There is no hot water coming out of any faucet except both tub/showers have hot water. Copper piping through out the house and the tank is 20 years old (yes, I know it should be changed). Can the piping be flushed out or should I just replace the tank?
I agree with DannyT although I'm not so sure about the aerators being clogged as the aerators would reduce the flow or diminish it altogether.
Are you getting water out of both hot and cold sides of the faucets? If you are getting hot water in the showers, run both faucets at once to check on whether the water lines are reversed. Also, check the cut off valves under the sink to make sure they are both open.
The house I just bought has 4 water heaters: 2 large and 2 small (I also have a small apartment on the 2nd floor). Are you sure there is not a 2nd water heater that may be turned off?