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? on chimney +/- angles and flue heights

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by Shari, Oct 30, 2010.

  1. Shari

    Shari Minister of Fire

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    1. Somewhere I read you lose a certain % of your overall chimney length for each angle you add to it.

    Example: I have the minimum Jotul suggests for a chimney, 14' - but - in that 14' I have two 45 degree angles. So, math wizards, what do I have in total chimney length considering the loss from the two angles?

    2. Didn't I also read somewhere that if you have two separate flues exiting your roof the flues have to be 'x' number of feet apart or, if not 'x' number of feet apart, then one flue must be 2' taller than the other flue?

    I am questioning my install, so any answers would be appreciated.

    Shari

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  2. Todd

    Todd Minister of Fire

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    I think the rule is 5' for every 90 elbow, guess you cut that in half for 45's? I think there are other variables though. Is your draft sluggish?
  3. Shari

    Shari Minister of Fire

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    Draft is not specifically sluggish but just realized (due to doing a cleaning) there is 13' and then the two 45's in the liner. This causes me some concern considering the specs on the stove say 14' chimney required. I got the run around from the stove store saying the rule is 14' from the 'fire base' - "Fire base"? Who's kidding who here? Specs say 14' of chimney - it says nothing about 'fire base'.

    Also found out this week (because roofing contractors are up there) that the flashing around the liner was not caulked in.

    I'm also wondering about the stove and chimney flues being the same height, right next to each other. There is supposed to be a 2' height difference, correct? If not 2' height difference isn't there a possibility of down draft through the furnace flue when wood stove is running? We haven't experienced a downdraft - I am just questioning.

    Shari
  4. Todd

    Todd Minister of Fire

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    I have 2 side by side flues and they are about 3' apart. The upstairs flue is only about 6" taller than the basement flue and so far I haven't gotten any smoke reversal (use to with old fireplace). General rule is for the flue on the main floor to be taller than the basement flue but I don't know by how much.

    I was thinking of extending mine with an 18" extention I saw on Rockford chimney supply because like you I have only 14' from the top of the stove collar and also have a 45 then the liner gradually turns up and straight out the chimney. My draft seems ok so far but I think it could be better and it's still early in the season.
  5. begreen

    begreen Mooderator Staff Member

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    Ideally there should be about 6" to 12" difference in the adjacent flue heights. Craig's extendaflue has a product for this. Todd, don't you have one installed?
  6. Shari

    Shari Minister of Fire

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    Todd,

    Our flue(s) are only about 1' apart so I think you can understand my concern. I thought I read something here on hearth.com about feet apart/different heights.... Hmmm...

    My 14' would be measured from the flue collar also - but that 14' includes the two 45 degree angles.

    Shari
  7. Shari

    Shari Minister of Fire

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    Ah ha! Craig's webpage is where I read the information that adjacent flues should be different heights - thanks for jogging my memory.

    On Craig's website, here : http://www.extendacap.com/ that top photo on the right is almost identical to what we have now - which is not safe yet the woodstove liner was installed by a pro!

    Shari
  8. Battenkiller

    Battenkiller Minister of Fire

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    So a 15' flue with three 90º els has no draft at all?

    I have a 90º coming out of my stove, a round 90º up three feet and then a sharp 90º as the clay thimble enters the flue. That's three 90s. I have super abundant draft in a 25' flue... a lot more than the 10' worth you'd have if you subtracted 15' for the three els. In fact, I'm going to buy a manometer to measure mine since I think I may have too much draft.


    Yes, I believe there a lot of variables. You really need to analyze the entire system for resistance. It varies for different size pipe and style of elbows, cleanouts, caps, etc. A hard tee can have twice the resistance as a round 90º bend. One round 6" elbow has a resistance coefficient about times that of a foot of 6" pipe, but that doesn't mean it only has 1/10 the capacity. Far from it.

