The importance of thermal expansion tanks

A thermal expansion tank is required throughout some parts of Kansas City when you are installing a new water heater. This code was adopted in 2004 which is why most homes in the area do not currently have expansion tanks.

A thermal expansion tank may also be required by the water heater manufacturer if your house is on a closed loop system. A closed loop is often created when the water pressure on the outside of your house is higher than on the inside of your house.

A thermal expansion tank will require maintenance.

What is thermal expansion?

thermal expansion on water heater bradford white and thermal expansion

nipples pointing together on water heater pipes pointing toward each other

water heater split open from thermal expansionthermal expansion

water heater split open from leak Exploded water heater


To understand the importance of an expansion tank you first need to know about thermal expansion. When water is heated from room temperature to 120 degrees it causes the water to expand, just like when it freezes. This is referred to as thermal expansion. When 40 gallons of cold water is heated up to 120 degrees or greater it expands to 40.6 gallons. If this additional volume does not have anywhere to go it will increase the pressure inside the plumbing system.

High water pressure high water pressure from thermal expansion


Expansion tanks MUST be installed correctly

large Thermal expansion tank installed for multiple water heaters incorrect installation of thermal expansion tank


The expansion tank on the right is not supported properly and is likely to flood the basement over time. Once expansion tanks lose air pressure they fill with water and become very heavy.

No PRV mean you probably don’t need the expansion tank!

pressure reducing valve inside home PRV


Older homes don’t have much of a problem with thermal expansion because most have what is know as an open looped system. An open loop plumbing system allows water to move freely in and out of the house as pressure builds up. Other homes that have backflow preventers or Pressure Reducing Valves do not have this same luxury. These devices will only allow water to flow in one direction, making it a closed loop system.This is why most areas require thermal expansion tanks to be installed on all closed loop systems. An expansion tank is typically a 2 gallon tank that is pressurized with air to match the incoming water pressure. If the water pressure exceeds the pressure inside of the expansion tank, the tank will absorb the additional volume thus protecting the plumbing system.

Thermal expansion is the number one reason that water heaters fail prematurely. In some cases the expansion tank can also fail which usually means the water heater is next. When an expansion tank fails it becomes water logged. It’s a good idea to check the pressure inside of your expansion tank to ensure it’s working properly.

The downside to thermal expansion tanks is the maintenance that is required to keep them working properly.

How to properly support a thermal expansion tank.