Boy, there can be several reasons for spillage. Blockage in the vent. Open return air duct. House it too tight - house is depressurized. Overfiring burner. Poor combustion air provisions in the area of the appliance.

I would say that it's more of a "safety" issue than a "fire" hazard since you are dealing with combustion gases venting into the home.

And while plumbers should be able to figure this out, such conditions often puzzle them. Like many contractors, the blinders keep them from looking at the whole list of possibilities. But that's not your worry.

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Eric Barker
Moraine Woods Consulting, Inc.
Barrington, IL