Douglas Pohl

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since Feb 18, 2015
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Recent posts by Douglas Pohl

Season 2 topic suggestion: batch rocket mass heater - build it right the first time then tinker (ask Ernie & Erica to share content from their builders book)
3 years ago
Scott, take a look online at what is working small in Japan - might be to your needs - See:  and
4 years ago

will marconi wrote:all good information.. does the 8" batch box system have a longer burn time compared to the 6"? or does it put more BTUs in thermal storage faster?

Please engage mind before mouth or in this case "fingers".

Study some science first...
5 years ago
American Beech 22,700,000 btu per cord
128cuft = 1 cord

22,700,000btu / 128cuft = 177,000 btu per cubic foot

If a batch rocket firebox was for a:

6" riser:
8" wide
13.5" height
18" deep
8 x 13.5 x 18 = 1944 cubic inches

Cubic foot is 12" wide x 12" height x 12" deep = 1728 cubic inches per cubic foot

1944 / 1728 = 1.125 cubic feet


1.125 x 177,000btu = 199,000 btu (if you could pack the firebox solid, therefore say 80% = 159,000btu per burn load.

8" riser:
13.5" wide
22.5" height
27" deep
13.5 x 22.5 x 27 = 8201 cubic inches

8201 / 1728 = 4.7 cubic feet

4.7 x 177,000 = 832,000 btu x 80% = 665,000 btu.


5 years ago
Work backwards from a cord of a specific wood to calc btu/cuft then determine how many cuft in the batch box for one burning.

Here is where to start:

A cord is 128 cuft.

OK... calc away...
5 years ago
If you can imagine it you can have plans made and build it...

5 years ago
I prefer to not comment without complete engineering details... hydronic heating is "soft comfortable heat" that I use on my boat - your USStove was designed for heating - consider buying, renting or borrowing professional combustion instruments to optimize how you operate your USStove. Outside air is better than using the air inside your home and sending your warmed air out the exhaust (I assume your USStove is in your home structure and not is a detached shed.) A quick back of the napkin calc shows for a cordwood such as oak with about 30 million btu/cord that you are consuming about 675kbtu in 12 hours, hence 56kbtu/hr daytime or over a 24hr period just 28kbtu or about 8kw/hr/day which seems ok. Everytime you burn a log look at it as $0.75 a log or 9x = $6.75 (based on $300/cord?) or only $202.50/month which is good for winter heating - stick with your USStove and evaluate adding/fixing new insulation (rent a thermography unit like a FLIR TG-165 to locate cold or poor insulation) this summer before building a R-stove (not UL approved so your insurance company might say if a fire you have no insurance) unless you are going to consider a mass rocket stove heater or a masonry heater... beautiful but is resource intensive to fund and build but offers a long term homeowner payback and property improvement. If you are going to build a mass rocket heater - consider starting small for a dedicated space or room... see the above room mass rocket heater picture. Small can be beautiful!

I just re-read your first post... "R 25 wall, R 60 or better in ceilings. The wall insulation is closed cell spray foam as it is a good wind barrier. 3100 sqft. 3 floors total. 2 floors 36x36 and a 3rd floor centered 16x36."

$202.50/month for heating a 3100 sqft home... keep the USStove - its working well.

Correction to an earlier post regarding a square riser: if you build a square riser, i.e. using fire bricks - the circle-riser size (in the above Peterburg table dimension) must fit inside the square riser to be equivalent. i.e. a 4" inside-diameter (circle) riser must fit inside a 4" inside square riser.

5 years ago
Considering a "J" feed means hours of tending a fast burning fire... maybe adding new wood every 30 minutes... else consider a batch box rocket - look here:
Hope that helps you build it better the first time...
5 years ago
So many times a picture is worth a thousand words... here is a "cast" batch rocket picture from one of the forums... credit goes to that author - Peterburg? You can build using fire bricks too. Now imagine a steel barrel or propane/air tank over the riser with an exhaust on the tank lower rear side... the metal radiates the heat... much more design work needs to be explained to optimize this R-stove but its all in the forums mentioned so dig in and expend some due diligence elbow grease... don't think a barrel or tank is the only style... the below picture of a mass rocket heater is very attractive.

5 years ago
I'd recommend you consider a batch box unless you like sitting and tending a stove for hours on end - the fuel burns fast so you need to "store the heat" with a good thermal mass or start with a 30-55 gallon barrel or propane tank (single, double or triple high stacks). Most "J" feeds will burn the wood down in less than 30-40 minutes so you are chained to tending a "J" feed rocket stove... build a "batch box" which is much more friendly... but be sure to plan how you will store the heat from burning wood (about 8,000 btu/pound for wood/pellets) How many btu(s) do you need per hour for heating a space? Depends on how much thermal storage and insulation "R" value to stop the heat loss... make an educated guess first using math then experiment to see how you can improve the situation.

Here is the holy grail to a basic batch box rocket stove design.

The dimensions of the batch box rocket stove are:
The assumption is, there should be a common base number to which all the other dimensions are related. Thank the forum senior members for all of the hard R&D work.
That BASE number is calculated from the diameter of the exhaust riser diameter.
BASE reference number is 72.34% of riser diameter. i.e. 4" diameter round or square exhaust riser x .7234 = 2.8936 BASE REFERENCE NUMBER
Width of firebox is 2 times BASE.
Height of firebox is 3 times BASE.
Depth of firebox is 4 to 5.5 times BASE.
Height of 'throat' port from firebox to riser is 2.2 times BASE.
Width of 'throat' port from firebox to riser is 0.5 times BASE.
Tuning the "throat" dimensions controls the gas mixing and burn turbulence which will greatly affect the stove draw... watch the flames and listen for the air rushing 'rocket sound'.
Height of exhaust riser is 8 to 10 times BASE, measured from the firebox floor. NEVER SMALLER ELSE IT WILL NOT DRAW WELL.
The firebox floor consists of a narrow flat surface the width of the firebox. Left and right there are 45 degree slopes in order to concentrate the glowing charcoals into the middle as the wood burns down. Those 45 degree chamfers are inclusive part of the dimensions of the firebox. In addition, there’s also a similar shaped piece at the rear bottom of the riser.
The total air inlet is 25% of riser cross section area (csa). Riser can be round, octagon or a square fire-brick shape... square works ok but not as optimum as round. For a first stove build it square using fire bricks - works ok.

P-channel is 5% riser csa.

Main inlet plus window wash is 20%. Main inlet could be larger when starting cold and is situated level with the floor of the firebox.

P-channel should be as wide as the port or slightly more, for the calculation of the 5% you should take the width of the port, not the actual width of the duct. This duct is hanging over the top of the port the same distance as the depth of the duct. Build the box first then add the p-channel port and optimize with a few test fires.

The back of the p-channel which is resting against the firebox rear wall has been cut away over the height of the overhang.

Exhaust is never smaller than the diameter of the riser.

Search here and on for more build suggestions and details...
5 years ago