Over the last couple of months, every once in a while the house will start smelling like Propane Gas Exhaust Fumes. I discovered the source to be our Hot Water Heater. I don't know what causes this to happen but for some reason it does not vent up the chimney but the fumes come into our bedroom (hot water heater is located in the closet). We have the self-venting chimney and 99% of the time it works fine. But I know this can be dangerous should it happen in the middle of the night and we don't wake up. Any ideas?