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"Chimney Effect" draft problems

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by Atomic, Nov 27, 2008.

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  1. Atomic

    Atomic New Member

    Joined:
    Nov 27, 2008
    Messages:
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    Loc:
    NE MA
    Hello All,

    Brand spanky new member who is planning his own woodstove install. I have done research - stove is a used VC (read my manual - <1995, "like new, only burned on sundays) defiant encore 0028 w/ cat. The person I bought it from threw in the pipe and chimney (class A Selkirk 8" Sure-Temp equivalent - back then it was called something else, but Selkirk told me it was the same), enough for the install in my single floor sun room. The intent is to complement an oil forced hot air heating system for my downstairs sunroom, kitchen, dinning room and livingroom (I put returns for the furnace and larger vent ducts in and the sunroom just will not heat up in the winter because of the 8 windows and 2 skylights - thermostat in kitchen)

    Here is the question for the experts.

    My sunroom is a single floor addition to the rear of a 1872 Victorian (not an airtight new home) that is 38 feet at the front of the house to the peak. Half way back it is 2 stories (both front and mid house have 1:1 roof = steep) then drops to the sunroom with a 12' cathedral ceiling. I talked with 2 installers (one at a stove store, one an independant). they both stated concern that if I install the stove at the back of the house and use the basic 10-2-3 rule I will have "chimney effect" draft problems. That is the heat rising in the taller parts of the house will pull air from the sunroom and creat a downdraft. This downdraft will make getting a fire going difficult and importantly stink (unacceptable) out the house with woodstove smell when we do not have fires going.

    Option 1 is to install on back corner of sunroom to code (10-2-3) and have a chimney in the 15-16 foot range.

    If I do the following will I still have "chimney effect" and draft problems?

    - I can close a door a the top of the back stairs (off kitchen which is off sunroom) and the doors off dining room and living room to help contain the updraft. Woulld this make a difference or does enough air go through the cracks around doors (old house loose fitting doors)?
    - open near window with lighting or put in an out side air kit
    - put in flue damper (stop down draft when stove not in use)
    - put in extra x feet of chimney (nothing that requires a red blinking light, but and extra 3-4 feet)

    Option 2 is to locate the stove on the wall shared by the second story, go through the roof on the sunroom and up the exterior of the second floor wall and another 10 or so feet to get 10-2-3 on the second floor roof. this option will make the install much more difficult expensive (around $1000 more is an estimate i have worked up on materials and labor) because of the exterior run chimney will likely need a few 15 degree bends and considerable more height (4 the way I see it now - 2 to get close to the exterior wall for supports 2 to go around the eves - i may find exterior supports that allow me to run it straight at 2-3 feet from the wall - have not found those yet). this alternative would also be less desirable appearance wise and a much harder to clean (from the bottom only and around potenital bends?)

    I do not want to do option 2 unless I really have to, but am afraid of installing option 1 finding the stove is not working properly (& stinking us out) necessitating option 2 (and a second hole in my roof).

    Anybody have experience with an option 1 install? Opinions?

    Thank you for your help!

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  2. ansehnlich1

    ansehnlich1 Minister of Fire

    Joined:
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    Loc:
    Adams County, PA
    Hey welcome to the best woodburning site on the planet.

    That defiant catalytic is a big stove, I had one, it'll throw serious heat and one can get long burn times out of it.

    The optimum woodstove install is a central location within the home and a chimney the runs straight up through the interior envelope of the home. The further away from this type of install the greater the possibility of problems, IMHO.

    My bet is an outside air kit installed on the Defiant using your scenario #1 would take care of any "stack effect" problems in your situation. I am certain many more experienced members will be by shortly with further input.

    I don't recall, does that Defiant of yours have provision for an outside air kit?

    Another factor would be the location of your roof lines ABOVE the terminus of your scenario #1 chimney. There have been situations where air flow off of roof lines can push down chimneys, even though the 10/2/3 rule is adhered to.

