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Posted By 101x81,
Jan 17, 2011 at 12:54 PM
how the boiler btu rating is done and is the btu max take in account the boiler efficiency?
I'm not a boiler man by profession but I've done a lot of research on US boiler testing vs. European boiler testing.
When looking specifically at solid fuel boilers the US is really lacking an accurate test method which analyzes all relevant aspects.
However, the EN303-5 European test specification is very thorough (they even analyze the fuel being used) as they have been building solid fuel gasification boilers for a very long time.
I have several documents including a 10 page test report on the Effecta Lambda 35 which I was unable to attach to this post due to their format. However, if you email me - firstname.lastname@example.org I would be more than happy to email the test report documents to you.
As far as I know, BTU ratings define the energy output, not the energy input. Efficiency is a different measurement. BTU output tells you whether the boiler can keep up with a heat load, not how much wood it will take to do so.
From my experience (a single data point), my boiler seems to provide a peak output that's pretty close to the rating - within measurement error for sure. On mine, the average output over the course of a typical fire is about 75% of the rated output. YMMV.
This is also my experience generally, although during high burn the boiler output can exceed the rating. I think the 75% is pretty solid, unless you do partial mid-burn loadings to keep closer to high burn status. Mid-burn loadings pose another set of issues and are best avoided.
In the US there are 3 or 4 standards in use.
One of them is the EPA Hydronic Heater Testing Program Requirements (Phase I and II). The test program is available for free; see this link http://www.epa.gov/burnwise/pdfs/owhhphase2agreement.pdf
Then there are some ASTM standards which I can not provide to you, they are copy righted.
All US testing protocols are more or less the same in essence, some details are different.
The OUTPUT BTU's are the BTU's transfered to the water. This is measured by logging the Flow (GPM), Deta T and Time.
The INPUT BTU's are the BTU's available in the fuel. For the EPA hydronic heater program red oak with 8550 BTU/Lbs on a dry base is taken as the reference value (higher heating value).
The efficiency is the Output divided by the input.
Also for the EPA hydronic Heater testing protocol, the amount of fuel is determined as follows: you calculate the firebox volume in cubic feet and multipy this by 10. This gives you the total weight of wood you need to put into the boiler to perform the high burn and all other test runs. Example: say your firebox is 6 cubic feet x 10 = 60 lbs of red oak plus/minus a tolerance. 60 Lbs of red oak translate into 60 Lbs x 8550 BTU/Lb = 513,000 BTU's (wet). Take into account your moisture content, lets say 20%, this gives you roughly 410,400 BTU on a dry base and this is your reference value.
Then you burn the wood at full speed for the high burn, let say during 4 hours. This gives you around 102,600 BTU/hour input.
During the test you log delta T, Flow and you calculate the heat to the water, lets say 80,000 BTU's/Hour to the water.
So your efficiency is 80,000 / 102,600 = 78%
The "funny" thing about the EPA protocol is that you can achieve nearly 100% efficiencies. See list of phaseII wood boilers.
So to measure efficiencies it's not the best testing protocol out there.
In general, a test protocol based on "stack Loss" method is a better, more accurate protocol, but more expensive to test.
Hope this clarifies the topic a little bit.
A true gassifier will be 80% plus, 90 maybe with all the toys on the high tech ones.
An OWB, more problematic due to operation and fuel. 35% to maybe 50% for the gasser variants? I have not seen any reliable numbers on the latest CB models. Comments that they are 50% more efficient would get them to 50%.
I think Garn have a treatise of the EPA testing on their site, bit of a joke. They use sawn lumber from memory.
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