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:
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 BTUActual 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....