Code Refrance: 2013 CPC Section 608.3 and 2013 CMC Section 1005.0.
Whats inside the expansion tank
What is an expansion tank & why do I need one?
An expansion tank is a cylinder tank about 2.2 gallons in size for most standard home water heaters and much larger in size for commercial or larger homes and is typically now required for use on all water heaters up to 50 gallons, larger expansion tanks for larger water heaters. They are also used for the installation of tankless water heaters.
Expansion tanks are built with a rubber bag (bladder) inside the tank and is about half the size of the tank it self, this bag is filled with air pressure that should match the pressure of the incoming water into your home's main water supply piping from a municiple water source, homes on private well are usually not required to have an expansion tank.
An expansion tank is required if a plumbing system is “closed”, which means that the back-flow devise that normally runs between the house and the main water supply is now blocked at the meter or other locations by a check valve, this (check valve, pressure regulator, back flow prevention devices) allows water to flow from the water meter to your home but, not backwards... blocking the extra pressure from thermal expansion of your water heater.
When water is heated, it starts to expand the water up to an average of 20% or greater which causes a rise in water pressure, this increased pressure must go somewhere. If there is nowhere for the pressure to go, it places extra stress and strain on your water heater and your home’s plumbing system. It can also cause your water heaters T&P safety valve to slowly leak allowing mineral deposits to collect and block or seize the valve.