Thermal Expansion

Thermal expansion tank is the little tank

Thermal Expansion of Potable Water
When water is heated it expands. This much we learned in grade school. So if water comes into your home and gets heated by the water heater, expands, it just pushes back out the pipe to the city main. But what happens when you trap the water on your side of the system? Well, lets say you install a pressure reducing valve to take the city pressure down from 120 p.s.i. to something more appropriate for a house, say 60 p.s.i. and you have just cold water piping. With me so far? You installed a pressure reducing valve that segregates the 120 pounds in the citys pipes from your now 60 pounds in your house. In effect, you just made two systems. Okay, as long as all you have is cold water, your pretty much fine. The city has their 120 pounds, and you have your 60. Your both happy. Now, lets add a hot water side to the system. Thats obviously your side of the system. You have 60 pounds of water pressure and your water heater kicks on. Its heating the water. The water is trying desperatly to expand. Does it have anywhere to go? Nope. So what happens is the pressure on your side starts climbing. It can climb higher than the city side. It can and will climb high enough to make the relief valve dribble on the water heater. It can pop a supply tube off a fixture, and you best hope your home to control the flood. It can collapse the center flue of the waterheater. Now thats an ugly scenario with that one. Talk about starting a chain reaction, just collapse the center flue of a water heater and see what happens. Better make sure your home owners insurance is payed up. You'll need it if that happens.
But, yes there is a solution. A thermal expansion tank. Its some place for that expanded pressure to go. What it is, is a tank with an inner tube in it. Same inner tube idea as your car tire. Its full of air. Remember in grade school, while you can't compress water, you can compress air. Now, when the water heater turns on, the expanded pressure pushes down on the inner tube and keeps the pressure rise in check. Easy huh!
For more information on thermal expansion, here are two links to Amtrol's information.  Basic Info   More Advanced Info
I have used many brands of expansion tanks, and I think Amtrol makes the best. Just my opinion.
As for maintenance. All manufacturers say the tanks are maintenance free. Bull!!
There is this thing called osmosis, and it causes the inner tube to loss anywhere from 1 to 5 p.s.i. a year. So its a good idea to check the air charge in these tanks yearly. They should be charged to the same pressure your regulator is set for.