Trying to calculate coal usage for my stove

I just bought a new stove and am ready to return it due to the coal usage, I have a Baker Mini-furnace and am using 100lbs a day with surface temp at 300f. I have seen where others have calculated how much coal their stoves are supposed to burn in a 24 hour period using variables such as; fire box dimensions, coal BTU per pound, etc.
Can anyone help with supplying me with the formula, or a web link to it?

Jiminy Crickets! I'm not sure where you are located but that's a lot of coal to burn in 24 hours to only get a 300* surface temp. I'm sure I don't have to tell you something is not right with your stove. I've always heard good things about Baker's. I notice this is posted on the Hand Fired Coal Stove page but you say you have a Baker's Mini Furnace. I'm not familiar with Baker's enough to say if you're in the right place or not. You may get a better response on the Furnace page.

Do you have any pictures of your stove? The chimney, pipes, etc. What type coal are you using? Do you have a barro on it? Do you have a stove front thermometer and a stack thermometer? How much ash is it producing and is it fine or chunky? Before you take the time and energy to take the stove out, give us a chance to help you find out what's wrong. I'll be watching for more information about your set up. Lisa

Burning 100 pounds and with a stove top temp. of 300 it sounds like your stack temp. is too high unless this is a very large furnace. Measure the temperature about a foot from the stove on the smoke pipe. Very high draft could be stealing your heat. Much too early to blame the stove. With more information and a picture if possible then members can better suggest solutions.

You have a good size stove, according to the product page, it has a 465 cfm blower on it.



If the blower is washing the heat off the stove like it is designed to, 300F doesn't sound out of line for a stove-top temperature. But still, 100 lbs per day is a lot of btus going somewhere.

I have a list of questions to kick things off and help us determine what's going on. Pictures are great as well, and will save you lots of keystrokes.

What are you trying to heat with this stove? basement, living room, etc.
Do you have any idea what the heat loss on this area is?
How is the heat distributed? duct work, floor grates...
What size and brand of coal are you burning?
What kind of chimney setup do you have? Does it have a strong draft?
Do you have a barometric damper installed?

welcome to the forum, have you burned coal before? are you using a barometric damper, how much coal will fit in the stove?how much are you putting in? what size chimney, config( thru wall up 15') masonry,I see the flue is 7" is it continueous? it could be your letting to much air thru the stove(cant happen with wood),Are draft controls on load door shut, if not they should be. do you have a co detector,are you measuring your flue temp? you want to play around with air controls on load door get your fire burning nice(TAKE TEMP) than close ash door dampers completly, fire should drop almost immediately,(if it does'nt this could indacate gasket issues.)next open 1 damper 1/4- 3/8" leave other closed wait thirty minutes(TAKE TEMP) i use a wireless BBQ thermometer got at home depot for 30- charmglow , i can watch stove temps from kitchen, stoves in basement, i can see if stove is making heat or steady or dropping open 2nd damper 1/8-1/4"let it burn for another 30 than record temp again, answer what you can , youll get your answer from the forum

I really appreciate the fast response, and everybody's willingness to help. But, I really need to mathematically figure out how much coal my stove should use in a 24 hr period based on fire box size, BTU per pound of coal, etc.

I know I have not provided background info on my stove, which is the part everyone is interested in, I may have asked the same questions. I have already been through those steps.....

In an effort to answer the questions at hand and hopefully redirect things back to my original question, this is the background info:

I have been burning coal for over 10 years with no problems ever, and am well aware of draft requirements, how to burn coal efficiently, etc. Stoves are the ONLY source of heat for our house, so we bought the Mini-Furnace(which is hand fired) so we could pipe directly into duct work that will be installed next spring. Our house is about 2100 sq/ft and will be about 3000 sq/ft when we expand the one section.

