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A "Poor-Man's" Block Off Plate

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by cowtown, Oct 6, 2011.

  1. cowtown

    cowtown Member

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    I won't get into specifics, but the installers where I live will not put in a block-off Plate. However, I have convinced one installer to pack the flue in the damper area with insulation (roxul) around the chimney liner - I think this would do the trick, any reason not to do this? This is obviously not as good as a full blown block-off plate, but I think it would be a good 80:20 fix (80 percent of my problem solved with 20 percent effort).

    Any thoughts out there?


    Dan

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  2. pen

    pen There are some who call me...mod. Staff Member

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    So long as it stays it place it will work. I suppose the worse case scenario is that it would fall down and you'd have to make a plate to keep it up later.

    pen
  3. Bubbavh

    Bubbavh Feeling the Heat

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    That is much better than nothing. I see no problem with that, as I have mine packed above my block off plate.
  4. stejus

    stejus Minister of Fire

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    That's all I did with mine. My clay flue was back filled with insualation after the liner was installed so there's no heat loss going up the chimney. I just stuffed some insulation around the fireplace damper and it's been there for two full burn seasons and no problems.
  5. Stump_Branch

    Stump_Branch Minister of Fire

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    Plates really only there to hold the insulation from falling out. Mines so darn tight it stays up there, then again not many folks get insulation from an industrial chemical furnace either. About 2in thick.
  6. North of 60

    North of 60 Minister of Fire

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    It would be nice to seal in the nasties as it will be airborn forever. Guess what that blower will be moving. Hmmmmmmmmmm
  7. fireview2788

    fireview2788 Minister of Fire

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    That's what my chimney guy told me to do. He recommended something else but I'll be using Roxul.



    f v
  8. Milton Findley

    Milton Findley Feeling the Heat

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    If I ever have to do it again, that is exactly how I am going to do it. I think that it is a 98:2 fix given the space you have to work with.
  9. EJL923

    EJL923 Feeling the Heat

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    I just did this last night, and had the wife take pics as im covered in Roxul and other stuff. Ill post the pics when i get home, but i made a real simple block off with no tapping into the masonry required.
  10. cowtown

    cowtown Member

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    Just a quick "thanks" to all who added to the tread. I think I am on the right path.

    Can't really type much now as I am at work (shhh, don't tell).

    Thanks again.
  11. golfandwoodnut

    golfandwoodnut Minister of Fire

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    I did this just a couple of weeks ago. My installer not only did not insulated around the fireplace he also did not properly seal the cap so I know I was losing alot of heat. The Roxul seem to be making a big difference. I also want to insulate at the cap end if I can ever get up that high (I need a 40 foot ladder or a bucket truck).
  12. karl

    karl Minister of Fire

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    First, I don't see how that is going to be blown around by the blower. There's a convection jacket between the two.

    As for the block off plate not being needed, I disagree. When you read about weatherization and stopping air infiltration, they tell you to look for dirty insulation. That's a sign of air moving through it. Which is basically leaks in the bottom of the house and leaks at the top causing air to move up and out like a chimney. We are dealing with chimneys. We are trying to stop them from doing what they are designed to do.

    A few years ago, I added insulation to my attic. I had r-29 and I was adding an additional r-30. I came to an area where the existing insulation was very dirty. I pulled it up and found a hole in the drywall a little bigger than my fist. I couldn't see it from inside the house because my kitchen cabinets are right below it. It turns out there are wires running through the hole, but the hole is way bigger than what is needed.

    My point is. I had air going up through this hole, as shown by the dirty insulation. This is with a 70 degree house and probably a 40 degree attic with a 8 foot rise. Imagine how much air is going through that rock wool when you have 200 degree air around the stove and 20 degree outside and a 15 plus foot rise.
  13. pen

    pen There are some who call me...mod. Staff Member

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    You make a great point Karl. However, what this guy is doing is the "poor man's" version. Is it perfect? Absolutely not. Is it better than nothing? Absolutely! Is it safe? Sure.

    If it were me I'd make a real one as you suggest, but if it's time to get things heated up right now, my schedule doesn't allow, etc, etc, then what he is doing won't hurt a thing and he can get to making it perfect at some other point.

    pen
  14. rakuz66

    rakuz66 New Member

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    So, in my set up I have an insert with a 6" ss liner. I have no block off plate or insulation surrounding my ss liner. Just a cap on top of the chimney. Should I commit to having someone install a block off plate and dropping insulation around the ss liner all the way up the chimney? thanks.
  15. fireview2788

    fireview2788 Minister of Fire

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    But the chimney is sealed with the only opening being the SS liner through the stove. You are insulating the area around the pipe to reduce the amount of radiant heat filling the air cavity and thus forcing it back into the room. You shouldn't have an air current around the outside of your liner, should be dead air.


    f v
  16. Milton Findley

    Milton Findley Feeling the Heat

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    An interesting question rakuz66, and one which needs an answer. I will take a stab at it. The chimney flue is the interior of the liner. It is running inside another pipe, or set of tiles stacked and mortared, or bricks and mortar or wattle and daub. The cap sits on top of the chimney chase, which is the aforementioned tiles, etc.

    In operation the interior of the flue gets hot, and it gets hot. The surrounding air gets hot, and you want that to happen because you do not want the liner to get too cool as the gasses get cool, and the components in the smoke condense on the walls of the flue. You want a warm flue, all the way to the top.