    Bet ya a buck peeps on here are gonna want to see PICS :)
  3. Atomic

    Atomic New Member

    Joined:
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    Loc:
    NE MA
    Good point on the pic.
    I will post them up when I get home.
    Thank you
  4. oconnor

    oconnor Minister of Fire

    Joined:
    Nov 20, 2005
    Messages:
    1,062
    Loc:
    Nova Scotia
    By "chimney effect" I assume you are refering to "stack effect", the situation where the house is acting like a flue. Where you install the flue isn't going to change any stack effect in the house - if your house is leaking air out the top and refilling at the bottom, you have stack effect. What it will affect is whether stach effect can reduce your draft - I am being picky on the language here intentionally. If you have a stack effect problem, you should be air sealing your house regardless of how you heat it, 'cause you are heating the outdoors.

    As for where to stick the flue - an internal flue is best, as it retains the heat in the flue, because it is shielded from cooling, which allows it to better maintain a draft at low fire situations - start and end of the fire cycle.

    Also, a straight flue is better for draft than one with bends - helps keep the flow rate up by limiting any mechanical resistance.

    A taller flue is going to draft stronger than a short one - stronger is better to a certain point, then it can be excessive.

    So what is my point? - Choose a location that gives the best balance of all these factors.

    Your option 2 has one flaw I can see - I don't believe you are permitted to use 30 or 45 degree elbows on an external install - check the manufacturers instructions. I recall that from a Selkirk manual. Lots of folks do it and get away with it, but I think the idea is that with bends and an outside flue, you have two strikes against ideal draft - bends and cooling, as well you are creating a place for creosote condensate to accumulate.

    In option 1, you don't have to stop at 16 foot flue - you can still add more chimney to improve draft and get out of any turbulence that may cause wind effect issues.

    Give the "Guide" link in my signature a read - one of the chapters has a good bit of detail on issues that effect draft - stack effect, mechanical depressurisation (using your dryer), wind effect etc. Hope that helps.
  5. Atomic

    Atomic New Member

    Joined:
    Nov 27, 2008
    Messages:
    9
    Loc:
    NE MA
    Yes, I can get an outside air kit for this stove to help with either option 1 or 2. That should help getting a fire going and maintianing draft in low fire situations (die down)

    I read the portion of the signature guide on house presurization and it seemed to focus on basement installations and air tight houses. I do have an older house, but do have storms, not all as tight as ideal, but better than none. I am sure i have some leakage on the second floor, but can minimize both second and third floor draft somewhat by closing doors (not sure how much that helps, which is why I am asking). To put a woodstove in the room I want there is no chance of running through the center of the house and up through the third floor roof.

    I have included pics so you can see what I am talking about. I am not looking for perfect, but don't want a headach (ie. big backdraft) let me know if these pictures help you give me advise as to weather is is just a bad idea or servicable (i know it will not be ideal).

    Option one is in the closest right front corner of the single story room closest. Not sure how high I can go with supports on a chimney system here, but not concerned with a 10 foot chimney poking up.

    Option two would be in the back left of the single story running up the exterior wall of the second floor back wall to the left of the window and above the roof line fot he seond floor peak.

    It is approximately 15 feet from the back of the single story to the exterior wall of the second floor and another 18 from back of second story to back of third story.

    Opinions would be most welcome.

    Thank you,
    George

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  6. Atomic

    Atomic New Member

    Joined:
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    Loc:
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    see it this works

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  7. oconnor

    oconnor Minister of Fire

    Joined:
    Nov 20, 2005
    Messages:
    1,062
    Loc:
    Nova Scotia
    I would be inclined to try option 2, and build an enclosed chase around it to blend it in with the house.

    Closing doors isn't going to change any stack effect by a great deal, as the door isn't airtight, but it is an easy way to stage the heating of the house. Opening the door after the stove has been on for a while would let the cold eair flow freely out of the given room. If you did build a chase around the flue, you could even incorporate a passive cold air return from the room on the third floor. Code issues come into play there, but if designed carefully (thinking about firestop dampers and proper distances for vents etc being seperated from the stove, etc) you could provide a path for cold air that would normally have to flow down stairways into the room where the stove is to take a short cut into your sunroom.

    Again, there are codes applying to air venting between floors that apply, as well, don't think of trying to run hot air up the duct (this will contribute to depressurization), focus on cold air returning to the room.
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