The water heater is in a 3 car enclosed garage. it sits in the back corner against the home. It is open to the attic and about 18" off the ground. The attic does have a couple of openings in it. the closet door I used a hollow door and drilled three rows of 1/4 inch holes at the bottom and top. I did find that the vent pipe had come loose at the stack that is mounted on the water heater top. I put some tape on the for now. I read that in the garage you are not supposed to use single wall pipe? and could this small gap have set off the co detector inside the house? it is located just on the other side of the wall from the water heater?