If you have had your water heater replaced in the last 10 years, your plumber probably told you that you also needed an expansion tank or valve.
Given the extra cost, you were probably curiouss about what it does and why it is necessary. Today I'll try to answer some common questions about expansion devices.
What is thermal expansion? When water heats it expands. Since water is not compressible, the extra volume of water must go somewhere. Thus the need for an expansion device.
Why didn't I need a thermal expansion device before? One of the dangers of plumbing systems is the contamination of the drinkable water system. One way this system can become contaminated is through "reverse siphoning". Reverse siphoning can occur when your water company is working on a water main. Opening the main can cause negative pressure on the water supply system resulting in the water flowing backwards. If this happens and someone has a garden hose laying in a puddle the water in the puddle can be drawn into the drinkable water system resulting in contamination.
About 10 years ago, In the attempt to avoid contamination, county and municipal water systems around the metro-Atlanta area began installed valves known as back flow preventers at water meters. Prior to the installation of back flow preventers the extra volume of water resulting from expansion in your water heater could be simply pushed back into the county or municipal water system. With the back flow preventers installed the area for expansion of heated water was drastically limited.
Doesn't my temperature/pressure relief valve do the same thing? The simple answer is no. Temperature/pressure relief valves are designed to respond to unusual temperature and/or pressure conditions. They are not designed to function repeatedly, as are expansion devices. In fact repeated or constant dripping from a temperature/pressure relief valve can result in a build-up of mineral deposits in the valve that could keep it from functioning properly in an emergency.
Can the lack of a thermal expansion device harm a water heater? If not relieved, excess pressure in a water heater can cause constant dripping from the temperature/pressure relief valve. If you have a gas or oil fired water heater, the excess pressure in the heater can cause the flue inside the heater to collapse, resulting in a spillage of combustion gasses, including carbon monoxide, into your house. This can be life threatening.
Can I install my own thermal expansion device? I recommend having thermal expansion devices installed by a licensed plumber, but if you are handy the installation of a thermal expansion tank or valve is relatively easy. The tank or valve gets installed in the cold water piping system; just make sure you follow the manufacturer's instructions for installation.
It is also critical to choose the correct size expansion tank for your home. The size of tank you need is determined by the size of your water heater and the pressure of the water in your house. The size of your water heater is easily found on the label plate on the front of the heater. Determining the water pressure in your house piping system can be a little more difficult. You will need a pressure gauge, which will screw onto an outlet like a hose spigot, the cold water line for your clothes washing machine, or the drain valve on your water heater. Since it is common for the hose spigot on the front of your house to be at street pressure, you should take the house pressure from the rear spigot. Once you have determined the size of your water heater and the water pressure in your house, you can use a chart to determine the size expansion tank you should buy. These tables are available on line or you can simply ask the sales person where you purchase the tank.
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