Pinpointing an Air-Pressure Interaction

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.

Mirror Check for Spillage

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.

Wrong Returns

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.

Smoke Pencil

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.

Smoke Pencil Test

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.

Remedy: Block the Return Grille

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.

Confirming the Solution

After sealing up the problematic return grille, the author let the water heater vent cool off, then re-started the system and checked the vent for correct draft. This time, the vent performed properly — problem solved.

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