I purchased the Mini-Furnace about two months ago, it has a 7" liner with cap from top to bottom, 7" thimble and pipe to the collar. The stove has a barometric damper which has been set to .05"WC by a manometer. There will be pics posted at the end of this post.
As for draft, there is NO PROBLEM with draft. When the chimney is COLD it is pulling between .01 and .02"WC of draft, most setups either have 0" of draft or have a downdraft. When the stove was up and running the draft was up around .12"WC w/o the barometric damper, hence the addition of the damper.
My house is located in a small valley in the trees, now I know some may think I have a draft problem form my location, but I don't. I know this because I have a VC Vigilant located upstairs that burns perfectly, about 40-50 lbs every 24 hrs with surface temps 300 - 600f (w/o a baro!). The chimneys are the same height and located about 10ft apart. And yes, the stove I am having a problem with is hooked up to an outside chimney. But, I have never had a problem for the past 10 years until I switched stoves.
While burning wood I have to keep the drafts open about 4-5 turns each, while burning coal they are open to 1-2 turns each. The upper drafts were set by the Bakers at two turns each off the bump stops (it is impossible to completely close the upper drafts on these stoves.)At these settings the door temp is 300f and the stack is about 150f.
I did find a problem with the ash door, it sags about 1/2" and prevents the seal from being made. To get around it I have been lifting the door when shutting it, this seems to work.
I told the Bakers what was going on and they came down to my house, and I really appreciate their effort. They made adjustments to the hinge/ door, it seemed to work for a couple of days then back to the same problem. From what I can see set-pin holes were drilled too big. I haven't gotten much of a straight answer on the fuel consumption, they think I may have a "turbulent" draft because the smoke occasionally swirls when it leaves the chimney and suggested I buy a Windbeater cap. Those caps are for correcting downdraft problems only.
Most recently this past Sunday, the grate shaker shaft that penetrates the side wall seized. WD-40 has loosened it up for now. Others who own the same brand of stove have had this problem more than once, so I had my father take a closer look since he was a machinist and now works in engineering. He said it looks like either the shaft or sleeve was machined either too large or small respectively. Then suggested having .001" removed from the shaft and polishing both surfaces.
I have not had time to report the latest info to the Bakers with Christmas and work. I can see the potential of the stove, and would like to keep it but if the fuel consumption can't be lowered and the door dealt with this is not the stove for my needs.
I am at the point where I want to figure out how much fuel this stove is supposed burn, then make my final decision.

With all of this laid out, can someone please either send me the formula figure out fuel consumption, or direct me to a link/ website where I can figure out this answer?
Here are the pics:



This is a picture of the draft level when the chimney is COLD:

It sounds like you have your stove set up correctly. I'd try a few things, figure out a way to seal the over the fire draft controls.. maybe some rope gasket?? I think your stove has a few compromises built in for burning wood. Maybe if those compromises were 'fixed' the stove may meet your expectations. Eliminating the over-the-fire air will allow the stove body to be higher, and controling the fire with only the ashpan draft controls should work better.

I honestly have never seen a fomula for a hand feed firebox. There are just too many variables. Draft, depth of fire, firebrick or not, full base shaker grate, or partial base with 'V' sides for the shaker grates, sealed firebox, over fire draft, quality of coal, ash content, and I'm sure many more I can't think of.

I know you can figure out with a stoker, how much coal the feed will supply to the grate per hour, whether this coal all gets burned is subject to another batch of variables.

Coal also can be a wildcard variable in coal usage. I've had coal that is lighter in weight, and burns fast, and I've had heavy coal that burns slow and hot. So even though most sources say that anthracite burns at an average of 13,500 btu per pound, I'm sure I've experienced a low of about 11,000 and a high of around 14,000.

Maybe someone has a fomula for a handfed firbox.. ??? It will be a real headache..


Greg L

Thanks for the input Greg, I have in fact tried to block the top drafts. This proved to be "explosive" at times and did not help it burn any better, if anything it burns better with some air on top of the bed. I am not used to applying this much air to the top of the bed, with the old stove I kept the top drafts open half the amount of the bottoms. This always worked great and is good point to start from for most stoves.

I am interested to hear from other Mini-Furnace owners with their experiences.

If I recall right, Doug had posted some formula for BTU output determination based on the grate/box config. Like Greg said, there are a lot of variables but if you can determine about what it's capable of by design, the other variables would be relative, coal's BTU content etc. I just PMed him so I hope he jumps in on this.

You stated your Vigilant is on the first floor. Is the Baker in the basement? Uninsulated masonry basement walls will absorb alot of heat due to their high thermal mass. Ducting most of the convective heat upward will eliminate some of this. You'll still have some radiant heat staying in the basement because there's no insulation around the furnace. You could build a sheet metal plenum around the stove to collect most of this heat. Have you included the basement sq ft in the 2100 ft sq estimate?

