Basics of Water Heater Expansion Tanks

Water heater with expansion tank on top

The Spruce / Kevin Norris

In a closed plumbing system, a standard tank water heater can stress your plumbing pipes and fixtures through the normal thermal expansion that happens when water is heated. This can be a problem in any closed system where water is heated. Both water heaters and boilers for home heating systems can be prone to this kind of damage unless measures are taken to prevent it. With a plumbing system, a water heater expansion tank can help minimize the risk of pressure damage to the plumbing system. (This is normally not a problem on modern on-demand, tankless water heaters—only traditional tank-style heaters are subject to this problem.) 

What Is a Water Heater Expansion Tank?

A water heater expansion tank is a safety device (sometimes called a thermal expansion tank). It is an overflow receptacle that alleviates the pressure caused by the normal thermal expansion that happens when water is heated.

The water heater expansion tank serves as an overflow receptacle by absorbing excess water volume that occurs when water is heated, as well as absorbing fluctuations in the incoming water supply pressure. Since water expands when it is heated due to thermal expansion, the water heater creates extra water volume every time it heats water. It's estimated, for example, that the cold water in a standard 50-gallon water heater expands to 52 gallons when heated to 120 degrees Fahrenheit. This extra water volume can create excess pressure in the plumbing system, and if the increased pressure is enough it may over time cause damage to the water heater, plumbing fixtures and the water pipes themselves. 

It should be noted that all hot water boilers and hot water heaters have temperature and pressure relief valves. There are required by code and by law to prevent injury during abnormal pressure and temperature changes. They are automatic and will relieve water at the appliance, but an expansion tank is essential for the systems to operate within the design parameters.

Closed Water Supply Systems

In an open system where expanding water can push back into the city water supply, there is not often any problem. However, many homes have a closed water supply system that is equipped with a one-way valve such as a backflow valve, check valve, or pressure-reducing valve (PRV). In a closed system, this extra water pressure from thermal expansion is most likely to cause damage because the extra water pressure cannot push back into the city water supply and therefore has nowhere to go.

In many cases, even if you use a city water supply (whether there is a check valve or PRV present), the increased pressure from the heating of the domestic hot water can cause the relief valve to disperse water. Thermal expansion tanks or cushion tanks may be required on the incoming cold water.

Most cities require that residential closed plumbing systems be equipped with expansion tanks. If you have some form of backflow preventer installed on the main water line, you may be required by law to also have a water heater expansion tank installed. Check the local building code or call the building and safety department to find out.

How a Water Heater Expansion Tank Is Installed

If you do not have a one-way valve installed on the main water line and therefore do not have a closed water system, you may still benefit from a water heater expansion tank. Expansion tanks are normally installed above the water heater on the cold water line before it enters the water heater; however, they can really be installed anywhere on the cold water line prior to entry into the water heater. Having an expansion tank helps prevent dripping faucets and running toilets by keeping the extra built-up pressure in the system from reaching the fixtures and prematurely damaging them.

Water heater expansion tank prices start at around $40 and range up to nearly $200; pricing is largely dependent on the size of the tank. For most residential installations with 40- or 50-gallon water heaters, a simple 2-gallon tank is fine. The tank is quite easy to install with a normal 3/4-inch threaded connection. A tee fitting will have to be installed above the water heater if one is not there already.

If you are hiring a plumber to have a water heater installed, it’s a good idea to get a price for the installation of an expansion tank at the same time. It may not be much more than you are already paying if it is done at the same time. If you are installing a water heater yourself, consider installing an expansion tank even if it is not required, since the benefits far outweigh the work and cost involved.