At current prices the average natural gas water heater costs $10-12 per month to maintain water temp. Before someone starts shouting, I said MAINTAIN not RECOVER. If the unit is in use, obviously it will take more gas to heat the incoming water than it will to maintain the temp of the water stored in the unit.
If it hasn't been done already, a simple water heater blanket/wrap will do more than you realize insofar as helping efficiency. Also, wrapping the exposed piping with thermal foam wrap will help. People don't realize just how much heat is lost due to the copper piping conducting heat away from the storage tank. In case you didn't know, copper is twice as efficient at thermal transfer as aluminum, which is twice as efficent as steel. All the copper piping you can see under your house is acting like a big radiator pulling heat out of your water heater's storage tank.
Also, just turning down the thermostat on the heater will do more than you realize. Combined with a blanket and some simple foam wrap, just turning the thermostat down to minimum can easily cut the gas use on an idle water heater a whopping 60-70%. Plus, you will have quick access to hot water if you ever need it by just turning up the thermostat. The latent heat in the system will also give an added measure of freezing protection when the mercury really starts to drop.
I don't know that I'd mothball the whole thing. It seems that unused things deteriorate faster than those used in moderation. And some simple steps will keep the gas use to a minimum while still allowing you to access the water when needed.
Brad
