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Who else has done this? Increasing heat from insert...

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by recppd, Oct 13, 2006.

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  1. recppd

    recppd New Member

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    As you can see by my signature I own a Jotul Kennebec insert. The insert's design is basically a cast iron stove surrounded by a sheetmetal shell with approx. 1" of airspace inbetween. That airspace allows cool air to be drawn in from under the stove (below the doors), circulate around the stove, and then exit above the doors all nice and hot. As with most inserts, it also has the optional blower installed to assist with more air flow into the room.

    In any case, I got to thinking....(scary, I know...)

    Since the sheetmetal shell heats up naturally from radiant heat coming off the stove, the inside of the fireplace (or firebox if a pre-fab fireplace) also heats up. Why waste that heat? So what I did was cover that "shell" with a 1" thick blanket of 2100 degree Kaowool. No air intakes were blocked, for obvious reasons. The results were quite encouraging as the exit air temp raised by 50-80 degrees, depending on the air control lever setting.

    Has anyone else tried this? I'm surprised it's not already a recommended practice...

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  2. Mike Wilson

    Mike Wilson New Member

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    Are you serious... 50-80 degrees... HHHHMMMMMmmmmmmmmmmmm.............................

    To quote my friends over at Guinness.... BRILLIANT !!!

    Sounds interesting. How many times have you tried the "wool on, wool off" temperature check?
    My only concern would be damaging something through overheating, but I doubt that is a great possibility. I have an exterior fireplace, and the actual masonry of the chimney is 3' by 6'... so I am heating up a lot of brick for nothing.
    Hmmmmmmmmmmmmmm................................ you have me thinking. Report some more! Pictures?


    -- Mike
  3. BrotherBart

    BrotherBart Hearth.com LLC Mid-Atlantic Division Staff Member

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    I did it this summer with my Sierra insert. I put a two inch blanket of mineral wool behind, on each side and over the top. Prior to that the convection air coming through the shroud over the top of the insert would bend a butane lighter flame without the blower running. After the insulation job it now blows the flame out.

    And the air is noticably warmer.
  4. recppd

    recppd New Member

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    Yep, the temps were correct. I waited until the fire was established in both instances and measured the temps with the blanket off, and then on. I relate it to wrapping a water heater for extra insulation. Also, FYI, the stove (insert) top temp didn't raise by much, or did it ever get close to an "overfire" temp.
  5. Rhone

    Rhone Minister of Fire

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    I think that a bad idea, though, a modification will certainly work. Your insert was tested WITHOUT being surrounded by insulation. Think what happens during a power outtage, when you depend on natural convection. You've now insulated your insert, that sucker is going to get super heated and probably go past the specifications it was designed, and I can't imagine overfiring an insert surrounded in insulation.

    My insert manual if I recall states there must be at least a 1" air space around the insert. That means, a 1" space around the shroud. A modification will work, my mason insulated the walls of the fireplace and I insulated the blockoff around my insert, so my insert is surrounded by insulation but none of it is in contact with it. If your insert comes wrapped in insulation, that's a different story. My brothers came that way and he put it in wrapped in insulation and all, otherwise I think you should move the insulation away from the insert and move it to the walls of the fireplace.
  6. Mike Wilson

    Mike Wilson New Member

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    Rhone is on to something there... what about your idea Rhone, insulating the firebrick, I assume... What was used, and how did you stick it on, high temp silicon? Also, what about insulation with that silver reflective stuff on one side...

    I have the insert open for cleaning now, so now is my time to try this.

    -- Mike
  7. recppd

    recppd New Member

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    Actually, since I put the insulation in place, I haven't had to turn on my blower. Now I know it hasn't gotten cold around here yet (maybe 45 at night), but I also haven't had to put the blower on yet to get some heat like I did before. I also notice a stronger air flow as well (convection).
  8. Rhone

    Rhone Minister of Fire

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    I used mineral wool insulation, it's 2100F+. It's pretty rigid stuff and you'll find it at building supply stores. KaoWool sounds like the same stuff, it goes by many names.

    My mason cut to shape, put it against the firebrick walls, drilled holes through it and the firebrick trying to get the mortar joints, and put simple tapcons with fenderwashers just to hold it in place. The stuff is more rigid than fiberglass. A warning with tapcons, they only go in 1" or so and that's all you get before they break so it's important if you get 2" insulation you use probably 2 3/4" tapcons and tapcons come in all sorts of unusual sizes, none seem to be standard.
  9. recppd

    recppd New Member

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    I should also mention this...

    My Jotul is inside a pre-fab fireplace with an exterior cold air intake. The cold air enters the firebox from openings all around its front, and not from a central intake. The result is that the cold air envelopes the insert. My Jotul has a 3" air intake at the back of the insert, but since it's not piped directly to the outside it has to draw air from the firebox. The ice cold firebox. So instead of insulating the firebox (which would still contain cold air) I insulated the "shell" to keep the cold air away from the insert and only allow it to enter through the 3" opening.

    I hope that clarifies things.
  10. Mike Wilson

    Mike Wilson New Member

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    I may try what Rhone is doing.

    Other thought on this, let me know what you think of this idea:
    My insert draws air from the firebox, through the back of the insert. No outside air connect (I actually sealed it off on the outside), just air from behind the insert/through the surround plate.
    If I insulate the firebox, I assume that the air temperature in the brick firebox behind the insert will increase. Therefore, the stove will take in warmer air than it did pre-insulation. If the intake air is warmer, would she would acheive secondary combustion sooner/longer than by drawing in cold air? Just thinking. I know the temps required for secondary won't change, but if the injected air is warmer, wouldn't it happen sooner, and therefore increase efficiency? (I run it 24/7, so the masonry behind the stove never cools off).

    -- Mike
  11. recppd

    recppd New Member

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    Actually, what you'll be doing is drawing air from the house, into the firebox, and then into your insert. This means that replacement air has to come from somewhere, which means cold air through receptacles, windows, doors, etc. I didn't want that to happen, so I let the combustion air enter through the firebox rather than the rest of the house. But, it enters the firebox ice cold in the dead of winter, which is where the insert sits. I know, I know, I know, the firebox gets heated during normal usage, but why not insulate it from the incoming cold air and allow more heat into the house?
  12. Mike Wilson

    Mike Wilson New Member

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    Yeah, I know where the air comes from, and I am not going to change it. I'm in a pretty large place that's not exactly air tight, and I am sitting on a beach, so its windy as hell... bottom line is that the air use for my stove is a minimal issue for me. I was just thinking about warming the air coming into the firebox.

    -- Mike
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