Can an on-off switch be installed on a gas water heater or must the heater be electric or a combination of gas & electric to do so? I am away during the week frequently & would like to save energy. Is placing it on a "vacation" setting the only thing I can do to save energy?

There is no on off switch for a gas water heater, you can completely turn it by shutting the gas valve and the cold water shut off valve, setting on Vacation is the best, you will be on the safe side. When you set it on vacation you will be saving energy and also when you return back home you will not go through a lot of hustle just set your gas valve to your desire temperature that's all, See the Image below, good luck.

John

Placing heater on "Vacation" setting is your best option. Heater will not activate, only pilot is burning. Other wise, you could turn off gas shut off valve at the heater - but you will kill the pilot as well. That means, you will have to re-light the pilot every time you come back home from the trip.

If you travel a lot, you could consider installing Tankless Hot Water Heater. They have no storage tank, no burning pilot. Hot water is on demand.