BTU output question

BTU output question

PostBy: Coalfire On: Fri Feb 19, 2010 8:00 pm

Hello everyone,
If you have read some of my other post you know I am going to go back to a hand fed next year. Well I was looking at a DS Machine Hot air circutlator Fire box size 16"x20" (320 sq inches) with a rated 130Kbtu output. My old gibralter CFS had a fire box real close to the same sq inches, but was only rated at 70K btu, so there is my question, what is up with that? I also looked on websites for other companies and got there exterior dimensions now I now the fire box will be smaller cause of the fire brick but it is a close estimate Harman magnafire sf 20.5"x35.5"(727 sq in)120Kbtu, Alaska kodiak 23"x23"(529Sq in)100kbtu, Hitzer model 354 heater firebox size 24.5"x14.5"(355sq in) 70Kbtu. So it would seem your btu per sq inch does not hold out among manufactures, does anyone know why the differant #'s, also one other question The harman 120k btu sf is rated for 3000sq ft, however there TL 300 wood stove 75kbtu can heat up to 3000 sq ft. I thought btus were btus no matter which fuel how can less heat the same amount of area? I asked two dealers and they did not know, so hopefully someone on here will know.
Thanks in advance, Eric
Hand Fed Coal Stove: DS Machine 96K btu Circulator
Coal Size/Type: Nut

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Re: BTU output question

PostBy: rberq On: Fri Feb 19, 2010 8:21 pm

Saying a stove will heat x number of square feet is ridiculous, and I just ignore claims like that. How high are the ceilings, for example? How good is the insulation? Is the house in Virginia or in northern New York? Square feet of floor space may be identical in all cases, but the BTU needs are vastly different.

I don't know what to make of firebox size vs. BTU output. I have always thought, for a hand-fired stove, that weight and surface area of the stove were roughly indicative of heating capacity. For my little Harman, running the blower or not running it also makes a noticeable difference in output. I always assume that the manufacturer is lying (exaggerating?) a little about BTU output. My rule of thumb in wood and coal stoves is, take the manufacturer's number and assume that 70 percent of that is reasonable for everyday use -- after all, you don't want to be running the stove flat out all the time, so that you have to shake down and re-fuel every six hours.

The fact that you have been burning coal for some time should give you a good starting point. How many tons did you use in an average year? How many BTUs does that represent per day? Multiply the per-day figure by about three to allow for very cold days vs. the season average. Then pick a stove that says it can output more than that number of BTUs in a day.

Or, just take the BTU rating of your current stoker (if it has been adequate) and pick a hand-fired stove with a similar rating.
Hand Fed Coal Stove: DS Machine 1300 with hopper
Coal Size/Type: Blaschak Anthracite Nut
Other Heating: Oil hot water radiators, propane

Re: BTU output question

PostBy: DOUG On: Fri Feb 19, 2010 9:37 pm

Here is a neat formula to use for obtaining a good idea of the Btu burning capability of a hand fired coal stove. Use the grate area, not the size of the firebox, fire pot, or fire brick.


Here is a calculation for you that will interest you. I showed this to Larry and he was amazed that it was so close to the actual laboratory test results. I got it from a book Solid-Fuel Furnaces and Boilers, by John W. Bartok, copyright 1982.

Grate Area (sq.ft.) X percentage of combustion efficiency X pounds per sq.ft. on grate X Btu output of fuel

The large Chubby has a grate area about 12" round. To get the sq.ft. of grate area, we take 12/2= 6, 6X6=36, 36X3.14=113.04, 113.04/12=9.42, 9.42/12=.785 sq.ft.

The percentage of combustion efficiency for most coal stoves is 65%, unless stated otherwise, 50% most wood stoves unless stated otherwise, the Chubby states between 85% to 90%.

The pounds of fuel per sq.ft. of grate per hour are: Anthracite Pea 5, Anthracite Nut 8, Hard Wood 20.

The Average heat output of fuel is: 12,500 Btu's for Anthracite and 6,500 Btu's for Wood.

Now that we have some definitions, lets do some calculations.

