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No Block Off Plate-Thermal Mass Generating Masonry Fireplace

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by Mr A, Nov 22, 2012.

  1. Mr A

    Mr A Minister of Fire

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    It is my first year using my Jotul C450, first burner ever. I tried likk hell to get the recommended block off plate installed, just no room to do it. I understand the concept of a block off plate to keep the thermal mass of the chimney from absorbing the heat of an insert, to heat the room faster. For my existing masonry fireplace, no block off seems to be an advantage. The thermal mass of my fireplace keeps heating the living room long after coals have burned out. Optimized-IMG_20120304_115157.jpg

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  2. DAKSY

    DAKSY Patriot Guard Rider Staff Member

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    Just think how much MORE heat the fireplace could absorb/radiate if a lot of it wasn't going up your chimney...
    jeff_t likes this.
  3. webby3650

    webby3650 Master of Fire

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    I'm glad that it's working out for you! That's a nice looking insert. I don't think a block-off plate is a must have in order to get good results. On mine and ones in the field, after the liner is down I seal off the top by stuffing extra insulation around the liner and then the top plate is sealed down. No heat is exiting the top around the liner. Some heat will be absorbed by the stone, but even without the plate, you will see great results!
  4. Mr A

    Mr A Minister of Fire

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    Yes, I have a foot of Roxul stuffed around the chimney liner at the top, then cap on top of that. I talked with a few local shops and they described installation exactly like you did, only they all say they use fiberglass insulation. I get plenty of heat and the heated stone and brick is a bonus.
  5. webby3650

    webby3650 Master of Fire

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    Well they aren't really supposed to use fiberglass, but I suppose I have never seen any problems from using it. I just wouldn't do it.
  6. Mr A

    Mr A Minister of Fire

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    I do have the top sealed, so it is not drafting. I came across some comment mentioning a masonry heater. Mine was not built specifically as a masonry heater, but a open fireplace. I am getting some benefit of thermal mass from the fire brick used to construct the fireplace. The back of the fireplace is in the garage, and the bricks are warm up to just above the smoke shelf. I read the Hearth Wiki on how to make a block off plate. The 20 gauge sheet metal is functionally just to hold up the roxul insulation. I have no room to to try and cut the hole just right in the sheet metal, get the liner through it, and then into the insert. It was difficult to get the liner into the insert, and still I didn't get the liner to flex enough to position the insert so that the surround could be installed. I am now just going to stuff the space above the insert with Roxul. A tight fit should keep it up there and do the same job as if I had the sheet metal in there. These face stones were popular in the 70's. Later on I may dig into the fireplace and construct a designed masonry heater core.
  7. Mr A

    Mr A Minister of Fire

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    I stuffed the damper area with Roxul, sealing the chimney from below. I did have Roxul stuffed around the liner at the top, under the chimney cap. It's not fancy or skillful, but what a difference in heat to the room. The hall thermostat is 5 degrees higher than usual in the morning after overnight burn. The living room is very warm, I have to open a window now. I wasn't heated out of the living room before. I am making smaller fires, the heat lasts longer. I'll will not have to burn as much wood as I was.
    Joful and DAKSY like this.
  8. weatherguy

    weatherguy Minister of Fire

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    It makes a huge difference, my office is 3 degrees warmer since I did mine, the room with the stove is probably 5 or so warmer. I have a big stone fireplace thats the stoves in and like Daksy mentioned, it retains more heat and longer with the block off thatn it did without.
  9. granpajohn

    granpajohn Minister of Fire

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    Anyone debating the benefit of a block off plate should just read this post.
  10. Mr A

    Mr A Minister of Fire

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    I wanted to add some more to my block off experiment. The temps have been consistent, 40's overnight, 50's- 60's during the day. Using the hall thermostat as a guage to measure block off effectiveness, with an outside temp of 56, thermostat reading 65, I laid onto coals 3 splits of cedar. Hall thermostat reads 76 a few hours later. I have never been able to raise the thermostat past 71 without the block off. Temps have warmed to 60 today, I have all the windows open trying to cool off.
  11. wjb111

    wjb111 Member

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    far.JPG

    I have had the Alderlea T5 insert for 1 year now. Absolutely love it, but it does seem to lose a lot of heat up the chimney without a block off plate. They installed a 6" SS flue but there is no insulation up top or in the flue area. Not sure if Home Depot has Roxul but my plan is to remove the trim plates & stuff some up there.

