When a water heater backdrafts, it's a safety hazard. Also, it's almost always a condition that the homeowner doesn't know about, and it will almost never set off your carbon monoxide alarms.  This happens when the exhaust gases from an atmospherically vented water heater spill out into the room, rather than safely leaving the house through the vent. Exhaust gases contain carbon monoxide and high levels of moisture, so this is always a condition that should be corrected.