Click here to Find a Contractor in your area.
Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit


347347 Posts: 82Member
edited May 2016 in Gas Heating
Hello All,
I know I'm missing something here, having a brain lock
Customer called with a spill switch that was tripped, they reset and I went out to take a look.
140,000 btu boiler and 40,000 btu water heater. 6" wye with a 4" connected for the water heater, boiler was installed about six years ago, I was told their was a new liner installed.
Put my draft gauge in the 6" boiler flue will making steam and had very little draft(just passed the 0 line). Boiler shuts off and damper closes and the draft gauge goes up to 2.5" wc.
My question is why would it rise after the damper shut and not during it running?


  • HatterasguyHatterasguy Posts: 6,058Member
    Take a straw and suck on the straw with the bottom end open. Not much resistance.

    Now, put your finger over the bottom end. Significant resistance.

    Your gauge is telling you the chimney is pulling on the closed damper.

    Normally, the chimney makes sufficient draft to pull on the boiler slightly when the damper is open. Yours does not and it's necessary to investigate why.
  • Jamie HallJamie Hall Posts: 8,971Member
    To which I might add that it wouldn't take that much of an obstruction to kill the draught while the boiler is running, but still allow the chimney to pull well when it was off.

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • GreenGeneGreenGene Posts: 290Member
    New liner I assume in chimney, did they put a cap on it to keep out nests? Could be squirrel, racoon, bird, you never know.

    I had a boiler go down once because baby racoons died in the top of it blocking the passages, then I looked in the chimney and saw the nest and quickly slammed the pipe back on until I was sure mom was gone.
  • GreenGeneGreenGene Posts: 290Member
    "boiler was installed about six years ago", was that the last time the bottom T was cleaned out?
  • ChrisJChrisJ Posts: 9,208Member
    Hopefully it's rare, but there's a chance it could be like mine was.

    They stuffed a 6" liner down a chimney that only had a 4xsomething opening and it ended up in 3 different pieces because of an offset in the chimney. So the chimney looked lined, but there were at least two gaps in the liner.

    Basically, the liner made the chimney completely unsafe. Far worse than it would've ever been if it was left alone and I'm not saying that was safe either.

    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • Firecontrol933Firecontrol933 Posts: 73Member
    Did you check for sufficient combustion air? Door to outside open and then check again?
  • 347347 Posts: 82Member
    Thanks for the replies.
    Hatt, I hear what your saying about the straw, just don't know why more resistance when the damper is open, boiler has a built-in draft hood off the back and all is clear.

    Jamie, found no oddveous blockage.

    Green, chimney has a cap, I did not take the flue apart to check for blockage at the tee.

    Chris, I may have a chimney guy come in to take a look down and see what's going on.

    Fire, the room is open. Bye the letter of the code their is not enough air (nobodies basement ever has enough). I thought of that and opened two windows in the boiler room and nothing changed.

    I don't understand this happening after 5 years that's what's puzzling me. Nothing was done to the house to change anything with the flue.

  • HatterasguyHatterasguy Posts: 6,058Member
    347 said:

    Thanks for the replies.
    Hatt, I hear what your saying about the straw, just don't know why more resistance when the damper is open, boiler has a built-in draft hood off the back and all is clear.

    There is more resistance when the damper is closed. That's when you have a very slight vacuum in the flue.

    Once the damper opens, the vacuum disappears. This is a sign of poor venting. If the chimney is working properly, it will draw air out of the boiler. Hence, you will see a slight vacuum in the flue.

    When it doesn't draw air, one needs to look for the reason. It could be a slightly plugged flue or it could be a flue that is very short in length or it could be a flue that is oversized as compared to the boiler and the flue gases cool too quickly to provide draft.

    A boiler can operate without draft, however, it's a dangerous condition. All it takes is a bit of positive pressure due to wind conditions and the flue gases backup into the basement. Hopefully, they trip the spill switch..............if the boiler has one.

    Otherwise, they trip the CO detector..............if the building has one.

    Otherwise...............well, you don't want to know otherwise.
  • 347347 Posts: 82Member
    I will look into the possibility of a blocked flue or over sized one.
  • HatterasguyHatterasguy Posts: 6,058Member
    edited May 2016
    347 said:

    I will look into the possibility of a blocked flue or over sized one.

    You mentioned "6" wye with 4" connected to the water heater".

    Presumably, this implies that they share a common vent before finally reaching the chimney.

    This is generally discouraged, and in some areas not permitted, but I will let others comment on whether the draft is compromised in such a condition.

    Generally, individual venting of each appliance right to the chimney is preferred.

    You could perform a test:

    Shut down the water heater, remove the vent, and plug the 4" hole.

    Start the boiler and check draft.

    Report back with result.
  • JackJack Posts: 1,044Member
    How tall is the chimney? How long is the vent connector and how many 90's, wyes, etc. How much rise off of the boiler before the first fitting. You can disconnect the vent connector and with a good flashlight and inspection mirror get a good idea of what is in there. Having worked for 5 yrs and now showing problems? Blockage or perhaps the vent damper is not opening all the way. One of the most effective ways to improve operation of a natural draft system is to insulate the vent connector. B-vent for you. The Wye for the water heater should be at or as close as possible to the vertical flue.
  • 347347 Posts: 82Member
    I'd like to add a picture of the smoke pipe, just cant figure out how to do it.
    chimney is about 25' tall. 1 6" 90 going into the wye, about 18" of rise. 4" has 1 90 and run of about 2' with basically not rise.
Sign In or Register to comment.


It looks like you're new here. If you want to get involved, click one of these buttons!