swampwiz, The answer to your question is that the best thing to do would be to at least turn your gas water heater to "pilot". Here's why: It won't use any gas keeping the tank warm for three months while you are gone. The other reason is a little more esoteric... if your water is turned off at the supply, the water has no where to expand when it's heated unless you have an expansion tank. This can cause the pressure valve to open enough to reduce the pressure. Turn the water heater to pilot when you leave... it will only take about 30-40 minutes to heat when you get back home. For electric water heaters, I suggest shutting off the power if the water is off. If the water level gets below the top element it will burn out.

Needs a flush. Common problem. Should flush yearly. To flush: Turn off hot water heater at panel if electric or turn off at thermostat if gas. Turn off water inlet valve. Hook up garden hose to drain at bottom, run to floor drain. Open T&P valve to allow air into system so tank will drain. Drain tank. After draining, turn on water inlet valve and let water run for a while out the garden hose to flush out more sediment at bottom. After flushing, close drain and remove garden hose, close T&P valve, let tank fill, then turn on electric breaker or thermostat. Note: Any time you open the T&P valve it may become stuck open due to mineral deposits. Always have a new T&P valve available before opening. T&P valve should be tested yearly also.

What is all the hot water needed for? A gas heater doesn't have to be in a well ventilated space because waste gases must be vented to the outdoors. It just needs source air and that's very easily done. Running a gas line is never a structural issue...it's a money issue. If two 50 gal units is a small fortune to operate...wouldn't one be 1/2 a small fortune? Still sound like a lot. I mean it's not like the second unit would cost 2x,4x,10x more than the first unit. That is kind of the deal if you want lots and lots of hot water on demand...it costs money. Solar can get you lots of pretty cheap hot water, at least DIY. The units you buy seem kind of expensive to me for what they are.

Turning off the water when out of the house on vacation is a good idea; however, you should also turn the water heater to the pilot or vacation setting on the dial. That will leave the pilot going, but it will not heat water. This is what you want during an extended absence, since you don't want the tank to be empty (from a leak), with the burner on. After your vacation, when turn the water back on and then turn the water heater back to the normal setting, it may take about a 1/2 hour to get reasonably warm water, and probably an hour or two to get really hot water. When you turn the water heater switch back to normal, in most cases you will immediately hear the burn fire up, because water in the tank has cooled off. If you don't hear the burner fire up, something is not working right. Bruce