Requesting Technical Review on Longwood Stove Design

Hi, We have just built a steel framed longwood stove at Open Source Ecology's Factor E Farm.

The basic rationale: an angle iron frame is fabricated to which one can weld "holder" nuts to clamp sheet metal into the frame, theoretically sealing the chamber.

See design/fabrication page here:
http://opensourceecology.org/wiki/Modular_Stove_Fabrication_Procedure

One can then build thermal batteries around the stove.

We used some of our CEBs to build walls on each side of the stove, and a base underneath.

Some of the team here decided that it would be cost effective and equally functional if the CEB walls were in direct contact with the burn chamber (Less metal cost, more conduction). See below.



The above pic is current, but the stove is still under construction. All steel panels are held to the frame by bolts/nuts welded to the angle iron.

We then plastered with a sand clay mixture in an attempt to seal the gaps between steel and brick.

My concern is sealing the stove, as the parts where the bricks/plaster meet the steel will shrink away from the frame while drying. Is there a surefire way to get an airtight seal on the parts where there is clay/sand meeting steel? Refractory cement perhaps?

Thanks for your time!

First and foremost: fantastic job on the info you provided! Process mapping, diagrams, costs, even installation.

But I have two major concerns:

1) You've got a massive firebox and only a 6" flue. That stovepipe is going to be extremely hot... plus, I would be concerned about proper draft. Since you're venting straight to the exterior of the structure, even after that long inclined run, you're still going to have some very high temps at your exhaust. Many years ago, I lived in a house that was heated with a modern manufactured wood furnace that had a 12x14x24" firebox--and a 10", triple-wall flue. (Even though it was a double-wall, all steel stove with a 3" airgap and a 100cfm blower, it was tremendously inefficient.)

2) I would assume from the pictures that this is an earthen/cob structure? What is the composition of the wall that looks to be the back of the firebox? Can it handle the heat that this monster is going to produce?

In general, even with your fastidiously documented process so far, I'm unclear as to what you are trying to achieve. It's obvious that you intend to burn 18"-24" logs in this... that's a lot of fuel for just one burn. In a nutshell, your design is a massive airtight stove set into a thermal mass. If that back wall is as stout as your direct-contact masses, then you will have a lot of retained thermal energy, and a ton of radiant heat from the exposed steel, but you're still throwing away a ton of energy--and unburnt fuel out your chimney. That's the main concern with the flue size: even though this stove will be sure to burn hot due to its size, its burn dynamics won't achieve high enough temperatures to burn all of the gases and you are going to be looking a yearly service of that flue to prevent creosote build-up. How large is the space that this is going in? Large, I assume?....really, large?

I hate to ask this question at this point in your process, but have you looked at some of the larger rocket mass heater designs found on this forum? I would be willing to bet that a RMH could be built into the same space, produce just as much heat (short term and long term), and would use a fraction of the fuel.

To answer your question about sealing the steel to the finished mass: furnace cement, although it is rated at 2000F, it is expensive and incompatible with earthen plaster. You might try high-fire clay slip, but you will still have some shrinkage. Any metal-to-masonry seal is always problematic--it also begs the question: why not just weld steel panels to your frame, check for leaks, then encase it in a thermal mass? Your efficiency gain is almost moot because your increased rate of thermal transfer to the masses means a decrease in the overall internal temperature in your firebox [less gases are burned] that would be provided by the missing three walls of steel.

wow, thanks for the insights. Your perspective helps clarify some things.

The house is 3000 sq ft, with 1000sq ft of the space independent of the rest of the house, meaning there are four rooms on the side of the house which are connected, but not accessible from the main space. We are looking at rocket stove bed or bench heaters for each room, to be constructed piecemeal (only one of the rooms will be lived in this winter).

The back wall of the stove is Compressed Earth Block plastered with sand/clay. How does one determine beforehand if a direct contact mass can "handle the heat?" I was concerned about having any bricks in direct contact since we are not sure of the composition of the soil used. But I was told, "lets experiment and if they dont work we will take them out and put in the steel panels."

The design and build of the house is documented here:
http://opensourceecology.org/wiki/Hablab

About the draft of the stove: I was reading your post to my friend, and she asks "if we just want to burn less fuel, why does the size of the flue matter?"

On the heat of the exhaust: If you see the "rails" on the top of the frame in the pic I posted, that is where we will slide in a sheet of metal to rest under the flue to serve as a flame deflector. Might this help to keep the exhaust temps a bit cooler?

Okay, now that I've seen the overall picture of your design, I'm impressed. Rammed earth bricks are a superior building material...period.

