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BTU equivalent for RMH

Roy Hinkley


Joined: Jan 22, 2011
Posts: 28
Location: S. Ontario Canada
I'm designing our new home leaning heavily on earth berming and passive solar.
Our building codes state you must have some kind of conventional heat source but I plan to do most supplemental heating with a wood fired RMH. I have an excellent supply of hardwood that needs little cutting, only transport.

Before I buy the plans for a RMH I'm curious what the approximate BTU rating might be for the 6 or 8 inch plans?
Ernie Wisner
volunteer

Joined: Oct 16, 2009
Posts: 788
Location: Tonasket washington
    
  23
BTU's are a factor of the wood not the stove. An Rmh puts off the same amount of heat as a normal wood stove with the same size of fuel load. the mass absorbs the waste heat. then re-emits that heat in the house. so if the combustion of the wood is butting off 20,000 btu and you are emitting in radiation about 10,000 of those btu's and the mass is absorbing about 9,000 and reemitting about 7,000. ok this is just rough because lots depends on temp of mass, emitting surfaces, inside temp of house, outside temp, type of wood, condition of wood ETC. the math is not esoteric just the variables. Erica and I are working on the maths that will work for those figures but there is a change between the way steel emits and masonry emits.

now most of our designs have the exhaust at the last 90. the one that goes vertical at 100 degrees F. this allows for you to be able to burn your stove into the shoulder seasons.


Need more info?
Ernie and Erica
Wood burning stoves, Rocket Mass Heaters, DIY,
Stove plans, Boat plans, General permiculture information, Arts and crafts, Fire science, Find it at www.ernieanderica.info


Roy Hinkley


Joined: Jan 22, 2011
Posts: 28
Location: S. Ontario Canada
Thanks very much Ernie.
From another of your posts( http://www.permies.com/t/11727/stoves/limits-rocket-mass-heater ) regarding a greenhouse I see the 6 and 8 inch both have roughly the same exhaust length(45 and 50 ft max). The wood I have access to is of the highest quality, hardwood 3x4 up to 4 ft long used to ship steel coils. All else being equal it would fit better in an 8 inch. It's easy enough to cut to length, not so easy to split as it's full of knots, I'm guessing rejected for flooring and such.
Some assumptions:
I suppose it's just a matter of burn time and frequency to store the right amount of heat for the space?
Since the exhaust lengths are roughly the same in a 6 and 8 inch system I assume about the same amount of heat would be stored with the 8 inch but with a shorter burn time(larger surface area of the 8" in contact with the mass)?
Either way you'll use almost the same amount of wood?


Ernie Wisner
volunteer

Joined: Oct 16, 2009
Posts: 788
Location: Tonasket washington
    
  23
nope the 6 inch system has a feed and CSA that is 5X5 and 3/4 while the 8 inch is 7X7 and 3/4 (to give them the same relation) that two inch difference means alot in how much wood you can burn which equates to how many btu's you get. Good grade hard wood is going to let you burn less but its not going to let you get away with the fewer BTU's available for harvest. as a side note a 6 inch mass trying to heat the same space in the same period of time is going to be much hotter)

as for the distance of the two i think you might have missed the maximum for an 8 inch is 50 linear feet while a 6 inch is 40 linear feet; ten feet in this rhelm is huge. given the difference in surface area between the six inch and the eight inch. (this is also where i keep loosing folks because little changes have huge effects in RMH design.)

I am going to forecast that the next question is going to be one of scalability and i will say this much 4 inch systems for bench heating dont work well and 10 inch systems dont either because of a different set of problems. the practical choice is between 6 and 8 inch systems and i almost always recommend the 8 inch unless you have a very small space to heat.
Roy Hinkley


Joined: Jan 22, 2011
Posts: 28
Location: S. Ontario Canada
HA! Got you there. The next question will not be about scale. I've read enough to know there's some kind of sweet spot in the 6-8 inch range.

I'm looking at the plans and wondering about sitting on a slab vs a wood floor. In any design are you insulating from the slab floor? I understand one whole hell of a lot of heat is going to be sucked out if in contact with a slab.
Anyway, I'm ordering the Cabin 8" RMHeater plans. I hope to persuade our stick-in-the-mud officials it can be done.

Thanks for all the quick help.
Ernie Wisner
volunteer

Joined: Oct 16, 2009
Posts: 788
Location: Tonasket washington
    
  23
yes insulate under the bricks on a slab floor. its still going to soak up heat but you want it to not soak up much of it directly under the firebox..

he he he he you have to know you where tempted

the cabin 8 inch works very well (its in our house after all just 4 feet from me.) please follow the plans carefully. the proportions are very fine it this one and it should be a very very good stove for almost any one. hmm dont worry about the ducting run because i dont think most folks need to heat a bedroom like we are heating ours. On the other hand it works great
Roy Hinkley


Joined: Jan 22, 2011
Posts: 28
Location: S. Ontario Canada
Thanks Ernie.
This is for the bedroom section of a passive solar house. I figure the bench as a short wall right near the window to make a sort of "cold air well" that will warm the cold air falling off the glass as it flows over the bench and the mass will soak up some of the summer heat. The bedroom section alone is about 500 sq. ft.

Kevin McCune


Joined: Dec 31, 2012
Posts: 14
So how many lbs of wood can you burn per hour in a 6 and 8 inch system? This would tell you the BTU output. But since these rocket stoves reburn the flue gases I have to assume that they put out more heat per lb of wood that a regular stove does, even not taking the mass heat recovery part into account?


Kevin
 
I agree. Here's the link: http://stoves2.com
 
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