Hi, new to the forum, and I'm interested in building a
rocket mass heater.
I have a very large timber-framed room (very high ceiling/lofts). This room has a very
large central fireplace, open on two sides. I have a nice Englander WS on one side, but
would like to have a
RMH on the other. The stonework on the firelplace is very large, and,
of
course, the chimney is freestanding, probably close to 20' tall, not connected to any
exterior wall. So, to my question, I want to take advantage of all the thermal mass of the
stonework on the fireplace, and would like to transfer as much heat into it to "charge" it as
I can. While my description makes it sound like I have a majestic expensive house, the truth
is that it is an almost 40 years old DIY one built by two friends and looses nearly all of it's
daytime thermal gain to drafts and losses through the tremendous amount of glass surface area
of old-time single and double pane windows (and it takes 4-6 cords a winter to heat, and even
then not very well...). Anyway, now to my question. Most
RMH efforts are geared to very well
insulated, self-contained
heaters. My intent is to transfer all the BTUs I can to the existing FP.
I understand that
concrete block and sand are not ideal insulators - can I utilize this to my
advantage in this case? The problem is, of course, that I want to direct these "losses" along a path,
while still keeping all other surface areas from robbing my efforts of charging the existing FP. I
was probably going to use an 8" square steel intake, welded to a 9" dia. insulated chimney,
(very similar cross-sectional areas), so, I wasn't having cement blocks exposed directly to the flames.
(Also, my neighbor has some lengths of 8" sq chimney tile - would that be better for the burn chamber?)
One thing that I was wanting to do was build up a semicircle of brick or rock/cob around the barrel,
full height, about 3" away from it to try to draw off some of the higher levels of radiant heat into the
thermal mass, but I wonder if this will alter the swirling path inside the barrel because of the temp
difference from one side to the other. Also, I'm sure that will direct the remaining radiant heat more
narrowly, though I don't see a downside there. On the
cob - I've checked my
local soil survey, and most
of soils listed are described as clayey, but I've never really noticed anything clayey when I dig...Can
I use it anyway, or
should I buy it? (Trying to do this on the cheap, as I was out of work for nearly
1 1/2 years until recently, and now my wife is also). Also, another question is the distance from the top
of chimney to the barrel, typically I've always seen 1 1/2", but lately I've been seeing 2" occaisionally.
Is that for the larger stoves, or is it tied to the cross-sectional area somehow? And would it be necessary
to put another layer of steel at the top of the barrel so that it doesn't burn through with those super high
temps? One more question - I had intended to run 8" dia ductwork for the exhaust under the barrel for the
rest of the way, but will that be too much of a drop in cross-sectional area, (from .442 to .349)? And, even
if I decreased the burn chamber and chimney size to match this, how do you get around having to neck down the
exhaust hole under the barrel? If you center the chimney, you don't have room for a 8" dia hole. I imagine
most people compress the duct and make an ellipse, but you start losing area quickly when you do that. Anyway,
if anyone has any thoughts or ideas on how I can achieve this kind of derivation successfully, I would sure
like to hear it. This seems like a great forum with some very informed members. Thanks!
