Water heater garage installation

I am aware that water heaters with an open burn chamber should be 18 inches above the floor when installed in a garage. My question is this. If the water heater is installed in a mechanical room connected to a garage with a self closer on the door between the mech room and garage does the 18 inch rule still apply. In this case while the door does self close there is still a gap about .5 inch or better between the garage floor and the door. I would assume vapors can still enter the mech room making the 18 inch rule still apply. What does the building code say?

Your question leads me to another question. Is a half inch gap enough for combustion air? I don’t think it is. If it was my call, and it isn’t, I would say eighteen inches up and a louvre on the door.

I’m almost certain that no door can open back into garage, much less a louvered door.

Combustion air source provided separately. Thanks for the reply.