Once you fix the over fire air like Greg suggests and get the ash door alignment resolved to give a reliable seal, please measure the exhaust gas temperature just before the baro and let us know. Sealing that will keep the heat in the heat exchange area longer alowing for more efficent heat transfer to the air and not the chimney. Saying that, I read that Harmon hand feds have air wash into the fire box designed in too by not fully gasketing the glass.

The pic of the manometer, is that w/o a fire in the stove? If it's a cold reading, what does it pull during a mature burn. You can keep a fire going so I doubt it's a problem with swirling air around the chimney. Could be wrong but if you have good draft at all times, I'd doubt that has much to do with coal consumption.

Gas pops from a fresh load is something that we all deal with and get around. Some leave a corner of the red coals exposed until the blue flames are established. Others introduce air by leaving the ash pan door cracked open for a short period allowing O2 to make it to the space above the coal. You could temporarily let in some over fire air until the blue flames are going and then close them back down. It's a new stove and will take some fidgeting.

VigIIPeaBurner, AKA Dave, PM'ed me and asked me to provide some calculations for you.

Re: Thoughts/Questions on Stove Efficiency

Hi, guys. Here is a couple of calculations to play with. But a few definitions first.

COMBUSTION EFFICIENCY:The percentage of heat that is released during burning the fuel. Most units can attain 70-90 percent efficiency.
HEAT TRANSFER EFFICIENCY: The percentage of the heat that is absorbed by the air in the heater. This is the biggest limitation in most units.
OVERALL ENERGY EFFICIENCY: The combination of the previous two. It is the ratio of the heat that is supplied to the home divided by the amount of heat that was available in the fuel.

A measure of the effectiveness of a design that sometimes used is the ratio of heat exchanger area to grate area. Values of 15:1 to 25: are common with the lower ratio better for coal. This ratio is not commonly listed in manufactures' literature but can sometimes be calculated from the information supplied. A ratio that is low would indicate low efficiency because the heat generated by the fire could not be absorbed quickly enough before the gases left the stove. Too large a ratio would be efficient but could indicate possible draft problems. The amount of direct surface area (the fire shines on) is the heat exchange surface area.

Regarding combustion efficiency. The shape and volume of the firebox influence the combustion. Too large a surface area cools the fire and reduces the amount of volatile gases that are burned. If the surface area is too small, not enough heat is absorbed by the air heating the home and higher stack temperatures results. You lose 1 percent efficiency for each 25 degrees F. that the stack temperature is above 350 degrees F. To find the average flue gas temperature, take readings at the section of stovepipe before entering the chimney and at the top of the chimney. Add the two readings, divide by two, then subtract the outdoor temperature. That is you average flue gas temperature.

Just a few things to think about. DOUG

Can the unit burn the fuel fast enough to give the rated output? Combustion rates vary significantly with the type of fuel you are burning. To obtain complete combustion of a pound of coal, 200 cubic feet of air are needed.

Pounds of fuel per square foot of grate per hour:
Anthracite, Pea = 5
Anthracite, Nut = 8
Bituminous = 9.5
Wood = 20

Average heat output of fuel BTU/Pound
Anthracite, 12,500
Bituminous 13,000
Wood 6,500

Efficiency of combustion percentage
Anthracite 65
Bituminous 60
Wood 50

To calculate the amount of heat output based on the number of pounds of fuel that the unit will burn: MULTIPLY:

Grate Area(sq.ft.) x Combustion Percentage x Pounds of Fuel per sq.ft. of Grate X BTU/Pound

EXAMPLE: 1.5 sq.ft. Grate x .65% x 8# x 12,500Btu = 97,500Btu's for Anthracite Coal

EXAMPLE: 1.5 sq.ft. Grate x .50% x 20# x 6,500Btu = 97,500Btu's for Wood
Another formula to play with.

This is the link to the original post by Ashcat. Thoughts/Questions on Stove Efficiency

I hope this helps you figure what you want to know. DOUG

Here is another neat formula for calculating the amount of Btu's at a given temperature for radiant stoves.

A chart from "The Wooburner's Encyclopedia,1976" says: The amount of heat emmited per square foot is dependent on the temperature of the radiating body.

