What is Thermal Expansion?
Thermal expansion refers to the characteristic of water to expand when it is heated. Homes that were built in the last six or seven years may have noticed a large balloon-like tank connected to the piping near the hot water tank. This device is known as a thermal expansion tank. Its purpose is to protect your household plumbing in the event of a pressure increase brought about by the heating of the water in your hot water tank.

How would the pressure increase affect my household plumbing if I didn't have a Thermal Expansion Tank?
Before you can understand how it will affect your plumbing, you need to know how the plumbing in your house connects to the water main in the street. Water is delivered to your house through a service line. Before the service line enters your house it passes through a meter pit, which is usually located in your front yard. The inside of the meter pit contains a small check valve that prevents water from moving from your house back into the water main. Such an occurrence might happen if the main were to break, causing a local depressurization of the distribution system. The check valve prevents any possible contaminates from entering the main where they might affect other users on your system.

Water volume increases when it is heated, such as with your hot water tank. A typical residential hot water tank holding 40 gallons will increase by about a half a gallon when heated 70 Fahrenheit degrees. But if the water has no room to expand then pressure will begin to increase resulting in damage to your hot water tank or plumbing. However, if you do not have a check valve in the meter pit then your plumbing is unrestrained. The volume expanse brought about by your hot water tank will simply expand back through the meter and into the water main, and because it is unrestrained their will be no increase of pressure.

For homes that do have a check valve, the potential for causing damage to your plumbing system is much greater, but you may still go several years without experiencing any problems. Here are several reasons why:

•While your hot water heater is running, the pressure in the tank and plumbing is increasing slowly. If at any time you open a faucet, such as to fill a glass with water, the pressure is instantly relieved.
•The increasing pressure might cause a small leak at a fixture, such as a toilet or faucet, causing it to run or drip until the pressure is relieved.
•If you stop using water before the hot water heater is done running the pressure may still increase, but not by enough to be noticed.

How important is a Thermal Expansion Tank?
A Thermal Expansion Tank is a very important device that will help protect your plumbing as well as your home. The purpous of the tank is to give the expanding water a place to go so the increase in pressure brought about by heating the water will be controlled. When an expansion tank is installed the excess water enters the pre-pressurized tank. As the temperature and pressure reaches its maximum, the diaphragm flexes against an air cushion (air is compressible) to allow for increased water expansion. When the system is opened again or the water cools, the water leaves the tank and returns to the system. It is important to note that even with a properly selected and installed thermal expansion tank the plumbing pressure will still increase, but not by enough to cause a failure in the plumbing system.