Some advice please RMH to heat about 100 sq. ft

I need to heat about 100 sq. ft. of R-10 insulated (walls, ceiling, floor) wood framed space.  Would like to know how much "heater" I would need to keep the place warm (even at -30* F) for a day or two. The stove part would be 3 ft. by 2 ft. brick and a 6" system.  Don't have a lot of room so would like as small as possible, even smaller on the stove part if possible.

Any advice, suggestions or comments welcomed.

My feeling would be that for 100 square feet, you would not want any system with a major "instant heat" component like a barrel. Just the exposure of the feed tube or firebox door would keep you warm enough while the mass accumulated and radiated heat.

As size is critical here, I would consider a 4" batch box inside a masonry bell, which could be 2' x 3' or a bit less. If you want a bench for sitting/sleeping on (probably wise), the bell part could even be a bit smaller, and connected to a half-barrel bell cavity in the bench. The total internal surface area (excluding floor) of the bell(s) is important for extracting the right amount of heat from the fire for efficiency and good function. The website batchrocket.eu has a table for this sizing, and while 4" is not included, careful extrapolation suggests about 25 square feet of ISA. This would be an internal space around 2' x 6' x 1' high, which after adding wall thicknesses and accounting for the taller initial bell would give a bench 3' x 6' in addition to a bell over the core 2' x 2' x 2' taller than the bench.

A 4" batch box would be easy to build from firebricks, preferably "splits" that are 4 1/2" x 9" x 1 1/4" thick. I recently assembled one as a test which worked well and required no brick cutting: https://permies.com/t/59459/EZ-Tools-Brick-Micro-Batch#505805

Thanks Glenn,

I had looked at your's before and like the idea.  Using a 4" pipe would be better I think.  This is what I see if looking at the stove from the top:

6" for feed space, 2 sq ft for raiser and  6" for exhaust, would that work?


Using a 4" with a 6" outer pipe then a bell of 12" or so then a 'honeycomb' of brick around the bell to allow some heat out. The 4" exhaust pipe would be inside a brick chimney. The back wall of the heater would be brick.  How much heat do you think this would put out?

Don't know if this matters in the design but it will be on a plywood floor with 4-6" of perlite then a 3/4" mason board between the floor and RMH.

http://donkey32.proboards.com/post/15925/thread

http://heatkit.com/research/2009/lopez-rocket.htm

http://technologieforum.forumatic.com/viewtopic.php?p=1530#p1530

Thanks guys.  

Back to the drawing board and this is what I come up with:  12" wide x 36" long, give or take a few depending on brick size.  A 6" square fuel feed, 4" square burner,  4" inside a 6" raiser (both round) with a 8" square (inside) brick bell then a 4" round stovepipe exhaust inside a brick chimney. Two sides could have a 7' brick wall on them the width of one brick.

Now the questions.  Would this work?  How much heat do you think it would hold/give off and for how long?  How long should the burner be (center of feed to center of raiser)?  How high should the bell be (it will have a metal top)? And last should the stovepipe have sand between it and the brick?

I live in Alaska and our winters have been getting warmer as the years go by but we could have -20* F at times.  Would like to keep the place 65-70* F. and not have to keep putting wood in every 30 minutes.

A 4" J-tube system is very small in heating capacity, and would have to be fed probably every 10-15 minutes. A 4" batch box would have something like double the heat output per hour of burning, and would have to be loaded much less often. (6" batch boxes are supposed to burn for an hour or so per loading; a 4" would be less, I don't know how much.)

Insulation is even more important in small systems than in large ones, so 1" of insulation around the riser may not be enough. I would make it 4" inside and 8" outside, with 2" of perlite-clay.

Do you want purely a tall bell, or would you want any bench too? I really think a heated bench that you could at least sit on if not sleep on would make you very happy, and be comfortable without need for higher air temps.

I think a bare minimum bell size, given the 8" OD riser, would be 12" x 12" inside, thus at least 20" x 20" outside (only one brick thick). A 4" batch box probably needs about 25 square feet of internal bell surface to extract the right amount of heat, so a 6' tall bell 1' square would do it exactly. You could easily make it 5' tall and expand the bottom of the bell to make a heated seat.

For RMH purposes, masonry is said to conduct heat at around 1" per hour, so a 4" thick wall would be hottest around 4 hours after you start the fire, then start cooling unless you made another fire. It would probably stay warm overnight, but not to keep the space at 65F all night. Brick fins that extend more than a foot or so from the heated parts would probably be useless.

