Water Heaters Need Thermal Expansion Tanks

One of the most common questions I get is “what is that little tank on top of the water heater”? It’s a good question especially since it’s a recent addition to the list of requirements for tank type water heaters.

What is a Thermal Expansion Tank?

It’s there to absorb any increases in pressure caused by rising water temperature in a closed plumbing system. If you were to take a long, hot shower then shut off all water supply to your home, for example, your water pressure would rapidly rise to street pressure. That’s because water expands due to the rising temperature.

The thermal expansion tank is an air chamber designed to compress as the water expands thus preventing your water pressure from rising to a critical level.

Ever wonder why water heaters fail when you are away on vacation or in the middle of the night? It’s because that’s when you have a cosed plumbing system. If you are home it only takes a glass of water or a flush of a toilet to to release any pressure build up from heat expansion.

Still Need a Pressure Regulator Too

Remember, there is a difference between a thermal expansion tank (which prevents high pressure from heat expansion) and a pressure regulator (prevents excessive water pressure from entering your home from the source).

It’s very important that  your pressure regulator is functioning properly. Otherwise you will allow street pressure to flow into your plumbing system which potentially may cause major flood damage which can cost tens of thousands of dollars or more.

A $15 pressure gauge will help you to keep an eye on your pressure. I recommend checking it monthly. A good quality water heater that has properly functioning thermal expansion tank, pressure regulator, and a lower  heat setting will give you many years of service.

You can also replace the anoid rod about every 3 years for added protection.

little tank at top of water heaters prevent thermal expansion
Water heaters now require thermal expansion tanks on top to prevent thermal expansion from building up pressure. They’re different from pressure regulators.