posted 4 years ago
An important consideration for advice is the structure of your house. Is there a basement below the lower floor, or is it on grade? How strong are the floors? Which way do the floor joists run relative to the bench you are considering? If the joists are perpendicular to the bench, the load can be spread over many joists; if they are parallel to the bench, a few joists have to carry the whole load, which is not likely to be safe.
I second the idea of having only a short bench on the main floor, and a masonry bell rising up through the floor and exposed to both upstairs rooms. As long as you can make a solid foundation under the bell, you won't have to worry about overloading floor joists in several areas. I think this would allow you to more easily fine-tune the heat output to the different rooms. If one gets more heat than the others, add some mass thickness there and heat transfer will be slowed to that room. You would not have a long path for flue gases but a short one with little drag. With an existing chimney at the side of the house, expense there is irrelevant.
I would run the exhaust from the bell up from the lower floor level along the ceiling of the lower floor to the chimney, positioning the bell and combustion core closer to that side of the house to minimize the distance. You could then make a bypass from nearer the top of the bell to the chimney pipe for easier starting if needed.
Another bell possibility would be making one bell on the lower floor, with a plunger tube exhaust rising to a second bell on the upper floor. Raising or lowering the bottom of the plunger would adjust the proportion of heat that is kept in the lower floor, with the balance going to the upper floor. It would need piers to support the upper bell, but not as many floor joists would need to be cut away as to clear a full-height bell.