Flue connections

[Tom Kartheiser]

Does the flue connections for the furnace and the hot water heater need the special tape around the 3 screws at each connection?

What causes rust around these connections without the tape. I assume it's moisture, but why?

[Gunnar Alquist]

I believe that the tape is not designed/approved for wrapping flue pipes.

If it is mineral deposits that you are seeing, moisture is likely condensing on the interior of the flue pipe and running down to where it can get out. This can be caused by a number of things. The flue pipe might be pushed up near the top of a roof cap, which would prevent it from exhausting properly. If single walled, it might just be a cold pipe and replacement with a "b-vent" might correct it. Or, it could be a roof leak. Best to defer to a heating or plumbing contractor.