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Vent size reduction from appliances to connector and chimney liner

Joe1timer Member Posts: 3
Hi everyone,

I'm a 1st time homeowner and my chimney recently needed to be relined (blockage had caused boiler to turn off).

We have 2 appliances - a 40K BTU Water Heater and a 187K BTU Boiler. The boiler has a 7" exhaust and the water tank has a 4" exhaust. The Chimney height is 35-40 ft.

The chimney guy that came installed a double walled 5.5" smoothwall flex stainless steel chimney liner. He also installed a 6 inch connector coming off the boiler (reduced from 7" exhaust), and again reduced at the chimney liner to 5.5".

He insists that he's been in the business for over 20 years and everything was done correctly.

Anyone have experience with this?

Thank you in advance!


    EBEBRATT-Ed Member Posts: 13,423
    The size of the flue, vents and connectors is determined by the gas code used in your area. National Fuel Gas Code, International Fuel Gas Code or others.

    In general flue piping is "usually" not reduced in size, however,
    it can be if the venting charts allow it.

    The fact that the chimney height is 35-40' it my be possible that the installer is correct.
    Joe1timerj a_2
  • Joe1timer
    Joe1timer Member Posts: 3
    Thanks for the response! I looked at the charts on the International Gas Code and it looks like the BTU is borderline OK for the chimney liner but that the connector on the boiler is too small...

    Having said that, It also states (504.3.21), that "Vent connectors for draft hood-equipped appliances shall not be smaller than the draft hood outlet diameter." Neither of my appliances are fan assisted..

    These pipes have been in place for a few weeks now and the boiler hasn't reset, and carbon monoxide alarms haven't gone off. Should I be concerned about this from a safety perspective or is it pretty much ok to assume at this point that the current installation seems to be working?

    If there is room for concern, is there any recourse against the chimney guy? He just keeps insisting that he did it correctly..
    EBEBRATT-Ed Member Posts: 13,423
    I wasn't aware you had draft hood vented appliances .I don't have venting table with me. I would suggest making an accurate sketch of the flue piping, firing rate and contact @Tim McElwain on this site or at: [email protected]

    He is the absolute expert on this. No one better
  • j a_2
    j a_2 Member Posts: 1,801
    What brand liner did they use....there are some very helpful apps out there.....
  • John Mills_5
    John Mills_5 Member Posts: 946
    Don't count on UL listed CO alarms to protect you. The place could be so full of CO you're half functional before they go off.
  • Steamhead
    Steamhead Member Posts: 15,984
    AFAIK according to most Codes, if a reduction is made, it needs to be made at the "venting device", which is a chimney or a power venter or whatever. You'll need to check the Code that is in force in your area.
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • j a_2
    j a_2 Member Posts: 1,801
    Some model peerless boilers come with a reducer that's put on just above the vent damper......What models are u venting
  • Joe1timer
    Joe1timer Member Posts: 3
    All - thank you for the responses!

    The contractor agreed to come back and replace the connector on the boiler to match the 7" diameter outlet size.

    He still insists that the 5.5" chimney liner is enough though. I'm a bit confused by the code. It seems to say that the liner should be at least as big as the larger connector, but also says that reducing is sometimes OK.

    @j a, I'm venting a 40K BTU water heater, and a 188K BTU Crown Boiler JBF series. Both have draft hoods.
  • Tim McElwain
    Tim McElwain Member Posts: 4,588
    Send me a sketch of what you have and I will size it for you. My e-mail is [email protected]
  • Tim McElwain
    Tim McElwain Member Posts: 4,588
    Hi Joseph,

    It is really going to be tight. The 4” from the water heater needs to be a double wall connector which will handle up to 59,000 BTU’s (single wall is no good). The six inch also needs to be double wall and even at that it is very tight as it will only handle 271,000 BTU’s. I would need to see the specifications on the 5.5” smooth wall flex stainless steel liner. I do not however believe it will handle the full load when both are running that being a total of 227.000 BTU’s. A double wall vent 30 feet high 5” will handle a total of 185,000 BTU’s and a 6” will handle 266,000 so the 5.5” is somewhere in between.

    I would suggest before going any further however to have a full combustion analysis run on both the water heater and boiler. That will determine if there is a potential problem leaving it as it is.

    I would also have your local inspector put his approval or dis approval on it.
  • Dan467
    Dan467 Member Posts: 11
    you found the right code liner would need to be 7 inch you canot reduce when there are two applinces
  • Jack
    Jack Member Posts: 1,046
    There was a blockage in the masonry chimney. 5.5 may have been all that would go down the chute. That doesn't make it right. Was this job permitted? Tim, would a dhw priority bring it into compliance? Is this an internal or external chimney?
  • ChrisJ
    ChrisJ Member Posts: 14,377
    I had a similiar situation back in 2011, only, they couldn't even fit a 5" liner down mine so it had to be torn down and we replaced it with a B-vent.

    I would go with what Tim said. Get the local inspector to comment on it and get the appliances tested by someone with a combustion analyzer.

    If need be get the water heater replaced with a power or direct vented model that can vent out the side of the house. That would in theory make the 5.5 more than capable of handling the boiler alone. Assuming the inspector is ok with it.

    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