Unbalanced duct sealing and a badly located return duct grille caused negative air pressures in the mechanical room that overpowered a water heater vent's natural draft, creating a dangerous situation.
Hot combustion gases spilling from a water heater vent contain water vapor that will fog up a mirror — a quick way to check for spillage. In normal operation, the gap at the base of the vent allows "dilution air" to enter the flue to help with the draft.
In this photo, the small grille to the left of the mechanical room door would be sufficient to balance duct flows for the home's hvac system. But a much larger grille has been added inside the mechanical room, visible through the door — which creates excessive suction in the space, especially when combined with flows into the slot for the air filter, visible at the lower right of the frame.
The evidence of the mirror is confirmed by a smoke pencil test. Under worst-case conditions (mechanical room door closed and air handler fan operating), exhaust is escaping from the water heater vent when room air should instead be drawn into the vent.
Another view of the smoke pencil test, showing vent draft failure. Note the plastic washer for the hot water outlet pipe at lower right: typically, exhaust spillage melts this plastic part, giving immediate evidence of a draft problem. In this case, the author says, spillage was not melting the part — possibly because the homeowners were in the habit of leaving the mechanical room door open to allow entry for their cat, which also helped to relieve negative pressure in the space.
To eliminate the risk of backdrafting when the mechanical room door is closed, the author sealed off the oversized return with tape, as well as the slot for the hvac air filter.