Gravity power in an FGM-Bell masonry heater

Hello to all!  I hope to find someone on these forums who can give an opinion on the likely result of varying the design of a Kuznetzov-style heater.  The heater design that I am referring to is a bell that contains the firebox.  The bell being a stratification chamber for the gases, which are then acted upon by gravity to 'move them along.'    Please peruse the four- very simplified- sketches attached.  (Excuse my poor drafting skills.) "A" and "B" are well-proven designs employing a gravity engine to move heated air.  "C" and "D" are my proposed tweaks.  The sole issue I wish to resolve here is whether gravity will permit the proper operation of stoves "C" and or "D".  In other words, am I expecting too much from gravity?  I have consulted with a few stove 'masters' in Europe who I hold in high regard.  They have not expressed enthusiasm, and at the same time, they do not have an explanation for why they don't like it.  I understood from them that no one has tested gravity in this situation.  Since I really need that stove "C" and or "D" be viable, I want to build them to find out.  I will use mud bricks so that I can 'erase' and 'play' with the design.  Again, the sole objective is to confirm whether gravity will perform as desired.  If anyone on this forum is a fan of this type of bell heater, I would be grateful for your educated guesses as to whether my tweaks will give satisfactory performance from the stoves.  Thank you in advance.  

Perhaps, you might get the "C" design to function.
I see a real possibility of your gases stalling in the second "cool" chamber and venting backward out through your stove.

Gravity is not really the word you want it, is just stratification.
The hottest air rises displacing the formerly hottest air and moving it to the lowest level.
This is fluid dynamics. In a single bell, the action is simple: The coolest air locates an exit, and now it can rise again as the hottest air, creating the draft that you need.

From your drawings I see the cooler air stalling in the second bell and refusing to create a draft exiting that bell.
If that happens, try raising the exit higher in the second bell or use a movable chimney on the second bell capable of sliding up to the top and then being lowered slowly back down as the gasses discover the exit.

I assume that you need to provide heat to the other side of the room and be able to walk "through the bell". Am I right?
Version C would be better, but if version D would have to be chosen then I would make sure the openings between the bells and the channel are large enough and I would also insulate the channel from the bottom and the top (insulating would also apply to version C). You could also chamfer the entry thresholds to lower turbulences (drag) of the gases.

Cristobal, Yes, this is my heater design for a single-story, 20' x 40' post & beam cordwood structure in northern Maine.  The stove is dividing the space, and I need access between the sections.  The overall length of the stove will be about 30'; the width is about 2', and the height is about 4'.  I prefer to have it divided 2/3 & 1/3 to obtain any available advantage from higher pressure in bell #1 at the crossing point.    The channel between the bells would certainly be insulated, and the stove would have the usual 'chamfers' where necessary.   The stove is being built early on in the general construction.  That is, the structure's roof is to be built first.  The stove bells are made from clay mud bricks, so it should be pretty simple to make any changes, e.g., knock it down.  If it works out the way I believe it will, my next design will have two 'doorways.'   The experimental part concerns just how much the pressure in Bell #1, along with gravity upon cooling gases, as well as any occurring draft, will help out.  My hope is that the cooled gases from bell #1 will be pushed and pulled into the crossing channel and still be warm enough to rise up into bell #2, and then, again with the help of gravity, to exit at the end.  I will be very happy if "C" works because it is then a simple matter to raise the floor so that there be no step.

Alden,

Please read this thread:

Hybrid-Masonry-Boiler-Heater

It talks about multiple bell systems. It will help you with sizing the two bells.

Alden,

I was re-reading your last post and realized it would help if you posted the layout of your house and the heater. I'm just wondering why you would need such a massive bell (ISA of 25 m2/270 sq ft) to heat your home?

Cristobal, I only have sketches of the build.  The structure is very simple.  A one-story rectangle. 20' x 40'. Gable roof 5:12.  I have decided that fluid dynamics does, indeed, permit me to separate two bells.  I will keep everything on the same plane, sketch "C", with the flooring brought up to make the step disappear.  But, as I may have mentioned, the stove will be done up with clay-mud bricks to enable easy changes.

Thank you for sharing the drawing.
Were you considering the rectangular bell in the middle of the room/house? The drawing does not show  any other walls that these two vertical lines, so a heater in the middle would heat it nicely, because most of the heat would come as infrared radiation. This way you could erect a chimney in the middle of the house and your space would not be limited by a long bell.

Cristobal,  Those two vertical lines should be ignored- the entire floor is an 'open-plan.'  Sorry for any confusion.  Cris, the bell is only about 4' high, so envision what is pretty much a long countertop, two feet wide.  The resultant table top will not be very warm due to the insulation under it.    Only the bathroom will have an enclosable space.  Said enclosure will be something like a sliding window panel on the top of the masonry stove.  This permits the bathroom to warm up when not being used.  It is hard for me to describe it here, but maybe you will want a photo someday.   Except for the bathroom, there is no need for walls.  The shape of the masonry heater is meant to follow the shape of the room.  So now you can imagine why I want a cut-out or doorway at some point.  I do not know yet whether the stove will need a chimney.  If it will need a chimney, it will be at the end wall where the stove butts up.  I will not know whether a chimney is required until I complete the build of the stove.  Very often, multi-bell FMG horizontal stoves will function fine without a chimney.  (Perhaps just a 'T' to block back winds.)  I know how odd that might sound to you.  Try to apprehend the fact that chimneys are a tool made necessary when forcing gases to obey human designs in their use of fire.  In a horizontal stratification bell with its proper firebox and proper operation, I would say that almost always the chimney can be dispensed with.  I do recall, however, that sometimes, a temporary chimney will be attached for the period of time it takes for the tons of thermal mass to dry out.  Also, a draft inducer or cheap inline fan can be utilized during the drying period.  If you are interested in FGM, you may wish to pick up the books written by V,E. Grum-Grzhimailo and Podgorodnikov and others from the old days.  If you ever find yourself building your own house from scratch, you will have the perfect opportunity to build its heater early on- the best way to do it, in my opinion.

I'm familiar with these concepts through other European masonry heater books. However, having  chimney is not violating free gas movements rules, just helps them. Heaters or even bonfires may work without a chimney, but with a chimney they will work better:
-chimney will help keeping inside bell pressure lower than the room pressure and prevent smoke (toxic gases) leaks to the room
-it will increase speed of gases so it will help the combustion in the firebox
-it will expel toxic gases AWAY from the house - by the time they will fall to the ground they will dissipate and mix with fresh air
If chimneys were dispensable, factories, metal works or power stations would be the first to stop building them to save a lot of money spent on 200-300 m tall stacks.
I have already built a house from scratch (CEB for all walls) and built a nice tall chimney between kitchen and the living room. The house is permitted, but even if it was not I would still build a chimney. I only regret that I did not build another one on the other side of the house.

Yep.  I am not disparaging chimneys at all.  If I need one, I do one.  I was referring only to horizontal stratifying bells- Not factories, steel plants, etc.