Heating a big space efficiently

I am trying to find out how hot a p-channel needs to get to be effective.  I have been inactive for a number of years due to health but am moving back to my shop and toys.  The location is at 6500 foot elevation and gets cold enough in the winter to need heat.  Nearly all supplied by wood.  The building I work and sleep in is about 6000 square feet single level. I add a lot of insulation but the building has very old windows completely around it with simple two layer glazing. In the past I was able to heat it with a 3-barrel stove, the two barrel version of which was rated as 200,000 BTU per hour.  The problem with it was a bad tendency to soot up.  Winter last year when I was not around to see to the regular cleaning, a chimney fire resulted that nearly burned the place down (charred wood in the attic).  So I replaced with a more conventional wood stove from a friend.  Unfortunately, it also soots up even worse.  So I am exploring the possibility of adding a "rocket" afterburner to it to get cleaner burn.

I have an 8" Dragon cast burner unit in one shop (and a 3-barrel in another) from a number of years ago (I still see the same chamber images but apparently that supplier is no longer in business). I have built a number of rocket stove experiments outside with different materials and designs, but have  run into trouble with materials like cinder blocks and clay flue liners breaking--I assume from thermal shock. (yes i have also worked with fire brick and ceramic wool)  I have also built 55 gallon barrel size forced secondary heated air water heaters that heated water at about 300,000 btu/hr--the problem being that the steel burn chambers did not last long. My goal is to eventually end up with a 200 to 300 K btu/hr rocket type stove that is durable--if I live long enough.

A p-channel is what supplies secondary air to an already burning fuel stream in a J-tube or early batch box. It works with straight room air, or with preheated air. I think for your use, I would just call it a secondary air inlet, as you have a different configuration.

I think you would need to couple the afterburner so closely with the stove that the stove exhaust was still something like 500-700F for secondary combustion to take off. It would likely be more reliable with preheated secondary air. It might be easier and more reliable to build a standard batch box RMH than tinker with trying to improve your existing stove.

At 6000 sf on one level, I don't know if a single point heat source would cover the whole space adequately. What kind of construction is the building, what floor structure, and what is the existing chimney? What sort of room dividers are there?

To determine the size of a batch box RMH you would need, this page of the batchrocket.eu site gives nominal output in kilowatts throughout a 24 hour day, which you should be able to convert to BTU/hour as 1 kW equals 3412 BTU/hr.

Hi Chaz;
I agree with Glenn.
In the time you have been gone RMHs have progressed beyond J-Tubes.
A 6" batchbox with a double brick bell, can heat a fair size space.
In your case, you might look at two 6" batches, one at either end.

Here is a link to a superb build at 8000' in Colorado.  
https://permies.com/t/248275/Batch-Rocket-Double-Skin-Bell
Be sure to check out the build itself.

An 8" built similar to Glenn Littman's might be enough for your whole shop.

Thanks to all for your input.  If I did not currently need the heat I would consider a batch box build.  As noted,  I have adequately heated the space in the past with a bit over 2200K btu/hr.  I have short ducts with fans that point to either end from the center where the stove is, with a collector above the stove.  The building is stick frame. I added blowin fiberglass under the floor and above the ceiling with double reflective foil over air bubble to hold the below floor insulation up. I had 2" rigid foam added to the outside walls and covered with reclaimed aluminum siding.  The existing stove has a simple steel sheet for the top with the fire box directly under. The flue exits at the side top of the firebox. The top gets to 700 to 800 degrees F.  My intention was to run a P-tube across the stove top for preheated secondary air and bring it in right at the exit from the fire box. The secondary burn chamber would be immediately after the first chamber exit. Not sure about having it vertical versus horizontal. The primary objective is to cleanup the exhaust but it would be nice to recover some additional heat. I have not yet figured out how to do that.

a couple points. the building in question was originally heated by a pair of 110,000 btu/hr NG forced air units.  Assuming 80% efficiency, it would take about 30 pounds of wood per hour to get  a continuous 200,000 btu/hr (if my Math is correct).  The insulation has been improved considerably since these NG units were used (NG no longer available on this site). At this point I am more interested in heating air than mass. Has anyone ever tried sending the output of a rocket stove through an NG heat exchanger?  Seems like this might be a good compact way to get heat from the gas stream into a room as opposed to a barrel, bench or masonry bell.

More recent situation.  I need to heat about 60,000 cubic feet of space quickly.  I have seen several images of 4 barrel  batch-box rocket stoves that seem to have this general aim. However I have not been able to find plans or detailed information for them. Can anyone please direct me to such?

I have spent a lot of hours searching but have some physical limitations that limit my efficiency.

