What is an expansion tank?
An expansion tank is a small tank that hangs out nearby your water heater, and it should be piped into the cold water line going into the water heater. It is a tank with two chambers in it, one half fills with air, the other half fills with water. That said, you will notice one side that will connect to the water system with a 3/4″ male iron pipe threaded connection, and a small air valve, like on a tire on the other side. Inside of the tank there is a rubber wall or diaphragm that can move back and forth in the tank.
Why do I need an expansion tank?
The purpose of an expansion tank is to maintain a certain pressure, not to control or adjust it. Your water system will have a set pressure. In Utah that pressure is not to exceed 80 psi by code. Good water pressures range between 40psi and 75 psi, and can be controlled by a pressure reducing valve if necessary (see our info on pressure reducing valves).
The reason you need an expansion tank is because of Thermal Expansion. When water in a tank water heater is heated, the molecules speed up, and as they do they require more space. This is called thermal expansion. A 40 gallon water heater can create an extra gallon of volume when it is fully heated.
So you might be wondering, where does this extra water go? It used to be that the water would expand and push backwards into the city lines, however most cities have installed check valves at the meters, which is a valve that only allows water to pass through in the direction it is supposed to flow. With the check valve the water in your system is locked in, and with nowhere to go, it will increase the pressure of the entire system, sometimes to dangerously high levels. High pressures can cause your temperature/pressure relief valve to open up and leak, or it can cause other parts of your plumbing system to leak. In some cases, it can even rupture the water heater tank!
The expansion tank gives that extra volume of water a place to go when the water is heated. The air chamber will compress as the water pushes against the diaphragm, and the diaphragm will move, making room for the extra water. Then when the water cools or is used, the diaphragm will move back. This way your system pressure can stay the same as your water heats.
Checking the Expansion Tank
To check your expansion tank you simply need to take the cap off of the air valve and push on the pin inside it, like you would checking a tire. If there is air comes hissing out, it should still be ok. If water comes out, then the diaphragm inside has ruptured and the tank should be replaced. If nothing comes out if it, then the tank has let the air out and it would need to be refilled. Expansion tank air pressure should be just a few pounds below the static pressure of the system. If the tank cannot maintain air in the chamber, it should be replaced.
How to replace and expansion tank:
If you already have an expansion tank, replacing it is a simple process of shutting off the water to the house, twisting the old expansion tank out of its place, and threading the new one in. If you do not have an expansion tank, you will need to create a connection for the expansion tank on the cold water side to the water heater.
Make sure use plenty of Teflon tape and thread sealant compound. The hardest part might be getting the old one to spin out of the fitting. The air pressure in the expansion tank should be adjusted with an air compressor to a few pounds below your system pressure before it is installed.
To check your system pressure, you will need to use a gauge attached to the system somewhere while the water is on. (Gauges can be purchased at your local hardware store or plumbing supplier with a 3/4″ female hose connection that can be attached to your washer taps or water heater drain.)
If your pressure exceeds 80 psi, you will need a new pressure reducing valve at your water main. DO NOT install an expansion tank if your pressure is above 80 PSI because it will not do any good, and will likely be destroyed by the high pressure. Remember, expansion tanks cannot control the incoming pressure to the system. Their job is to keep the system pressure where it is at while the water heater heats the water.