    Bill B.
  12. Hogwildz

    Hogwildz Minister of Fire

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    The problem with no block off, is that while your stone is absorbing some heat similar to a masonry heater, a masonry heater is centrally located in the home, with only the upper portion of the flue exposed outside the home. Most of the heat is radiated back into the home. On a fireplace, if it is on an outside wall as most are, heat is being absorbed, and a good bit is being lost to the outside through the stone. This is where a block off plate helps contain heat to the portions or stone below it. Is a block off plate a must, no, does it help most of those who ran without one, then installed one and noticed the difference, yes. The insulation at top is good and will help. But the mass of stone all the way up to there will be absorbing heat, and transferring it outside. If your system seems to do the job, then don't sweat it.
  13. weatherguy

    weatherguy Minister of Fire

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    Lowes carries it, I dont think HD does. I just installed mine this summer and wow, did I notice the difference right off the bat, my office was always 70, occasionally 71, now its 73-74, I havent checked all the other rooms but its noticeably warmer throughout the house.
  14. Armoured

    Armoured New Member

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    I am still planning the full block-off plate but did stuff the smoke chamber/smoke shelf part of the fireplace chimney with rockwool and fitted some around the flu and it seems to make a difference; I think the room gets warmer quicker and can maintain a much higher temperature.

    I still have a fairly deep space behind my insert (6 to 10 inches), and I'm thinking of putting rockwool with the reflective facing on the firebrick wall at the back of the fireplace (which is outside wall of house). The insert itself has bare cast iron there and the firebrick is certainly absorbing a lot of heat - although I have checked the chimney outside and it must be very well insulated, as no detectable warmth.

    Anyway, any reason not to put some rockwool sheeting on the back wall of the fireplace? Even with rockwool on that, there should still be room for air to circulate behind and around the insert.
  15. Mr A

    Mr A Minister of Fire

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    i would think the only reason to not insulate the back of the fire box is that I dont think it would matter, but I'm no expert. On the outside wall of my fireplace bricks, there was no heat felt by hand until the smoke shelf area, pre roxul installation. I can comfortably place my hand near the sides and bottom of the insert, post roxul, but not the top. The flat stones of my hearth were 2 inches above the floor of the fireplace. I mixed up some perlite as aggregate with portland cement thinking the bottom needed to be insulated. Well, it is now , but I don't think it needs to be. A well built masonry chimney from the last 30-40 years should be insulated with an air space between the firebox and outside bricks,-building codes starting requiring it-same with the clay liner flue, just no way to determine if it was actually built to code without digging in to it to look. Home Depot and Lowes can special order it after a lengthy amount of time trying to find the right person that actually knows what it is and where to order it from. I have been there, done that! I found mine at a building supply listed on the manufacturer's website. I have several batts ,16"X48", left over, $cheap+shipping, inbox me.
    Cheap used Roxul for sale
  16. OhioBurner©

    OhioBurner© Minister of Fire

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    Not all are going to benefit the same. I put a block off plate on mine and thinking about taking it out. Its very hard to accurately say the plate was the cause of an apparent increase in room temp. Now if you were loosing heat out the chimney then sure... I know mine is sealed well at the top. Sure the bricks themselves will sink heat to the outside, but they are going to do that no matter what. One thing I did notice pre-block off plate was that after my stove was off for several days, the bricks were still noticeably warm. I havent done the same comparison afterwards to see though. But for folks like me with an interior chimney that is sealed at the top, does it really mean a huge benefit? Maybe the stove will run a little hotter, but that heat is still there regardless, just a much lower rise over a longer time and greater area (I mean like the stove may run 50 degrees warmer over its few square foot area, but the bricks may just run 5 degrees hotter over 10 times the area if you get what I'm saying). Heck if you dont have an insulated liner maybe it would be an advantage to let the chimney get hotter?

    For me it doesnt really matter I havent noticed a big difference. But its a pain to pull the stove now since with the block off plate I have to pull the whole chimney liner up to pull the stove out. Not a biggie as I hardly ever need to pull the stove. But I think I was off just a hair bit on the hole in the block off plate and now the stove sits out almost a half inch. Also since that insert likes to burn HOT every time it gets up there in temp I wish I didnt have all that insulation sitting a couple inches above it making the problem worse as the secondaries are turning into little torches and the baffle and front tube start glowing... Maybe I'll have to jot down some measurements and remove the block off plate and see if it was making a difference.
  17. Armoured

    Armoured New Member

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    Thanks; well, it may not help much, but shouldn't hurt. I do see the firebricks get fairly warm and since on the outside of the house - even in a chimney that seems to be VERY well insulated - it certainly isn't adding to the usable heat.

    And appreciate the offer, but think shipping Roxul here wouldn't be so effective -a nd I know where to get it.
  18. Armoured

    Armoured New Member

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    I think if the chimney is in the interior of the house it should make less difference, since the heat absorbed by masonry will radiate into the house. I often see fireplaces with interior chimneys designed to carry air around firebox and up along the masonry with air vents at ceiling level, with the idea to circulate the heated air around better (obviously might need a ceiling fan).

    Unfortuantley in many cases chimneys built on outside wall, so that's the issue.

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