All water heaters are different and venting for "make-up" or combustion air is based on the manufacturer's recommendation and state or local code.

The reason/logic for venting is that you have no way of knowing what the infiltration of fresh air might be or where it might come from.

Below is a link to an example........only an example (see page 13)

Here is a link that might be useful: Typical installation for a water heater

This post was edited by audiomixer on Tue, Feb 19, 13 at 1:30

If the house was built with the hot water heater in the garage and you have never experienced a problem, then you are probably OK. The air combustion rules are becoming an issue for appliances inside tightly sealed houses.

How tight are the garage doors? If you feel any draft around the door seal then I wouldn't be concerned.

i would be nervous having a gas water heater in an enclosed garage.

Why you say that, weedmeister?

An open flame in an enclosed area with flammable gas all around.

You need to get the water heater up of the floor.

18-24 inches IIRC to avoid gasoline vapors.

And well protected from vehicle impacts.

This post was edited by brickeyee on Wed, Feb 20, 13 at 14:22

Gas water heater installation in garages is very common in my West Coast location.

Protected from impact - yes, raised off the floor, no longer necessary. For at least the last 10 years, new water heaters come equipped with a design to prevent flammable vapor ignition. Also, gas appliance equipped garages are supposed to have floor and rafter level fresh air openings, which (theoretically) should be able to disperse a gas or fume buildup.

Residential gas explosions aren't common, and the ones I recall involved propane, especially at occasionally used mountain vacation homes in remote locations.