    There is a secondary air flow, between the liner and the chase. This flow needs to be minimized because the flow is generally going to be from your living space to the underside of your chimney cap which is unlikely to be hermetically sealed. Air flowing outside the flue will cool the flue. Most people attempt to minimize the amount of air flow lost up the chase through the space between the liner and the chase. This is most easily done by packing properly rated
    insulation around the top and the bottom of the liner as it enters and exits the chase. The air surrounding the flue will heat up, and dead air is insulation without a bunch of non flammable fibers in it. In such an installation there is no air flow to speak of, and even those who worry about that little bit could not subsist on it. It is certainly less than that lost through your attic insulation due to the small area and the density of the packed insulation.

    Other folks hermetically seal the cap in place, and fabricate sheet metal plates to plug the bottom of the chase and then they insulate those. This keeps hot air from filling the smoke chamber of an old fireplace and escaping up the chimney between the liner and the pipe. Others do all of that, and insulate the liner as well, but in your case, that bird seems to have flown.

    I do not know what is right for your installation. In my climate, and with a steel chimney chase, (an old air cooled triple wall chimney), I needed all the help I could get to keep the flue warm all the way to the top. Last year, the air cooled triple wall was still air cooling, this year, I have blocked the convection so things will stay warmer around my insulated liner.
  17. cowtown

    cowtown Member

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    Really appreciate all the feedback from everyone. I see it the same as Milt.

    The purpose of the Roxul is to keep the radiated heat from the insert to force it into my living space and not up the chimney (between the liner and the masonry flue). The Roxul will also prevent hot air from my house to go up the same space.

    It was mentioned in this thread that the insulation woudl get blown in to my living space by the blower - i don't think that this would occur.

    Thanks again for everyone and their feedback - one last question, can i simply wedge the Roxul up where the damper is or will i need some sort of support (e.g. run some wire there to hold up the roxul)?
  18. daryl

    daryl Feeling the Heat

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    All I have to say is that that insert blowers do not blow air in the old firebox. They blow out the air outlets on the stove.
  19. North of 60

    North of 60 Minister of Fire

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    I understand, They pull in room air and blow it back out. Or try to. They are NOT sealed. Blower shafts, motor housings etc...
    will still pull that stuff through on a cheap squirrel cage blower. Most blowers are at the bottom of the insert and thats where that stuff settles. When it gets moved out onto the floor space it settles. In carpet it gets stirred up every time it gets walked on. Children and most people like to be near or on the floor at the insert sucking up the heat and what ever else is there. I have seen swabs that have been taken in front of an insert due to this issue. Guess what was in it? Thought I would share this and dont care if you doubt it. Carry on.
  20. begreen

    begreen Mooderator Staff Member

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    Essentially, you have described the poor man's version of a block off plate.
  21. daryl

    daryl Feeling the Heat

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    I would be curious what those swabs showed in a test. What kind of fibers where found? A house is filled with different fibers. Carpet alone is one of the biggest sources of airborne particles out there, it is like a big magnet for anything that is in the air. And people and pets stir those things up.
    I just think that the amount of particles that come from a (poor mans block off plate) are minimal at best. To say a cheap 180 cfm fan can carry enough particles from behind a steel surround out into the room seems a little odd. It's not like people or pets are back in the firebox stirring things up.
    I think people get a little carried away with whats floating around in the air. It would be nice if everything we breathed was nothing but clean pure air, but that isn't reality.
    Carry on respectively.
  22. cowtown

    cowtown Member

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    Well just went out and got the Roxul.

    I will let everyone know how it turns out. Thank you for all the input.

    If I do find that fibers end up coming out of the blower, I will be sure to let everyone know.

    Thanks again.
  23. mhrischuk

    mhrischuk Guest

    Those are good points for a blockoff plate.
  24. karl

    karl Minister of Fire

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    You don't have dead air in there. You have plenty of convective air currents

    In that case the hot air is not lost to the outside but the heat still is. Still air has an r value of 5 per inch. Fiberglass bats have an r value of 4.3 per inch. So why do we bother to insulate our walls? Because we can't keep the air still.

    With a chimney sealed at the top. You will have very warm air travel up the chimney close to the liner. The air gets to the top and then moves away from the liner and towards the outer part of the chimney. It is then cooled by the chimney which is exposed to the outside air through the brick, As it cools it falls back down along the outside to be heated again and rise back up close to the liner. These convective air currents move the heat up to the top part of the chimney where the heat escapes outside even if the air doesn't.

    Attached is a picture I used to use when I was a flight instructor on how a thunderstorm works. It shows whats going on. It's based on the same principle hot air rises and then cools as it gets higher. The adiabatic laps rate for our atmosphere is generally considered to be 3 degrees per thousand feet. That means 70 degrees on the ground -30 at 30,000 thousand feet. That's enough to shoot the cloud up an additional 10,000 feet or more after you reach the tropopause. That's where the temperature drop essentially stops.

    In nature 100 degree swing over 30,000 thousand feet will move massive amounts of air (the moisture helps this quite a bit). In our chimney 150+ degree swing over 15-20 feet?? I bet you could feel a breeze in there.

    Attached Files:

  25. mhrischuk

    mhrischuk Guest

    So we need to figure out a way to seal it off and create a vacuum!

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