If you've had your water heater replaced in the past 10 years you've probably noticed in addition to purchasing a new water heater your plumber informed you an expansion tank or valve was needed. Since this device is an added cost you were likely curious about what it does and why it is necessary. The following are answers to common questions about expansion devices:
- What is thermal expansion?
To gain an understanding of thermal expansion devices a simple law of nature must be understood: When water heats, it expands. Since water is not compressible, the extra volume created by expansion 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 drinkable water. One way this system can become compromised is through “reverse siphoning." This can occur when your water company is working on a water main. Opening the main can cause negative pressure on the system, resulting in water reversing direction. If this happens, and someone has a garden hose lying in a puddle, the puddle can be drawn into the drinkable water system, resulting in contamination. Ten years ago, county and municipal water systems in metro Atlanta began installing valves known as backflow preventers at water meters. Prior to this, the extra water from expansion in your water heater could be pushed back into the county or municipal water system. With backflow preventers, the expansion of heated water was drastically limited.
- Doesn't my temperature/pressure relief valve do the same thing?
The simple answer to this question is no. Temperature/pressure relief valves are designed to respond to unusual temperature and/or pressure conditions. They aren't designed to function similar to 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, the 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 a life-threatening condition.
- Can I install my own thermal expansion device?
Although I recommend having thermal expansion devices installed by a licensed plumber, the installation of a thermal expansion tank or valve is relatively easy if you are handy. The tank or valve is installed in the cold water piping system; just make sure you follow the manufacturer’s instructions for installation. It also is critical to choose the correct size of expansion tank. The tank size is determined by the water heater size and the water pressure. The water heater size easily is found on the label plate on the front of the heater. Determining the water pressure can be a little more difficult. You will need to purchase a pressure gauge from your local hardware or home-improvement store. The gauge will screw onto an outlet like a hose spigot, either 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 determine the water heater size and water pressure you can use a chart to determine the size of the expansion tank you should buy. These tables are available on line or you can ask the salesperson where you purchase the tank.
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