Expansion Tanks, what are they, and do I need one?

When replacing your water heater, it is recommended that if your current water heater does not have an “expansion tank”, you add one to the new water heater installation. You may find yourself wondering, “What is an expansion tank?” and “Why do I need one now when I never required one before?”.

What is an expansion tank?

An expansion tank is a small tank that is installed on the cold water line into your hot water tank. It is designed to alleviate excessive pressure that is built up when the water in the tank is heated, causing it to expand.

Why do I need one now when I never required one before?

In recent years, the plumbing code has changed, requiring a “backflow preventer” to be installed in the cold-water line going into your house. This prevents water from feeding back into the main water supply. Before the days of backflow preventers any expansion caused by the water in the tank being heated would simply flow outwards toward the water supply. Over-pressurization never became an issue due to this. Now that the supply lines are mostly equipped with backflow preventers, when the water expands it causes high pressure situations due to the system essentially being a “closed system”. High pressure systems can cause problems without an expansion tank including water being discharged from the tank’s relief valve, and early failure of the water heaters tank body due to cracks in the glass liner. Most municipalities plumbing codes are now requiring expansion tanks on water heater installations due to this reason.

As you can see, not only is it a code requirement, it is also a good idea to add an expansion tank to your water heater installation. It will protect your family from a potentially dangerous situation, having hot water discharge from the relief valve, as well as protect you from having to prematurely change your water heater due to a leak. If you have a situation where there is not room for an additional tank in your mechanical room, we have alternative solutions, such as an expansion valve, which takes up much less space.