I bought a used hydronic boiler at an antique shop to replace my old one. It came with a flue that reduces from 6" at the appliance box to 5" after a draft hood and a vent damper.

My chimney vent pipe is 6" Type B. Should I replace the draft hood to not be reducing 6->5 and consequently get a commensurate damper?

Additionally, is the damper function really that important? Can I remove it and have the pipe non-damped?

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  • You have so many questions about this old boiler, you might want a professional to install it. You can't just hook everything up and just fire it up. You need to do a draft test to make sure it is venting properly. You need to do a combustion test to make sure the burners are getting enough oxygen and burning completely otherwise it will produce to much smoke /soot/carbon monoxide and waste fuel.
    – Gunner
    Oct 12, 2021 at 17:53
  • @Gunner, so what, that's the way I've been doing things for years and it all works out for me?
    – amphibient
    Oct 12, 2021 at 18:16
  • 2
    "so what"? You have been driving a car for years and does not mean you have proper driving habits. Without the proper tests, you might kill you or your family. What you said is the difference between a properly trained /licensed person and yourself.
    – Gunner
    Oct 12, 2021 at 20:27
  • I agree that if you don't know the answers to your questions you probably should not be doing a complete gas boiler installation yourself. But the questions are still reasonable.
    – jay613
    Nov 12, 2021 at 13:38
  • 1
    It doesn't look reduced in the photo. You sure?
    – jay613
    Nov 12, 2021 at 13:51

2 Answers 2


I would convert to the 6" "B" vent asap and carry it to the chimney connection. As far as the type of pipe to be used for the boiler connections, which was your next question, that should be schedule 40 (standard black pipe). Where you connect the copper to the black steel pipe I would use dielectric unions. Move the pump to the discharge piping if it not already there. Most systems came with the pump mounted on the return piping to the boiler, (the old way).


To attempt to answer your questions:

  • The damper's purpose is to conserve fuel, and cost to you. Considering you already have one, it would be a shame to throw it out. They are not generally mandatory but perhaps your AHJ requires one in new installations.

  • Whether this furnace must have 6 inch venting or can accept 5 inch is something you need to read in its installation instructions. That you have not read them, and probably do not even have them, is supportive of the suggestion in the comment that this is not a project you should undertake yourself. But still, the documentation will tell you venting requirements. I believe some boilers similar to this one permit some defined reduction in vent size, but I believe this particular one does not, and in fact even the current installation of 6 to 5 may not be allowed.

  • So, if it's not allowed, should you use what you've got, and stay at 5 inches, or switch back to 6 inches with a reducer installed backwards, or should you throw out the unsuitable damper and get a correct one, or should you throw it out and go without one using all six inch pipe? The manual and your local code requirements will together provide those answers, along with your appetite to invest about $150 in a correct new vent damper as a means to saving an unpredictable and unmeasurable amount in heating costs over the course of time.

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