Hello
To all of you who participate in this forum, sharing knowledge and ideas – Happy New Year and thank you for your willingness to help each other!
It will be my first attempt on building double bell masonry heater and I will appreciate any input and criticism.
Here is the plan:
The MH will be placed in the middle of a two story residential house with wood frame structure.
In the plan of the first floor (where the MH will be build) there is a concrete slab with concrete foundation provided for the Mass Heater. This allowed space is 2.10 m x 1.20.
Drawings :
alchemyarch.wordpress.com/2017/01/07/masonryheater/Basic design elements are :
Batch box fire chamber , with parameters for a 15 cm core system. Actually I am calculating the whole system as a 15 cm (6”) however the length of the batch box should be longer to allow for 50cm pieces of wood to be loaded.
On top of the batch box I plan to build a H=80cm heat riser from calcium silicate insulation plates or from lightweight refractory bricks.
First bell will be built around the heat riser with its long axis, perpendicular to the axis of batch box-heat riser. First bell should be double skin – with inner layer firebricks and outer of solid red bricks. On top of the first bell (if budget allows ) I plan to set up a 8cm thick solid plate of volcanic tuff stone. Such stone I did use few years ago in a wood fired bread baking oven for the heart and dome and it works very well.
First bell and second bell are partially overlapped. In the base of the second bell/top plate of first bell, there is a valve for “cold start” sliding in and out and allowing for direct release of gases from first bell to the exhaust pipe on top.
Second bell is slightly narrower than first and it is build as single skin out of solid red bricks, except of the lower part of the side wall facing the opening from the first bell. Perhaps the outer surface of the second bell could be rendered with clay based mix.
In front along the long axis and sideways from the batch box I plan to build a heated bench which has opening to the lower transition between first and second bell. Bench will be not cross-flow but rather working as a “trap” for hot gases – have seen such system to work.
Total inner surface of first bell is 3.9 m2
Total inner surface of second bell is 4.8
Total internal surface 8.7 m2 off course this is approximate calculation.
The back side (opposite from the door of the batch box ) of the mass heater should be insulated from the outside , because doubles a wall which has wood frame inside and doesn’t need to be heated.
I have a line of questions
The internal surface is bigger than suggested by chart on BatchBox.eu. My assumption is that my entire back wall of the heater will be insulated, therefore in regards of this part of the calculations there should be a reduction(I can withhold the aprox 3 m2 from the calculation because the heat from this back wall will retain IN the heater. Any suggestions on this “”oversized” design ?
My other question is in regards of the double skin being applied only on the first bell – my assumption is that there is the major stress from the extensive heat. Do I have to worry for cracks from the longer and extensive usage in other parts?
I would also appreciate any input on the heated benches of the adjacent (appendix) type- Is it worth the effort to bridge a whole wall to allow openings to this bench ?
In the cross – section I am showing a white oven , placed directly on top of the batch box – it will be a small one , vaulted on top and well insulated – perhaps some could bake a loaf of bread under such conditions ?
I have some doubt on the pattern of hot gasses flow - can I / should I place some more obstacles from the first bell out and up into the second bell to gain more heat transfer to the brick shell ?
I have to place clean outs on appropriate places throughout. Perhaps on the bottom end of first bell and where the valve in the second bell is ...
Thank you in advance.