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The image below shows the top of a gas fired water heater, the part to the right is the vent for the water heater, it has openings to allow air to be drawn into the vent pipe allowing there to be a positive draft. if the vent is clogged, by any number of things, like snow a nest or other. the fumes from the burnt gas will spill back into the house. some gas companies require spill switches to be connected to the draft hood, to shut the water heater off, if the fumes start to come out of the vent hood, i don't know why all gas companies don't require this safety device, it saves lives. Anyway this water heater does not have one so the water heater is pouring fumes that contain carbon monoxide into the home. How do I know this? see the pipe to the left, covered with black foam insulation? notice how the insulation is melted? that's how I noticed. I make a point of checking all gas fixtures for a positive vent whenever I service anything in a persons home.
Snow buildup on the roof covered the top of the vent pipe preventing it from venting outside, so it vented inside, no one was harmed but the potential was there. Have spill switches installed on your gas water heater, even if tour gas company does not require them.