Due to limited space the longest it can be is 36" and the widest 20".  Doesn't matter if its a j-tube or batch, whichever one puts out the most heat.  I could do a 6" system but in your other post you said a 4" would be better.  Would having 2 bells work better? One about 7' high and the other about 5', both 12" x 12" id?  The temp. doesn't have to stay 65* all night but shouldn't get below 50* if possible.

Thanks again.

Chad, a single Bell, single bell single skin could be doable. With batch. May be a bit expensive, as you will have to use a fiber riser.

Satamax:  A 4" or a 6"?  Would it hold heat  to keep the place at 50* F by the end of 24 hours?  Why a fiber raiser, why not just two pipes with insulation between them?  And would a round metal bell with bricks around it (square) be better than just a brick one?

A ceramic fiber riser has such good insulating qualities that 1" thick is sufficient, allowing you smaller internal bell dimensions.

The outside dimension limits you give mean that a 6" batch box would not be able to shed all of its heat before exiting to the chimney. A 4" batch box is likely to be roughly equivalent to a 6" J-tube in output per hour.

The same, with a taller bell.

Thanks for the video, that's what I'm thinking of.

I thought you were on that path. That's why i posted the previous links.  Which are all tall and narrow experiments.

I was calculating.  20x36, with bricks laid flat, that's approximately an inside rectangle of 10x25 in which you could fit a 4 or 5 batch. With a very tall bell. With a 2m tall (6.56') You would be about 35sqft of isa. Which would be quite nice. A cast iron top could give you the quick heat.  And even the firebox could double as a small oven. If you make the firebox stick out a smidge. You can even have a tiny cook top on it. The advantage of the batch, compared to the one in the video above, is that you can increase the size of your mass above the firebox, instead of having to feed through the top.

The other big advantage of batch, especially in Alaska where you need heat all day every day for months on end, is less frequent feeding and faster charging of the storage mass when you are burning.

I emailed Lars and he thinks it would be to much for 100sf.  He is going to sent me the plans.  Will use a 4" batch feed. As I will be going vertical with the exhaust (4" pipe inside brick or just brick?) should there be a double wall between it and the bell or can they share a wall?

I really appreciate the help.  

The exhaust should be cool enough that there is no need or benefit to using brick. It would be a 12" x 12" minimum volume. I would use a 4" stovepipe with a 6" pipe around it stuffed with insulation where you go through the roof and above. Or if you can get "official" 4" insulated pipe cheap it might be worth saving the bother of fabricating.

How hot is the exhaust when it first comes out?  Could ducting rated at 420* F work?

We can't say without knowing exactly how you built it, and even then could only guess. It requires testing and experience in the particular conditions. But a bell of the size Max described (appropriate to the combustion core size) should absorb most of the usable heat, leaving only 200-300F exhaust gases at most. If the exhaust to the chimney is hotter than that, you are wasting a lot of heat and should modify your bell to be larger or taller.

Here is what I think the bell area would be:

4" sq raiser made of 1"  thick insulated material with 3" between that and the outside brick. That would make the outside of the bell about 20" sq. (12" sq id).  Would it help to have double brick?  Given this size how long should the burn tunnel be from raiser center to front of batch feed opening?

OK change of plans.  We are going to build a 14' x 14' A frame cabin (about 15' center height).  So can go with a bit bigger RMH as we have more to heat.

In building the stove could I use 16" x 8" x 8" cement blocks with the holes showing then a one brick high (2 1/4") "wall" around the top filled with perlite then a 1/2" sheet of mason board on top of that as a insulated base against the plywood floor?

Chad, a drawing is worth a thousand words.

8" concrete blocks with the holes horizontal and allowing airflow from one side to the other, plus the perlite layer, would do excellently for keeping your wood floor safe. I would mix a tiny bit of clay with the perlite and dampen it so it stiffens up a bit and will definitely not squeeze out from under the loads.

Thanks Glenn, that's why I would use a cement board on top, to keep anything from squeezing out.

Satamax, wish I could post a drawing.

Glenn,  as we have been talking about using one in a small space I think I will start with your design and go from there.  The one thing I can't seem to find is what is the minimum clearance around the sides including the exhaust. That's using all brick.

Code requires about 3' or more of clearance to combustibles, unless the masonry shell is at least 8" thick, when it can have 4" clearance. This pdf has a nice graphic (on page 23): http://heatkit.com/docs/Man00.PDF

As I suspect code numbers are less relevant to your case than actual safety, I think a small single-skinned bell would probably be safe with a foot of minimum clearance to sides and back. You might want to add a metal heat shield with 1" air space behind it to the nearest wall. I would try to maintain the diagram clearances near the firebox door.