The large Chubby in the wide open damper, most heat up the chimney: .785 X .65 X 8 X 12,500 = 51,025 Btu's
The Large Chubby in the half open damper, .785 X .85 X 8 X 12,500 = 66,725 Btu's
The Large Chubby with the Damper mostly closed, .785 X .9 X 8 X 12,500 = 70,650 Btu's

So, Larry's figures are correct. This is a really neat formula that can be calculated for any stove. There are other numbers for different fuel types to run these calculations. The only thing is when trying to calculate other stoves, we can't assume just by closing the manual damper that the efficiency will go up. The efficiency ratings have been started to get published more by manufactures, especially now that the government tax credits for wood stoves have been instituted.

Okay, to answer your size question of the Jr. Chubby, it is 28" high with the legs, 14" round, 16" round on cast iron top, 23" to the top of the rear flue. I'm guessing that the weight is about 100Lbs. At least that is what it feels like to me.

Now let's run some numbers on the Jr. Chubby. The Jr. Chubby has a grate area around 8" round. That is .3489 sq.ft.

The Jr. Chubby in the wide open damper, most heat up the chimney: .3489 X .65 X 8 X 12,500 = 22,648.5 Btu's
The Jr. Chubby in the half open damper: .3489 X .85 X 8 X 12,500 = 29,656.5 Btu's
The Jr. Chubby with the Damper mostly closed: .3489 X .9 X 8 X 12,500 = 31,401 Btu's

There are other formulas to calculate the heat output of stoves too. Here is another one.

Re: Stove BTU output based on temp and surface area?

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.

This formula will get us close and usually assumes 50% to 65% efficiency stoves though. Here is an example for the Jr. Chubby:

The Circumference of a 14" round pipe is 43.98" X the height of the stove body, minus the legs, is 23" X the area of the 16" round cast iron top 201.06", then multiply the total in sq.ft. by the outside temperature of the stove.

So, 43.98 X 23 = 1011.54 + 201.06 = 1212.6"/12/12= 8.42 sq.ft. X let's say 600 degrees F 2850 Btu's = 23,999.37 Btu's

That's pretty close. Neat, huh? :idea: ;) :)
Stove/Furnace Model: CLAYTON 1600

Re: BTU output question

PostBy: oliver power On: Fri Feb 19, 2010 9:56 pm

Don't under estimate the HITZER 50-93. They can really crank out the heat. If it wasn't for needing electric to run my hot water circulator pumps anyways, I'd never have a stoker. I'd take a good hand fired stove any day over a stoker model. I always thought that, due to possible power outages. Now, after running my stoker boiler for two weeks, I find it has nothing over my 50-93. I'm talking stoker vrs natural draft here. It's not the performance that separates the two. They perform about the same. It's the fact that the stoker model depends on electricity. I can see why you're going back. In my opinion, when it comes to stoves, it's hand fired all the way.
oliver power
Stoker Coal Boiler: KEYSTOKER Kaa-2
Hand Fed Coal Stove: Hitzer 50-93 & 30-95, Vigilant (pre-Vigilant-II), D.S. 1600 Circulator
Baseburners & Antiques: MANY (Mostly when burning wood)

Re: BTU output question

PostBy: freetown fred On: Fri Feb 19, 2010 11:23 pm

Yep,I'm sold on my Hitzer 50-93--there is one corner the heat doesn't get around that well,but I got one of those little corner--archway fans & pretty well have it covered.This is a 200 yr old farm house--we have the back shut down,but are still heating close to 2000 sq. ft. I've only got 1/2 a basement where the hot water heater is(elec) w/ a night timer(off peak price)and no place for duct work(all post & beam) this hand fired works excellent---12 hr shakedowns--24 hr hopper fill---I'm thrilled w/ it---as far as BTU's,I guess she's putting out plenty--stove room 78# rest of the house 70-72# constant :D
freetown fred
Hand Fed Coal Stove: HITZER 50-93
Coal Size/Type: BLASCHAK Nut/Stove mix

Re: BTU output question

PostBy: Coalfire On: Sat Feb 20, 2010 8:28 am

Hi everyone,
Thanks for the info. I know what size of stove I need for my house Ive been burning coal for a long time. Just when I was looking for a new stove the btu ratings were annoying me. I guess some manufactures over rate and some under rate. I wish there was a standard test sequence so you know apples to apples. When I talked to Amos at DS machines he said there is roughly 400btus per sq inch of burn, and that seems pretty accurate. And yes Oliver power I agree its Hand fed all the way the stoker has a little ease to it, but I miss the heat from the hand fed :)
Have a great day everyone, Eric
Hand Fed Coal Stove: DS Machine 96K btu Circulator
Coal Size/Type: Nut

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