There is a good article about building a three-barrel shop heater at batchrocket.eu. That one uses a cast core, but you could equally well build a firebrick core instead.


If you really need the heating capacity of four or more barrels radiating, you could simply insert a single- or double-barrel bell in the exhaust stream of the three-barrel bell, which would be hot enough to allow more heat extraction without compromising draft, assuming the core is sized big enough. Batchrocket.eu has formulas for sizing cores and bells according to needs.

Thanks Glenn. Some how I missed the 3 barrel build.  I have an 8" Dragon core with three vertical barrels in another building. The downside is it does not hold much wood and requires frequent feeding to warm its space much.

If I interpret Peter's data correctly, an 8" system should support up to 5 barrels.  A 6" 3 barrels. The challenge here is that my expensive newly installed triple wall stainless flue is only 6". I do have an old 6" flue next to the old one so I think I could run the exhaust of an 8" system into both to get the equivalent of an 8" flue.  The exhaust temp of a multi-barrel bell should be cool enough and clean enough not to worry about chimney fire. Not sure yet if I have enough ceiling height to stack 3 barrels.

I am currently experimenting with a 25 kbtu indoor propane heater to see how much that heats the area when cold outside.  Hopefully the Btu requirement versus temperature will scale relatively linearly.

Has anyone made a chart or list of BTU/hr for different types/configurations of rocket stoves?  I have seen what appears to be KW or KWH per day from a couple firings per day but I am not sure how to convert that to BTU/hr for continuous firing which is how most heating appliances like gas furnaces are rated.

I would multiply the BTU/hr by 24 to get BTU/day, then convert that to KW/day or whatever units are appropriate.

Glenn,
No problem with that but I am trying to go the other way. Most specifically at the moment, how many BTU/hr could you get with continuous burn of a 6 or 8" batch box rocket stove?  Don't want mass in this case, just fast heat. I am currently working on determine how many BTU per hour are needed to keep she space relatively comfortable. Once I have that in hand, I need to figure out what size BBRH I need.

Where I have seen KW mentioned, Is that kw/hr if continuously fired? Generally noted for 2 firings a day so does that mean 12 hours per firing, in which case one firing would last an hour so 12 times the kw would be the KW per hour rate?




































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Have given the situation OP describes a bit of thought, and it occurs to me what is needed is to graft the rocket concept onto a central heat distribution system. I am aware of one direct application...........gasification boilers.........and another older system that uses hot air convection......same as with most forced air heat systems.

Commercial made gasification boilers.......both updraft and downdraft can be purchased, but are fairly expensive. The most common in US are the outdoor boilers. Nearly all of those are batch burn of firewood. But move on to the euro market and there are larger boilers that use biomass.......aka.....wood chips with automated feed systems. Very expensive.

As a kid, the home we lived in had a wood or coal fired forced air furnace. My grandparents had a big two story home that had a similar furnace in it. I don't know the mechanism, but a fire in the firebox heated an enclosed area above, and then the hot air was pumped thru ductwork like any other forced air furnace. I can't remember how the heat exchanger was setup. My dad built a 28 x 40 shop and it is also heated by a stationary wood burning floor furnace, with blower. No other  ductwork or heat distribution. Not even radiant heat as the firebox has an insulated jacket around it.

I found this photo on another site of a one of build a guy uses to heat his uninsulated metal shop. Both radiant and convection heating......

He runs a hot fire in the heater, and heat exchanger are the pipes that run thru the firebox. But imagine a batch box running off to the side and exhaust routed thru this like a bell........and heat output ducted so it could be pumped around?

Lastly, am curious if anyone has ever used wood chunks.........about tennis ball sized chunks in either a J-tube or downdraft gasifier? One that could be employed as a continuous burn?

This is a commercial boiler setup that comes as close to continuous burn potential as any. Use wood chunks vs. sticks of firewood?

Now that I think of it I grew up in a century old drafty frame house without insulation that my father had retrofitted with a wood burning forced air furnace. It was in a relatively mild climate but consumed 2 cords of wood a week in the coldest weeks. Under the right conditions/equipment, I could cut, load and haul about 4 cords of maple in a day. My second floor bedroom single pane window would accumulate ice on the inside on the coldest nights.

I used chunks of wood in the gassiffier units I built.  Had only a very small door for loading.  Again, the metal burn chambers burned through after a relatively few firings.

The current building does have forced air gas furnaces, one for each end.  When I first tried them, concluded it would cost $70 a day of propane, which is now even more expensive.
The problem with a rocket stove is that I have not figured out how to relatively simply get hot air into the existing forced air system. I also am really hesitant to run two wood burners for the building.  Up until this year, I have been using a central triple horizontal wood stove with an open collector above and ducting/fans pointed at either end. Again, the problem with this was dirty burn and chimney fire. I have thought of an external boiler with heat exchangers in the forced air systems but the cost of commercially available systems has been more than I have been willing to swallow.

