Recently I had a new furnace flue pipe cap put on. It goes inside pipe and not over it. The person I hired said “yes it is seated down in the inside pipe that have clips that keep it in place.” The storm collar rests around the outside of pipe... My question is should the cap be over the outside of pipe or is it sometimes done on the inside? He put screws and caulking to keep the cap in place. I don't know any better, but would think it would go over outside of pipe. Thanks for your assistance.
Brad Sutter from Sutter Roofing Co in Flordia had the following response to this question:
Without a picture, its hard to know for sure what the person is talking about. I provided a picture to share and reference, which may help. Typically, on a hot air exhaust, like a dryer vent, hot water vent, or kitchen exhaust you have an insulated assembly or “pipe within a pipe” similar to the unit shown below. This allows the heat to escape without getting the exterior surface heated beyond what the roof flashing can handle.
The cap is the top piece that prevents rain from entering the pipe. It usually removable for any cleaning needs. The mounting clips can be on the interior or exterior side of the pipe. As for function, the cap typically extends over the edge of the pipe to prevent rain or snow from running or blowing in.
There is sometimes a secondary rain collar installed at a lower point above the roof flashing to prevent rain or water from running down the pipe onto the top of the primary roof flashing. That piece is also shown below.