How high off the floor?

I have two questions if someone could please answer for me. First a furnace in the garage behind a vented door. How high should it be off of the floor? Same for the water heater sitting next to it. Both fuel sources are gas. Thanks Mike.

Do you have any pics? Wondering about the vented door
comment. Are the furnace and water heater both inside a room that
is part of the garage? Is the water heater a FVIR model

Ignition sources need to be 18" above the floor


The burner, not the bottom of the appliance.

Most furnaces are well above that when sitting on the floor.