion tank manufacturers. The calculators request additional information. "Cold Water Temp" should be set to 90 degrees Fahrenheit, according to Watts Water Technologies. While the incoming water may be colder, it is mixed with the hot water in the water heater. It is rare that a water heater is completely emptied and has only cold water in it.
"Tank Pre Pressure" is usually factory set at 40 PSI. This pressure is adjustable by adding air to the expansion tank, up to the maximum rated pressure (typically up to 80 PSI).
The calculators provide the minimum size expansion tank expressed in gallons. Choose the next larger expansion tank if between sizes. If just slightly over the nearest size, adjust the Tank Pre-Pressure value to determine if the nearest expansion tank can work. Do not exceed the maximum pressure rating of the expansion tank.
Consult with local building department for local or state codes.