The video added to this thread is interesting. It looks complicated and expensive. I have not figured out where the flue exhaust is on it.

Thanks again for the response thoughts.

I understand batch box firings generally last around an hour, so if a "standard" daily output is based on two firings, multiplying daily yield by 12 for continuous firing would make sense. I would probably cut that in half for uncertainty in actual length of a firing plus the unlikelihood of actually getting 24 hours of continuous burning. You might need a considerably larger bell/mass to properly absorb all that heat; the hotter the mass gets, the less of the combustion gas heat it will be able to absorb.

Thanks again, Glenn.

Yes, I agree real results likely considerably less.

I found an error in the version of Peter's spreadsheet as I was making a modification to it to include a 12" set of calculations. The box depth for 8 inch
showed the same as 7". The table on Peter's batch box website is correct so I don't know where the one I printed out came from.  Anyway, a word of caution for anyone trying to make an 8" system from the spreadsheet I printed.

Good day Chaz,
Don't know what your capabilities, limitations or thoughts are, but this ol' boy put some creative work over a few seasons into incorporating efficiency into a nice hybrid system. I imagine you've seen it, but if not, maybe it will help.
Because you sounded to be in a hurry, I'm guessing you needed heat.....like two months ago.
Hope this finds you well and warm, good luck, Mike.

Mike,
I did not see a link or way to get to what you referenced. Or is it you?

Yes, I need heat as the current homemade conventional wood stove (made by someone else) is not coming close to doing the job but  I am actually looking at something for next winter. I am now somewhat handicapped but have a fairly extensive metal shop and have built a variety of rocket type experimental stoves in the past.  I am getting old so want to minimize the amount of experimenting for a large box batch rocket stove.

https://permies.com/t/73673/Batch-box-RMH-Jotul
Now appeared at the bottom of my page, looking at your thread.

My apologies, had it bookmarked, forgot to attach.
Listening/looking further at what you've already done, I'd say you are past this.
I am a carpenter and mechanic, lot of oilfield work, some work as a millwright. Have a sawmill and decent basic metal fab tools/abilities. When we are up north we live way off grid, fully wood heated. I have a lot of work next few years as we renovate/ build. Plenty of ideas for change.
Not anywhere near ready to give up my old two level Fisher woodstove, but may modify it a bit. Would very much like to get more heat out of less wood/particulate. That's my interest. Standing next to that 400F cooking surface with coffee/breakfast on when it is really cold out, is one of my life's joys. Crawling under my house to build a foundation for one of these brick monstrosities, will not be. But, I am open minded to change.
I'd be very interested in seeing what you have/had and why you think it did/didn't work.
Good luck.

trying to get to some of my old images of things I have tried but have not yet figured out how to get any of them here.  Can't do a drag and drop apparently.  Am not where my shops at present (6500 feet altitude in center of NM) so can't take a current image.  Before rocket stoves I was playing with forced air gassification.  55 gallon size burner under a big commercial previously natural gas water heater.  Was able to get 300,000 btu per hour heating and sometimes little visible smoke.  Main problem was steel burn chamber burned through after a few firings. Small opening made reloading hard. had to use small chunks.

Hi Chaz,
you can either put the photos as an attachment when posting a reply. Then they will show up at the end of your post.
Or you can link to a photo host using the Img-button and entering the URL.
For more info, feel free to visit https://permies.com/w/how-permies-works

As mentioned earlier, I started out with gassifiers. The image below shows the basic forced air burner with 55 gallon drum outer shell.

Still trying to find an effecient way of getting images up here.

Another image of gasifier water heater

Previous post attempt failed.
at the bottom you can see the small muffin fan the forces combustion air.

The image that is listed as bottom of burn chamber shows the holes in the bottom that let in primary combustion air. Not visible at the top rim are holes that let in preheated fresh secondary air for burning off smoke and such not originally oxidized.

The image of flame showing secondary air where you can see kinda holes on the far side and jets of flame on the near side. This is the fresh heated air coming in to the top of the burn chamber and creating the vigorous flame you see at the top of the image.

The disassembled gassifier image shows the top of the burn chamber in the top ring.  You can see the 2ndary air holes where there is light colored gravel below. You can barely see the initial burn-thru of the burn chamber. The middle container went around the burn chamber to provide the channel for heating the secondary air.  There was vermiculite insulation between middle chamber and the outer 55 gallon drum.

chaz van wrote:Still trying to find an effecient way of getting images up here.

