Understanding Outdoor Boiler Btu Outputs
"Most Outdoor Boilers Have Not Been Tested For Btu Output"!
If an outdoor boiler does not specify a Btu output value
it likely has not been tested or the manufacturer is too embarrassed to post the results!
This is why some manufacturers only state "this appliance will heat up up
to XXXX square feet"
because they have not actually tested or verified their products performance!
Next if the manufacturer has the Btu value listed:
a) is it a tested output Btu value or an estimated Btu value?
b) if it is not a tested value, is it an estimated input or an output value?
Btu output is determined by multiplying the Btu of the fuel input into the appliance by the tested
efficiency of the appliance, this can be a huge difference if the appliance is only 60% efficient!
c) what type and quality of fuel is the Btu value (input or output) based on, as this is effected by
the Btu factor (value) of the product being burned.
(Btu values are typically based on high
Btu value fuel, not the fuel the average consumer will actually be burning in their appliance)
d) is the tested Btu based on a constant sustainable draw over an extended period of time?
e) or is it a "temporary" maximum draw based on the "tested" appliance Btu output
stored Btu (1Btu = 1 degree Fahrenheit per pound of water) in the
water jacket of the boiler?
Stored Btu example: a 300 US gallon boiler with fluid of 180 degrees
Fahrenheit can effectively
store up to (300/.12*(180-32))=370,000 Btu
providing heat down to the freezing point
(great for a shop, warehouse,
etc.) but would only provide a practical residential heat storage
(based on room
temperature of 70 degrees) of (300/.12*(180-70))=275,000 Btu.
Hence the problem with determining the actual Btu output for an outdoor boiler.
EPA certified wood boilers have an EPA Btu output that was determined as a result of the
EPA Emissions Test. As with the EPA emissions test, the EPA Btu results are not typical of what
the average actual end users use of an appliances performance can experience.
(values are based on EPA test fuel, not the fuel the average consumer will
actually be burning in their appliance)
An example of an EPA 112,000 Btu
rating, a temporary 300,000 Btu draw down &
248,000 Btu rating is given here for the Wood Doctor HE8000 EPA Phase 2 Boiler
Coal Stoker Outputs:
It is an accepted engineering industry standard that you can only
effectively burn 50lbs of coal
per square foot of stoker head per hour. This
accepted standard and the Btu value of the coal
being burned effectively
limits the "Btu input" of a stoker head.
Keep in mind not all stoker heads are equal! Depending on the construction of the head the"actual surface area"
that is available to
burn the coal is not the same, on a cast iron
stoker head and ash ring you
will typically loose 2-1/2" of "actual surface area",
steel head and ash ring you will typically loose up to 4-1/2" of "actual
i.e. a 16" cast iron stoker head (13.5") has an actual surface area of .994 square feet and
effectively burn only 49.7 pounds of coal per hour. If the coal being
burned has a value of 8000 Btu
per pound the maximum "Btu input" of the
stoker head is 397,600 Btu. The problem is you
have to know the efficiency
of the appliance burning the coal in order to determine "Btu output".
appliance is 60% efficient, the Btu output on this 16" head would be
A good example of all stoker heads not being equal is in comparing the
cast stoker head and ash ring which has an "actual surface
area diameter of 18", to the Portage
& Mains 23" stoker head and layered steel
ash ring that has a "actual surface area diameter of 17"
A manufacturer's product literature can make their
product look like it has a larger Btu rating
then a competing manufacturer's
product - when in fact it actually has a smaller Btu output!