It's me again. I meant an under the air conditioner hot water heater. The current one is 31" high. It has an expansion tank, but I was told it wasn't necessary. I'm in a second floor condo. Is it really necessary and if so, why? Thanks a lot.

It is extremely important that water can cycle in and out of a hot water tank with changes in temperature. In the past, this was little problem as the water could flow back out the water meter or back to the pressure tank. It is more common now for public water systems to require a backflow preventer or check valve. They mess up the old system. The best solution is an expansion tank. They also avoid problems if you have to shut the water off to work on a line. Otherwise, you have to shut off the gas or electricity and leave a hot water faucet open. I can't see removing an existing tank. Backflow preventers are even more common in a multifamily dwelling.