Water heater backdrafting?

[tnpappas]

The water heater vent is tied into the furnance vent. The vent to the exterior is practically horizontal. This is a five year old water heater. Is the damage here being caused by backdrafting or poor connection of the water entry pipes?

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[Inspectorjoe]

The water heater shouldn't tee into the furnace vent connector. You said "vent to the exterior". Was there a chimney, or did it just terminate outside? If it terminates outside, they should both be separate. If into a chimney, they should enter the chimney separately, with the water heater connector above the furnace connector. If the walls and ceilings are drywall, clearance is an issue.

But I don't think the corrosion was caused by backdrafting. There has been so much moisture that it was puddling on the top of the water heater. The anode rod cap and the plastic bushing aren't melted on the side that faces the draft hood (the other bushing is missing). And the draft hood support legs are only rusted where the puddle of water was.

I think the corrosion is from a leak at the nipple that sealed itself.