new member, new project.

hello all,

I live in WV and built my own off grid home. I am striving for self sufficient farm.

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. Unlike most down the rabbit hole, I don't agree with a small home. There are many universal household tasks that are not scalable. I believe as times get harder more family and friends will need help, so I am setup.

I found you guys. I believe in the rocket mass heater concept, I have bought the rocket mass heater 3rd edition book and using it as a reference. Currently I have a wood furnace with water coils connected to propane hot water tanks as thermal storage/dual boiler option, with a pex hydronic distribution system. Everything works great.

Winters can be harsh. I currently burn 3/4 to 1 cord a month. About 4-5 a winter. My chimney exhausts the house at head height at bottom floor. I have a long 6" double wall insulated pipe running up past the eve.

So, by reading your design and accomplishments you all convinced me that I am wasting many btu's maintaining high exhaust temps. Now coming out of winter I am selling my furnace and insulated pipe. I am convinced I can get more efficient and burn less wood.

The hole in first floor at the wall "can" accommodate an 8" exhaust pipe. I could make it bigger, but that would be a project as its a concrete wall.

I plan on welding a custom stainless steel water tank as my thermal mass, and using it as a heat exchanger to my current propane hot water tank system.

Currently I am planning on your 8" system using all brick with a metal cooktop/clean out instead of 55 gallon barrel. (putting more heat to the exhaust/mass section)

Quality and life of the stove is most important to me.

1. is 8" big enough or should I go 10"?
2. if I go 10" system do you think I can reduce to 8" "at the wall" on the exhaust? assuming I serpentine enough to exhaust at cold temps where the air volume would be reduced due to lower temperatures. I am thinking 35'
3. if I use 10 system what dimensions do you recommend for the feed tube, burn tunnel, heat riser, length of exhaust?
3. thoughts, comments, questions?

I think this is great site. if anyone needs solar, wind, off grid living advice email me I would be glad to help. wdmarconi@yahoo.com

Whatever you build - compare with this spreadsheet of recommended dimensions - get it right the first time then optimizing is easy... http://donkey32.proboards.com/thread/734/peterberg-batch-box-dimensions

Thank you for the reply and information Doug. I am thinking of making the heater out of fire brick and using the common J shape rocket. I am still digesting the chart you pointed me too and how to it incorperate it in my design.

Do you have any kind size chart for a 10" exaust for in terms of J shape fire brick rocket? Specifically the feeder, burn tunnel, and riser?

On your sugested chart what is the "base" demension? Would that be the burn tunnel? thanks for your time.

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 http://donkey32.proboards.com/ for more build suggestions and details...

I concur with the batch box design instead of the J-style feed. Unless you plan to sit with the firebox and fiddle with the wood every 5 or 10 minutes while it is burning. If you like that, then go with the J-style feed.

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.

lots of information to digest. I am studying it. very cool! I wanted to put in a glass door anyway. I am working on my house load. Using the numbers you shared with me, and what has been currently working. Surely the rockets will be more efficient. I would be most happy to reduce my wood consumption. This is a link to my current furnace.

http://www.usstove.com/index.php?route=product/product&path=125&product_id=977

I drilled it and installed water coils for (2) 50 gallon propane water tanks as storage. On a typical 20 degree day I burned about (9) pieces of hardwood about 20" length 7" diameter. I would burn about 12 hours a day. When first lit I run wide open an hour, once its well lit I would damp it down the rest of the day. the hot water keeps house evenly distributed and warm at night when not burning.

I really like this batch box idea. Do you have any thoughts on running an external air intake from the outside? I am thing with this refractory cement I could mold a hole in for a pipe at the bottom instead of door. I can hear the difference in a exhaust fan when the outside door is opened. Also I crack the door when starting the fire as it helps get things rolling. less smoke etc. Thoughts?

Do you prefer the bricks over cast refractory? longest life? either way I will build it so I can replace easy.

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.

Also I crack the door when starting the fire as it helps get things rolling. less smoke etc. Thoughts?

it's far better to have the chimney exit at the highest point of the roof.

Do you have any thoughts on running an external air intake from the outside?

link

anyone know roughly how many pounds of wood fit in the 6" and the 8" batch box. I am working on figuring how much btu storage I need, and proper sizing.

thanks for all help

will

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:
https://chimneysweeponline.com/howood.htm

A cord is 128 cuft.

OK... calc away...

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

Hence

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.

BIG DIFFERENCE BETWEEN A 6" and 8" RISER FIREBOX - BUT YOU DON'T NEED TO LOAD IT FULL EVERY TIME - HALF WILL WORK....

I would prefer to go with the weight. You can measure that as you load the stove if you wish, and if you buy a moisture probe you can also take spot readings to estimate the moisture content. Water in the wood takes up available heat, and that lowers the total heat efficiency, which is fairly obvious, this is why burning green (wet) wood doesn't work as well as burning properly seasoned (dried) wood. Efficiency of the stove, and how much heat escapes up the chimney should also be estimated.

Below are three web sites that discuss these points, and offer some starting values, as well as explanations of the calculations:

http://worldforestindustries.com/forest-biofuel/firewood/firewood-btu-ratings/

http://mb-soft.com/juca/print/311.html

http://www.chimneysweeponline.com/howoodbtu.htm

Bottom line? Depends on the species of wood and how dry it is. And how efficient your stove system is in using the heat produced. Off hand, there appears to rough agreement that somewhere around 6200 or 6400 BTU per pound, and about half that if it is wet or green wood, is fairly typical. Better estimates will result is you know the species of wood you are burning, and the moisture content.

I like the idea of over sizing but controlling the output with smaller loads. Great idea. Also will slow me to keep cutting 20 inch long logs or so. That's what I am used to cutting.

does anyone see a problem with too small of a batch in an 8 inch system? filling it a quater full. Like not draw properly? My thoughts are over size and be aware of out side temps for amount of wood to put in batch.

I have read that people prime there system by lighting a peice of news paper in the exaust. I was thinking of installing a blower fan on the end of a T where the exaust meets the wall and wire it to a switch. Turn it in when trying to light. Any one try this?

Will

The standard peterberg batch box has been found to work best when loaded full. If there is considerably more mass storage than instant radiator, this just means you need to burn less often.


People have reported using switchable exhaust fans of various sorts for initial draft establishment.

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?

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...

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?

the second option

Hi all its been a while as i have been away for work. So i am going to be starting this soon. I am finalizing my plan.

I am looking for some insight on proper sizing/ volume of the bell compartment for a 7" system.

I am building a large water storage tank as my thermal mass the bottom of the tank will be the top of the bell compartment. I been working all the math to figure the right sizing of things for my system. However, I still lack the information on proper sizing of the bell area. As of now the plan for the tank is 4' in diameter and 4' tall. So there will be a 4' diamter surface area to act as a heat exchanger at the top of the bell. How tall should the bell be, to get the maximum heat exchange before exhaust? I am not limited to this size tank.

Thank you for all the great information.

Will