Thermal Expansion Tank
A thermal expansion tank has become a necessity in most municipalities due to their upgrading of the water system to protect the public health by installing a check valve at your water meter. This check valve protects public health by preventing water contaminates from entering the public water supply through your water service and water distribution lines by stopping it cold at the water meter by way of a one way spring loaded check valve.
As an example: Let's say that one pretty day your out spraying your vegetable garden or flower garden with a hose fed jug of fertilizer attached to your water hose. Suddenly with no warning you lose water pressure because of a county water main break 5 blocks away. With no check valve at the water meter that fertilizer in your jug gets siphoned into the county water main feeding your water meter which contaminates the entire water main. This has happened before!
Now back to where the Thermal Expansion Tank comes into play.
Before with no check valve on the water meter as water was heated inside your water heater the expanded water caused by the heating process would just make it's way back into the county water main getting used by someone down the line. Now with the check valve in place the expanded water gets stopped from entering the county water main just like the contaminates. So now, this expanded water is pushing it's way in the piping system wanting a way out. It will cause pressure buildup in your water distribution piping through-out your home in some cases causing extreme damage to toilet ballcocks, faucets, water piping and the water heater.
To prevent this damage a thermal expansion tank is necessary to give the expanded water a place to expand to. The tank has an internal bladder that allows the expanded water to push up into the bladder and stay until a faucet or other plumbing fixture is turned on allowing the expanded water to push out of the tank allowing a whole new cycle to begin once the water heater begins to heat.
To test a thermal expansion tank all that you will need to do is unscrew the shrader valve cover on the end of the tank. Then, test the air pressure inside the tank with a regular tire pressure gauge. The air pressure should be the same or just below the water pressure inside your home. You can adjust the air pressure the same as you would the air pressure in a bicycle tire as an example. If when you depress the valve of stem of the shrader valve and water spurts out then the tank is bad probably due to extremely high water pressure and must be replaced. Good time to check the home's water pressure at this time to find out if the Pressure Reducing Valve is working as well.