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Replacing draft vent into chimney

I had my boiler serviced a few years ago and the tech examined the exhaust and interior of the chimney. When he put it back together he used some chunks of the old concrete and slathered furnace cement all over it. It looked bad and there was a small hole in it, so Ive decided to redo it.

Take a look at the existing penetration after I've removed all of the cement. It is 4" pipe going into a 7" hole, so it would be a lot of void to fill.

I have test fit a 7"x6" reducer to a 6"x4" reducer with a 4" coupling. It seems to fit nicely in the hole and I plan on using furnace cement around the reducer and taping the rest of the joints with foil tape.

Is this an acceptable way to do this, or does the 4" pipe need to protrude into the chimney void by a certain amount?


  • SWEI
    SWEI Member Posts: 7,356
    Foil tape is not OK on flue pipe.
  • Milkman_Spawn
    Milkman_Spawn Member Posts: 6
    SWEI said:

    Foil tape is not OK on flue pipe.

    Good to know...Do you seal the joints at all then?
  • neilc
    neilc Member Posts: 2,703
    helluva handle
    known to beat dead horses
  • ratio
    ratio Member Posts: 3,625
    The chimney & flue piping are at a negative pressure compared to the room, so no need to seal the flue, as nothing can leave it into the room. However, the outlet side of all pipe, fittings, etc., should be crimped to fit inside the the inlet of the next piece. Your picture above shows some pieces installed backwards.
  • cableman
    cableman Member Posts: 69
    I made all my outlet pipes fit over the inlets this way condensate and what ever doesnt drip back out on the outside of the pipe. More critical on a wood burner. I stuffed roxel around the cement area and used a ss plate to cover it all, still need to secure it. I do have a full liner though.
  • lchmb
    lchmb Member Posts: 2,997
    your boiler should never condensate. Your vent pipe will not last long if it does. I dont like using furnace cement.. Go to Home Depot and get a bucket of premixed cement. Put a new thimble in, cement it in place and put your reducer into that...
  • rlaggren
    rlaggren Member Posts: 160
    Is it safe/legal and a reasonable idea to insulate flue connectors? I seem to recall some code on this but can't bring it to mind. Seems insulating the flue connector would send a more heat up the chimney which be good in cases where there is no flue liner.

    disclaimer - I'm a plumber, not a heating pro.
  • captainco
    captainco Member Posts: 793
    Yes the flue is negative but we want it to pull from the equipment not the room. The flue should be as tight as possible for this to happen but it doesn't have to be sealed airtight.