Hot Water Heaters Expansion Tanks, what do they really do?

Thermal expansion of water in a closed plumbing system can create a number of annoying and potentially dangerous problems. These include: unusually high pressure in a system, pressure surges, and the chronic or continuous dripping of a temperature and pressure (T&P) relief valve. In addition, dripping faucets and leaking toilet tank ball cock fill valves are also symptomatic of thermal expansion. More serious problems can also occur due to thermal expansion. When dangerous pressures are built up in a water heater, internal parts may fail such as the internal flues, fittings or water connections. If a flue way collapses it can lead to the potential release of toxic gases, such as carbon monoxide into living spaces.
Virtually, every modern plumbing code requires the installation of an expansion tank on hot water heater installations. The reason is simple. Water expands when heated. This extra volume of water needs to go somewhere. Before the widespread usage of backflow preventers, check valves and pressure reducing valves, this expanded water simply pushed the cold water back into the city water main. Now with these devices required in new construction and most permitted remodels we have successfully closed this system.
Where does the pressure to go?

Expansion tanks are really simple devices. They contain compressed air and a special rubber bladder. When your hot water heater turns on, the water within your piping system begins to expand. This expanding water slowly enters the expansion tank as the water is brought to temperature. Eventually, hot water is drawn from the system thru a faucet and the expansion tank releases the extra water into the piping system.

The installation of an expansion tank is a simple upgrade for any hot water systems. There are some things you need to know before running out and purchasing your unit such as existing water pressures, capacity of the hot water tank, and ensuring the unit you purchase is for potable water systems. I always recommend working with a licensed plumbing contractor.