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A couple venting questions

Mike_SheppardMike_Sheppard Posts: 483Member
I am trying to better understand a few things with venting boilers.

There are two situations I have come across in the field that I felt were not correct.

One situation had two old firetube boilers, and one atmospheric water heater all venting into a common chimney. The cat I firetube boilers were replaced with cat IV, positive condensing, boilers, and each had it's own vertical vent installed in the chimney using it as a chase. But the old atmospheric water heater was left connected to the chimney and in operation, meaning it is now venting into a chimney that has two other vents inside of it. Is my assumption that this is incorrect, correct? I am reading the 2015 IFGC code and cannot find anything allowing this.

The other situation was another replacement of firetube boilers with condensing boilers. Four new category II - negative draft condensing boilers were connected to the existing chimney, no liner. Can condensing boilers connect to a old masonry or clay lined chimney? Is there a specific liner that can be installed down the chimney to allow this?
Never stop learning.


  • IronmanIronman Posts: 5,041Member
    No on both counts. You can't orphan a water heater on an over-sized chimney; and you certainly can't use a chimney to vent it that's been converted to a chase.

    A condensing appliance (cat. 2 or 4), cannot be vented into a masonary chimney.
    Bob Boan

    You can choose to do what you want, but you cannot choose the consequences.
  • Mike_SheppardMike_Sheppard Posts: 483Member
    Thanks @Ironman ! That’s what I figured. Much appreciated.

    I do have one more question. I must not be understanding what I am reading properly. When exactly is a chimney liner required?
    Never stop learning.
  • IronmanIronman Posts: 5,041Member
    If you're referring to corrugated metal liner, basically it's almost always gonna be needed with an 80%+ appliance. The reason being that an old masonary chimney will be over-sized for the new, higher efficiency appliance. It will also have to be insulated anywhere it's exposed to cold.
    Bob Boan

    You can choose to do what you want, but you cannot choose the consequences.
  • ScottSecorScottSecor Posts: 245Member
    I agree with Ironman. As far as chimney liners go, we install them when the masonry chimney is on an outside wall (exposed to the cold). We've installed them on both residential and commercial jobs. Most if not all of the time we use these liners woth atmospheric boilers.
  • Bob HarperBob Harper Posts: 813Member
    Chimneys must be suitable for the class of service. A masonry chimney, if it has a liner that has been inspected and approved for that application, is suitable for CAT I gas, oil, wood or pellet fuels. Positive vent pressure must have listed venting. You cannot use the concentric space between a liner and the chimney to vent.
    There are a few interesting choices to consider for CAT IV venting: Polypro flex liners listed to UL 1738 and a hybrid 80/90 liner by Dura-Vent that incorporates a listed 90% liner inside an 80% type for concentric venting where a water heater has been orphaned. Check with your AHJ for approval.
    CAT II is dangerous because there is so little heat wasted up the stack its hard to generate reliable draft and it condenses.
    Your CAT IV stuff can use pressure stacks or a listed CAT IV liner.
  • Mike_SheppardMike_Sheppard Posts: 483Member
    edited June 26
    Thanks guys, really helping clear things up. Much appreciated. Slowly digesting everything out of this code book.

    For cat II, III, and IV, it says "as specified or furnished by the manufacturers of listed appliances." I am assuming for these appliances you would just follow the manufacturer's instructions and use their listed venting?

    As for cat II and draft, in the commercial world I typically see engineering firms specifying exhaust fans for them. I personally hate that option due to reliability. They'll put in 4 large condensing boilers, run a metal chimney up the masonry chimney, and use an exhaust fan to maintain proper draft. It creates a single point of failure and is usually very costly when it fails and takes the entire plant down.

    The last cat II job I had to look at was pretty messed up. Three new 2 million btu condensing boilers, and three 1 million btu water heaters all tied into a common vent which went into a masonry chimney which was 20+ stories tall. No form of draft control at all, no barometric dampers, no exhaust fans. They had to do an emergency start up because their rental boiler went down in the middle of winter one night. A tech got all the boilers fired up but they all went down as they satisfied. The draft was so high it was keeping the air switches made preventing the boilers from coming back on. Over +2 inches of natural draft. Their solution was to cut a big hole in a Tee in the common vent to act as a barometric damper. To my knowledge, over a year later, it is still that way.

    I am writing all of this down to make sure I don't make any of these mistakes myself in the future.
    Never stop learning.
  • ratioratio Posts: 2,013Member

    I am writing all of this down to make sure I don't make any of these mistakes myself in the future.

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