We have had a water spot appear and not go away in our master bath. After looking around I found out that the flue pipe, B-Vent, is dripping with condensation when the furnace is running.

I have changed the heat cycle on the thermostat to a slow cycle so it stays on and off longer to hopefully allow the flue to heat enough to get the gas out.

I also took a look at the rain cap to see if it looked good. It did but I also noticed it seemed to be bottlenecking the gas badly. When I lifted it up, a lot more steam pushed out.

So my question is, could a too restrictive rain cap cause condensation on furnace flue pipes? Should I replace it with a more open one?

UPDATE: Changing the thermostat to slow cycle helped dramatically but there is still a small wet spot. Going to try a new rain cap.


To answer your question I suppose it could although I have never seen it. I doubt your vent cap is too restrictive unless you have or have had in the recent past a bad sooting problem. I rather suspect the B-vent is not being kept warm enough. It has to be in a warm space basically all the way to its termination. No drafts. No cold spots. Warm. It should be run straight up or nearly straight up. And as short as possible and the very least number of bends. Bends are a killer. Happy Venting. P.

  • How long is the B-vent run from furnace to roof? Does it have any elbows in it? How old is this flue pipe installation and is this the first time it has leaked? Pay attention to what @Paul Logan said. The B-vent for my hot water tank runs straight up through the roof but it runs up about 12 feet through the attic space. The flue gasses would condensate so I bought 1" thick fiber glass pipe insulation to insulate the flue pipe. It wasn't cheap but it fixed the problem. Is this furnace a high efficiency condensing furnace? If you don't know post the brand and model number. – d.george Nov 1 '17 at 10:20
  • As for length, I'm not sure on total length but it goes from the basement through the main to the second floor and into the attic. There are two elbows next to the furnace and two elbows in the attic (both 45). My issue is that B-Vent specifically says to keep 1 inch airspace clearance all around it. That is why I'm trying to find other ways rather than insulate it. I did find Fyrewrap which seems like the safest bet for insulating but it doesn't come cheap =\ – T0t3sMcG0t3s Nov 1 '17 at 13:32
  • Yes the Fyrewrap is a good idea. Make sure that it can handle 300*F. – Paul Logan Nov 2 '17 at 4:56
  • Rock Wool is a product that I have used. It can take the high temperatures of close proximity to B-vent. – Paul Logan Nov 2 '17 at 4:58

In the end, the rain cap was smashed on further than it was supposed to go and was actually capping most of the gas in.

So the answer is: It was the rain cap

The rain cap design wasn't restictive but the incorrect placement of the rain cap was restricting gas flow causing the issue.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.