    I'd imagine the individual makers can provide resistance coefficients for each component they sell. You add all the resistances up to get a total for the system. Overestimate, because the pipe will have increased resistance as soon as it gets a coating of soot and fly ash on it. You have to consider your stove as well. Some stoves have a lot of internal resistance. Load configuration can hugely affect draw as well. And of, course, stack temperature within any system is the biggest factor. That varies a little bit from time to time.

    In short, I don't think there are any hard and fast rules or formulas that you can use. Consult your manual and talk to your local stove techs. They will be able to analyze your system a lot better than any formulas we give you. In the end, if you are really concerned, buy a cheap manometer and measure your draft. That's really the only way to be sure.
  9. begreen

    begreen Mooderator Staff Member

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    Usually the stove exit is not counted in the 90's, and 5'/90 seems a bit much. I usually figure a rule of thumb at about 3' per 90, which would put your system at about 19'. Draft requirements also vary a lot from stove to stove. Some need quite a bit of pull and others, like our 602 work on a basic stub of a flue.
  10. Todd

    Todd Minister of Fire

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    I use to have one before I relined the flue and installed a new cap. Now both flues have been relined, no more fireplace.

    Shari,
    I still have the old 8x12 extenda flue, it won't work for my install anymore, if you want it you can have it.
  11. Shari

    Shari Minister of Fire

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    Todd,

    That is a very generous offer - let me see what the installers say. I was a bit firm with them in that I wanted someone to check this out a.s.a.p. I will get back to you and let you know. Thanks again for the offer. :)

    Shari
  12. Shari

    Shari Minister of Fire

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    By the way, someone above said chimney length is measured from the flue collar - thanks for that info! The stove shop was telling me today it was measured from the 'fire base' - I knew that was wrong!

    Shari
  13. MikeP

    MikeP Member

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    Not saying that the stove shop is correct about your Jotul, but some stoves like the Englander 30 that I have, list in the manual, "15' minimum from the floor to the chimney cap".
  14. Battenkiller

    Battenkiller Minister of Fire

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    I've seen it stated both ways. Much of the established literature says to measure from the bottom. My old Vermont Castings manual says to measure from the flue collar, so that is where I measured from, even though the stove burns across the bottom and not from bottom to top when in secondary mode. Perhaps that's the best thing to advise folks when you've designed a stove that has three full-length internal baffle plates and a serpentine horizontal flame path 55" long over about an 18" vertical rise. :cheese:

    How many 90s just inside my stove, BG? Sorry, no... they're 180s. Maybe one 180º bend cancels out two 90s? %-P


    Needless to say, my stove requires a good draft to operate with the bypass damper closed.
  15. VCBurner

    VCBurner Minister of Fire

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    I have 13' from collar to daylight. Stove manual says minimum 16' is required. I also have the problem presented by some giant pines that surround the house. They cause a major dowmdraft during windy conditions. I need to extend a chimney and possibly add a vacu/stack cap to the top of it. What has happened with your install since the last post on this thread?
  16. Shari

    Shari Minister of Fire

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    Chris,

    I wasn’t having a downdraft ‘problem’ with the Oslo - I was concerned about possible carbon monoxide down drafting into my oil furnace flue. Apparently the installer and our city inspector both missed on the code: One of the flues must be at least 2’ taller than the other. I called the installer on it and they came out and added 2’ to my Oslo flue.

    I was a bit short in chimney height originally for the Oslo so by Jotul’s ‘rules’ I ‘should’ have been having a slow draft problem (but I wasn’t). I originally had about 12/13’ straight, to two 45’s. Now I have 14/15’ straight with two 45’s.

    Hope this helps.

    Shari
  17. Renovation

    Renovation New Member

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    I read 1' per 90 somewhere, which seemed a bit low.
  18. VCBurner

    VCBurner Minister of Fire

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    My stove's manual states 16' minimum from collar to daylight, looks like they may take different types of installs into account. Not saying that the 16' would work w/ a bunch of 90's and 45's. They do show rear exit flues w/ a 90 bend going in to the chimney. They also show a free standing top exit w/ a 90 over the mantel and another 90 up the chimney.

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