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 backflow 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…
To read the rest of the article which I originally wrote for Brewer Inspection Services please click here.
A thermal expansion tank should be installed by a licensed plumbing contractor.