When inspecting homes people often ask what’s that little tank above the water heater.
Answer: A Thermal Expansion Tank
Thermal expansion tanks have been required in most areas for the last 10 years or so when installing a new water heater. This code was adopted because of the advent of backflow preventers, water meter check valves and pressure reducing valves without a bypass being installed. When water is heated in the water heater it expands, since it cannot return to the public water supply preventing possible cross contamination it needs someplace to go. Thermal expansion is the number one reason that water heaters fail prematurely. In some cases water heater failure, due to thermal expansion, can have devastating results.
The water meter check valve and back flow preventer valve create a closed plumbing system which can result in potentially dangerous problems. Including the build up of high pressure in a the water heater (even with a “T&P” pressure relief valve). Plus it can cause other problems like leaking plumbing fixtures.
When water is heated it expands
in volume as its temperature rises. If the extra volume has nowhere to go it causes the tank to expand. In the old open system, the excess expansive pressure and water would simply flow back into the public water utility’s system.
“Dangerous conditions exist during thermal expansion long before the relief valve operates. Internal pressures repeatedly occurring during recovery periods can ‘stress’ the tank causing the tank welds and fitting connections to the tank to weaken.”
“The best solution to thermal expansion is to control the pressure the heated water generates within normal, safe operating range, well below the emergency setting of a relief valve. This will allow thermal expansion to occur, but without causing a dangerous increase in pressure. This is accomplished by adding an expansion tank.”
For more information watch this demonstration http://www.watts.com/pages/learnabout/thermalExpansion.asp
or read more at Missouri American Water
A thermal expansion tank should be installed by a licensed plumbing contractor.