Temperature of Surface Fairenhiet -------------- Total Energy Transfered BTU's per hour per foot

80* ------------------------------------------------------ 15 Btu's / Sq.Ft.
100* ------------------------------------------------------ 51 Btu's / Sq.Ft.
150* ------------------------------------------------------ 168 Btu's / Sq.Ft.
200* ------------------------------------------------------ 315 Btu's / Sq.Ft.
400* ------------------------------------------------------ 1230 Btu's / Sq.Ft.
600* ------------------------------------------------------ 2850 Btu's / Sq.Ft.
800* ------------------------------------------------------ 5430 Btu's / Sq.Ft.
1200* ----------------------------------------------------- 9370 Btu's / Sq.Ft.

I'll be looking forward to hearing your findings. DOUG

Thank you for the information, this has gotten me closer to the answer I am looking for. However, after reading all of the information and running the numbers I have other questions.
So, let's start with the numbers from my situation:

Firebox measurements from firebrick to firebrick:
LxW
22.25" x 13.5" = 300.375"/144 = 2.09 sq/ft of grate area

2.09 x .65 x 8 x 12500 = 136,500 BTU

Actual grate measurements minus the the shelves they rest on

18.75" x 13.5" = 253.125"/144 = 1.76 sq/ft of grate area

1.76 x .65 x 8 x 12500 = 114,400 BTU

I know there is a small difference, but I want to have as accurate numbers as possible so I'm not sure which one to use. Either way thats a lot of potential BTU output for a stove.


Now, using another formula found linked to this thread I am able to estimate the BTU production relative to weight of coal burned in a 24 hour period. As mentioned in the beginning of this thread I am burning at minimum 100lbs every 24 hours, so here is the formula:


(lbs. of coal used in 24 hrs) x (coal BTU rating) / 24 hrs
100 x 12500 = 1,250,000/24 = 52,083 BTU/hr At this rate I have a door temp of 300f and a stack temp of 140f.


Now after seeing the numbers something struck me as odd, the BTU/hr figure looked close to the total BTU rating of my 25-30 year old VC Vigilant 1400 putting out twice the amount of heat with half of the fuel......so I ran the numbers on that stove as well.


The Vigilant that I have is a 50,000 BTU stove, and in a 24 hr period I use 50-65 lbs depending on how far the t-stat is open. At 50 lbs the surface temp is about 300-400f, and at 65 lbs the temp is at 600-750f. This stove does not have a barometric damper, only a factory installed manual damper.
So, at 50 lbs/ 24 hrs the BTU/hr = 26,042
and at 65 lbs/24 hrs the BTU/hr = 33,854


These numbers seem a little low for the surface temps, but I attribute it to the manual damper on the stove. By closing the damper the gases are forced through an exchanger system on the side walls allowing maximum heat extraction.


The part that I am confused with is this; the Baker stove is using 100lbs of coal to produce 52,083 BTU/hr with temps at 300f and my Vigilant is using 50lbs of coal to produce 26,042 BTU/hr with the exact same temps!??! It doesn't make sense that a stove half the size is producing the same if not more heat than a stove twice as large and 30 years newer with supposed updated technology.


I have a theory and would like some feedback on this; the Mini-Furnace has the tallest amount of open air space above the fire box compared to all of the other Baker stoves, 12 -18" depending on where measurement is taken. Which would give the stove a larger surface area to heat from but it only makes sense that with an increased surface area more fuel is required to to keep the stove temp constant versus a stove with a smaller area above the fire box. Same concept as keeping a refrigerator cold; takes less energy to cool a full unit versus an empty one. I wonder if the stove body was 6 -8" shorter if it would take less fuel to maintain temperature.


To further explore this theory, I looked at the Baker stove a friend of mine owns. He has the Fireside II, the firebox is nearly identical in length and width (the width may differ by 1/2") and holds the same amount of coal - 100lbs. The only difference is the height above the firebox, his is proportionately very close to my Vigilant - about 6-8" of airspace above the firebricks. His burns about 50lbs/ 24hrs heating his whole house to 80f (I am happy with 70f) and is able to maintain the same temps as my Vigilant and the Baker Mini-Furnance. We have comparable sized homes, and both stoves are rated for 3000sq/ft homes.


My only thought is to try out a manual damper on the Mini-Furnace in an attempt keep the heat in the firebox a little longer to give the heat exchanger more time for extraction. I have read the lengthy discussion (or argument depending on your view!! ) on this subject and see both sides, but I have seen the difference they can make based on my own experience. This may provide an even comparison between this and my Vigilant, if it makes no difference maybe I need a shorter stove....

