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.