Hey Chaz.
You can put several photos in the attachments. With explanatory titles, that might work. As mentioned they will all show up at the end of the post.
Short posts with one photo each, as you are doing, I’d another option.
If you upload your photos somewhere (also once they show up in a post on Permies) you can include them into posts with the IMG button.

OK, here is my attempt to upload multiple images of the Dragon 8" J-tube with no mass and 3 radiant drums in one of my shops. welded up a container from 1/4 steel plate. Used short square steel tube under to keep off floor. filled space between containment and fire box with vermiculite.  

I like that gassifier barrel. Reminds me of a stainless steel porch firepit my mom gave us couple years ago. Pretty clean burn/reburn.
If that setup had a piece of casing instead of a 30gal grease drum inside that sadly defunct old air compressor, the chamber would have lasted. Chamber also needs a method to more quickly conduct heat to the thermal mass. Anyhow, cool I like  the idea.

If I understand it correctly the 3 drum stove sooted up. Don't fully understand these types of stoves and haven't yet tried to build one. So without getting into ratios of box/flue/mass/inlet whatever, that for the configuration you built, without a taller flue inside the drums, the smoke is losing velocity too quickly, getting lazy. Preheated reburn air, don't know.

You've spent some time around 3ph480v apparently. I have an Enerpak porta power and couple of rams. That press is priceless this day and age.

Tommy,
Was there a question in your last sentence?  Yes, I have a fair bit of experience with 3-phase power at both 2xx and 4xx volts. The 50 ton Enerpac has had several different 10K pumps at different times but none have been 3 phase although I have one pump that is.  For shop tools needing 3-phase I use variable frequency drives to convert single to 3 phase.

There was a muffin fan forcing both primary and preheated secondary air.  The smoke and other un-burned gases got burned at the top of the burn chamber where the ring of holes with fresh hot air mixed with what was coming up the burn chamber. This mixed combination moved on up into the tubes of the water heater. The flame action was vigorous and turbulent. There was no smoke coming out of the top of the water heater when it was running right.  There was no soot buildup until the burn chamber burned through. Virtually all the heat went up into the water heater tubes with vary little separation from the burn chamber so I don't see how you could move heat to the thermal mass much faster. There was no other thermal mass. The burner assembly was well insulated.  The "proof" was in the performance I got that closely matched the natural gas performance of the water heater. I never got a chance to measure burn efficiency but I feel it was pretty close to the theoretical you could get from burning wood.

The small drums of the burn chamber were obviously not enough for the temperature. Thicker wall material would have lasted longer but would have still oxidized through with time. The bottom line is that the burn chamber would need to be some sort of refractory.  This would reduce the preheating of the secondary air by I don't know how much. After this I made several attempts at a rocket burner in place of the gassifier but the chamber materials i had kept cracking/breaking.  I think due to thermal shock.  I ran out of "spare time" to pursue this further. At a future time I intend to post some images of these failed attempts.

Yes it was very much like a stainless outdoor smokeless fire burner I have that also used forced air.

Apologize.
My comments were not clear. Two separate thought trains. My lazy smoke idea was for the tall, three drum shop stove that apparently sooted up.
The barrel water heater burner looks like it worked quite well. My little burner is passive only. Short of my old RSF reburn 'efficient' woodstove it has been my only first hand exposure to any of this and part of the reason I became interested.
I have an antique two step Fisher in my house I'll never give up, love the heat. But, I will make it run more efficiently.  
I have a bulldozer and backhoe I want under cover, via garage extension, a bermed greenhouse and small outbuilding. All three need some form of wood heat
Anyhow, your work is very informative, good luck.

Tommy,
I don't know where you got the sooted up for the vertical 3 barrel and yes I misunderstood which unit you were referring to. Can't comment on the lazy smoke idea at this time. I don't remember what the temperature profile of the 3 barrels is. I will document it when I get a chance, hopefully in the next month. One thing I though about but never did was to embed thermocouples in my builds to record internal temperatures so I could really tell what is going on.

Got you on wanting to cover stuff up. My main issue is what the sun does to everything exposed. I am getting some simple quonset hut type open covers assembled this winter to put some things under including skid steer and backhoe. I have a lot of building but it is all used up and more for inventory. No garage as such.

I got started on a semi-buried greenhouse some years back that I never finished. I got a pair of trenches dug and put up metal frames for the glass but then never finished mounting glass. I had planned on a rocket heater for it. I have a huge amount (thousands of sheets) of solar panel glass. It is sort of frosted. The good and bad part is that it is tempered so it can't be cut to size.