I'm thinking about starting another thread to see if others are using a larger stove as a primary heat source, and ask for their usage amounts, stove brand and type, etc, for another comparison.

My goal is to have one stove to heat my house, and it seems reasonable to accomplish this with @50lbs/ 24hrs. I look forward to the feedback....

I think that you may be on to something there. I would try a manual damper installed before your barometric draft regulator and see how that performs. Here is a video from Larry Trainer, the designer of the Coal Chubby Stove, on how to operate a Chubby Stove. I think that this video will help you greatly. He uses both the manual damper and the barometric draft regulator to fire his stove. http://vimeo.com/user1238673

As for the grate dimensions, I would use the smaller, actual grate size, for the calculations.

The part that I am confused with is this; the Baker stove is using 100lbs of coal to produce 52,083 BTU/hr with temps at 300f and my Vigilant is using 50lbs of coal to produce 26,042 BTU/hr with the exact same temps!??! It doesn't make sense that a stove half the size is producing the same if not more heat than a stove twice as large and 30 years newer with supposed updated technology.

The Vigilants are radaint stoves; there are't bloweres and the heated cast iron surfaces radiate heat to warm surfaces and air simultainously. The Baker is a twin wall steel furnace (convective stove w/a large heat exchange area ) with a 465 CFM blower moving air between the interior surface the ehaust gasses contact (very hot) and an external steel shell to force heated air out from the envelope of steel surfaces. If you measure the outside steel surface, your geting the temperature of the external steel surface that's been cooled by the forced air stream. Two different concepts, apples to oranges.

Other considerations;

• If the Baker is in the basement, are you heating the basement area and uninsulated masonary walls?
• The design of the Fireside II vs the Mini-Furnace can is the size of the heat exchange chamber. See Doug's chart about surface temperature related to surface temperature. Furnace heat exchangers heat air to a lower temperature and a higher volume to move the same BTUs as a higher temperature air at a lower volume. What's the Fireside II's CFM rating, 465 like yours?
• Let us know what your manometer is reading at a mature full burn. Is it higher than -0.03-0.04? I think you stated the baro was set at -0.06 You might not need the manual pipe damper if you set the baro to open sooner.

My Mark III channels air through two 1"x 3" square tubes that run parallel to each other a the very top of the firebox. When my stove is cranking, those tubes still remove enough heat to lower the top surface temp to 300°. The very back of the stove is double-walled for the air channel -- that only runs 150° - 250° depending on where you take the temp from. With all these temps, the single-walled sides will be 450°- 500° a few inches below the top in the center, & closer to or over 600° a foot farther down, in line with the firebrick.

As was mentioned, trying to compare total heat output between the Baker and the Vigilant, using formulas that utilize surface temps is not a relevant comparison, apples to oranges. I don't have enough info. to make a complete comparison, but lets take the info that we do have and speculate. The Vigilant is rated for 50,000BTU at max burn rate. I imagine that would be running max recommended surface temps of 700deg. At that rate you're burning around 65lb coal per 24hr. Now this is where we are missing info, Baker doesn't give BTU ratings, but rather recommended sq ft ratings. One thing I do know is that Baker's design has tested above 90% efficient. witch is not hard to believe given the fact that my much simpler designed Coal Chubby tested at 85-90%.
My baker Fireside holds 60-70 lb of coal, is rated for 2500-3000 sq ft. burns at above 90% efficient and running at max recommended stove temp of 500deg. on load door, burns around 90-100lb. per 24hr. depending on what brand coal I'm using. I normally don't run it at max, and only burn 70lb. per 24 hr. at around 350-400 deg temp.
Now here is the speculation. Comparing the recommended sq ft rating to other stove company's stoves of comparable ratings, I would say my Fireside stove is around 90-95,000 BTU.
The Baker Mini Furnace holds around 100 lb. of coal, and is rated for 2800-3200 sq ft. I would speculate that stove to be in the 120-130,000 BTU rating. If you were putting out 50,000BTU at 65lb. in the Vigilant, To make 100,000BTU you would burn 130lb. What I am trying to say is that I don't think you are out of whack burning 100lb, for the heat you are probably producing.