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Monday, March 3, 2014

BTU Output: How is it Determined?

Several people have recently asked about how BTU output is determined. BTU/hr rating is basically a speed and efficiency rating.  The (1) greater the number of POUNDS OF WOOD CONSUMED PER HOUR, and (2) the higher the OVERALL EFFICIENCY, then the HIGHER THE BTU/hr OUTPUT RATING.

 For example, if we put a 20# load of Douglas Fir at 22.5% moisture (dry basis), then the dry weight is approx 16.3# (20/1.225 = 16.326).  At 8,522 BTU/per pound for Douglas Fir, our TOTAL INPUT is 138,909 BTUs for this load (8,522 BTU/# x 16.3 lbs = 138,909).  
Ideal Steel Hybrid:
If the stove consumes the wood in 2 hours at an overall efficiency of 82%, then its overall output is 56,952 BTU/hr [(138,909 input/2hrs)*.82 efficiency = 56,952 BTU/hr output].  
Secondary Air Only Stove (EPA Default Efficiency):
The same formula at 63% efficiency is 43,756 BTU/hr [138,909/2*63% = 43,756).  EPA tests use default efficiency values for wood stoves unless the stove has been tested for efficiency as part of the EPA test procedure.

It is possible to get a very high BTU rating with a small firebox, or a very low BTU rating with a large firebox.  BTU output doesn’t have much to do with firebox size (though burn time does). As noted above, BTU output has everything to do with (1) Rate of consumption, and (2) Efficiency.

Notwithstanding what most manufacturers say, if you use cordwood, BTU output generally goes down, because the stove consumes fewer pounds of wood per hour than with EPA cribs, in part because there is less surface area exposed than with EPA cribs. A manufacturer can make up an efficiency number (or use LHV, or just one burn rate, or just part of one burn rate) but in our experience, the BTU output numbers that EPA reports are usually not far from the truth.  When you see BTU/hr numbers that are double or triple the EPA numbers, they are often pure fiction.

Most people just put wood in their stoves and are happy if they are warm!  But higher efficiency is a good thing as you can see above.  As efficiency goes up, you either get more BTU output, or burn less wood, or both.


  1. Greetings,

    I am confused by the number given for BTU/hr output for the Ideal Steel Hybrid. I understand how the 56,952 figure was arrived at but I don't understand where the 52,090 figure comes from.

    Why does the derived number of 43,756 at 63% efficiency match the given value of 43,756 BTU/hr while the 82% figures vary?

    Also (and more important to me) is the 82% efficiency figure for the Ideal Steel Stove true on longer burns as well i.e. if the stove is damped down so as to burn the load of wood in 10 hours rather than in two hours would this change the efficiency of the stove? (Obviously it would reduce the BTU/hr output, but would the 82% efficiency figure still hold true?)

    Thank you for your assistance,
    A Puzzled Viewer

    1. Wow! Someone actually read this, and tried out the math! The 52,090 BTU/hr number is in error (and has been corrected). 52,090 BTU/hr would have been the output at 75% efficiency, not 82%. Probably the author playing around with a spreadsheet and not paying attention. In both calcs, the two numbers should be the same (the parenthetical explanation merely serving as explanation for hourly output).

      The IDEAL STEEL has a higher efficiency at low burn than at high burns. During EPA tests, it was 84.5% efficient at low burn, but only 75% at its highest burn rate. The same approximate numbers would apply to cordwood.