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I have an oil water heater (see related question) used for a hydronic forced-air furnace. There is a backflow preventer on the incoming cold water and return hot water, and a small expansion tank on a T in the cold water intake. The tank weeps a little bit through the pressure relief valve.
Is some sort of maintenance required on the expansion tank? Does it need to be emptied?
Expansion tanks can fail over time. There should be a valve stem like you have on a car tire to add air to the tank. Shutoff the water supply, open a faucet to release any pressure from the water on the tank, and pump up the tank to about 60psi (even better is to fill to your average water pressure at your home, but 60psi is typical). If the tank doesn't hold the pressure with an open faucet, then the internal bladder has ruptured and the expansion tank needs to be replaced. They are usually threaded, so you can unscrew the old tank, apply some plumbers dope or Teflon tape to the threads, and screw on a new tank (pressurize it first).
There are two styles of expansion tanks. The good modern ones have a bladder, and they are supposed to be maintenance free (although the bladder can fail). But the older type is just a header tank without a bladder. You are supposed to fill it halfway, the extra air in the tank provides room for expansion. But the air will dissolve into the water over time, and you'll have to drain some water out periodically to create a new air cushion.
In my house I replaced the header tank with a modern style with a bladder. One maintenance headache crossed off the list.
If you don't want to replace the expansion tank, turning off the fill valve would probably slow or stop the air from dissolving into the water.