As you mentioned, composition is the issue. You would be hoping for a high percentage of clay, if you want the back of the firebox to hold up over the long term, otherwise, I'm fairly certain that a 2.5ft thick earth wall will handle the heat...possibly too much. You might try test firing a single brick somehow and see how much it shrinks/cracks/burns/crumbles before you build a full scale fire in there. With such a thick compressed earth wall as an integral part of this stove, those masses on the sides are dwarfed in comparison to the mass that is that wall. I would recommend simply lining the back wall of the firebox with 1" thick refractory brick to not only regulate the thermal transfer rate into the wall, but to also protect it from abrasion from logs/stoking. What you could also do is increase the vertical run of the chimney and encase it the same material as the rest of the wall or a cob of the wall material, at least 6-8" thick, and dramatically increase the amount of thermal transfer into the mass of the wall-- However, the wall may have so much mass that it will take an awfully long time to 'fill' it with thermal energy. Plus, unless you've thermally isolated the walls from the ground, most of your retained energy will dump into the ground before it radiates back into the room.

As to the draft and the flue size issue: I'm going to have to stress that a 16cu.ft. combustion chamber is going to need much more CFM out than a 6" flue is going to provide--especially when operating at full temp. With a full load roaring at temp, you will probably get a big puff of smoke into the room if you opened the door because it will be easier for the stove to push all that air out the door than up the chimney. If you are committed to this incinerator, then I implore you to open up the flue. Remember: better draft = higher temp = more fuel is burned before it goes up the chimney. A flame baffle is absolutely necessary in a design such as this, but with only a 6" flue, you will be limited as to the amount of air that can be pushed through the stove at any one given time...and your creosote will build up much faster. Since the panels are bolt-on, wouldn't it be simple to remove the flue panel and re-fab it to at least 10"?

Chris, we just did a test run of the stove last night, and we indeed got some smoke back when we opened the door. If we were to put a small door in the main door that is 6", would that prevent the smoke coming into the room?

If not, what do you think is the easiest method of opening the flue? Would we need to completely replace the 6" with a bigger flue? We put a lot of work into fabricating the flue and it would be a drag to do it all over again.

Graham,

I'm sure a lot of time was spent on hand fabricating the flue, but let me ask you this:

If you were walking down a road and met someone coming the other way who stops to tell you, "Hey, look out! If you follow this road, you'll fall off the cliff up ahead! You should turn around and go back--there's a much better way to go if you take a different route", would your answer be, "Well, I've walked all this way down this road, I'd rather just head towards the cliff"?

Commercially available wood furnaces that are comparable to your design have a 40% smaller firebox (18"x24"x36") with an 8" flue.

Because of the burn dynamics of your device, and the size of the fire that you are trying to contain, I steadfastly recommend that you re-fab the flue to 10". That gives you the ability to put an 8x8" door on the lower half of the front panel and a small damper vent on the face of your ash drawer. Putting a vent on your ash drawer is key as this will not only give you draft up through your fire, instead of across the top of it, but it will also allow more of the coals that fall into the drawer to burn completely. The cardinal rule will be: always close the ash vent before opening the fire door to prevent the possibility of smokeback-- but with a 10" flue, you might not have that problem . You should also consider having a small damper vent on the fire door for draft options and to keep an eye on the fire without opening the door.

Chris Burge wrote:Graham,

I'm sure a lot of time was spent on hand fabricating the flue, but let me ask you this:

If you were walking down a road and met someone coming the other way who stops to tell you, "Hey, look out! If you follow this road, you'll fall off the cliff up ahead! You should turn around and go back--there's a much better way to go if you take a different route", would your answer be, "Well, I've walked all this way down this road, I'd rather just head towards the cliff"?

Commercially available wood furnaces that are comparable to your design have a 40% smaller firebox (18"x24"x36") with an 8" flue.

Because of the burn dynamics of your device, and the size of the fire that you are trying to contain, I steadfastly recommend that you re-fab the flue to 10". That gives you the ability to put an 8x8" door on the lower half of the front panel and a small damper vent on the face of your ash drawer. Putting a vent on your ash drawer is key as this will not only give you draft up through your fire, instead of across the top of it, but it will also allow more of the coals that fall into the drawer to burn completely. The cardinal rule will be: always close the ash vent before opening the fire door to prevent the possibility of smokeback-- but with a 10" flue, you might not have that problem . You should also consider having a small damper vent on the fire door for draft options and to keep an eye on the fire without opening the door.

Hi Chris, it has been a while since I read your response. Everyone at the farm considered and appreciated your feedback. Unfortunately, we were already well into the fabrication and the ones building it were understandably reluctant to start over. Plus winter was upon us. I have since left the farm, and have heard that the stove is having major problems with smoke back. They also refabbed the front door to have a smaller operable door with a lever from another wood stove, which did help with accessibility, but still smoking back. Just wanted to let you know your concern was well informed.