Home Improvement Skills & Specialties Plumbing

Basics of Water Heater Expansion Tanks

Water heater with expansion tank on top

The Spruce / Kevin Norris

A standard tank water heater can stress your plumbing pipes and fixtures through the thermal expansion or heating of water. A water heater expansion tank or thermal expansion tank is a safety device that can help minimize the risk of pressure damage to a home's closed plumbing system. This overflow receptacle alleviates the pressure caused by the normal thermal expansion when water is heated.

Water heaters and boilers for home heating systems can be prone to this kind of damage unless measures are taken to prevent it; however, this is not a problem on modern on-demand, tankless water heaters—only traditional tank-style heaters are subject to this problem.

What Is a Closed Water Supply System?

In an open system, expanding water can push back into the city water supply with a place to vent. However, homes with a closed water supply system have a one-way valve such as a backflow valve, check valve, or pressure-reducing valve (PRV). This extra water pressure from thermal expansion can cause damage because this extra water pressure 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 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 also to have a water heater expansion tank installed. Check the local building code or call the building and safety department. If you do not have a one-way valve installed on the main water line and do not have a closed water system, you may still benefit from a water heater expansion tank.

How to Install a Water Heater Expansion Tank

Expansion tanks are typically installed above the water heater on the cold water line before they enter the water heater; however, they can be installed anywhere on the cold water line before 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 to nearly $200; pricing mainly depends on the tank size. For most residential installations with 40- or 50-gallon water heaters, a simple 2-gallon tank is fine. The tank is relatively easy to install with a standard 3/4-inch threaded connection. A T-fitting must be installed above the water heater if one is not there already.

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

When to Replace a Water Heater Expansion Tank

A water heater expansion tank typically needs replacement between five to 10 years. You can extend the expansion tank's life to 10 years if you match the tank's pressure to incoming water pressure. It prevents damage to the bladder and leak issues at the valves.

Signs you might have problems with your tank include dripping water, which may indicate a valve is blocked by mineral deposits or debris. An internal bladder is used in newer models to keep water separate from the air pressure. Changes in pressure can damage the bladder. These changes can also enter the water's heating system, causing heat loss.

Check water pressure using a water pressure gauge, attaching it to the drain valve of your water heater. Check air pressure using a standard tire gauge, pressing it against the valve on the top of the tank. Adjust the pressure with a hand pump and raise the pressure as needed. If the pressure is too high, press against the pin valve to release air inside the tank.