Jim K: The “burn rate” in a GARN is, in fact, the Btuh output rating. I have calculated the actual heat input to storage countless times, and have not only confirmed the 420k Btuh rating, but have regularly exceeded it. I have well exceeded 450k Btuh in my GARN.
David: My impression is that the Garn figures are conservative. I have been corresponding with someone locally who has 2x WHS2000, and he is getting the rated output despite altitude and not having the best wood to burn.
I dreamed about this last night (my wife says I need to get a life!), thought about how my Tarm (140,000 BTU/H output rating) operates under varying conditions, and also remembered that I saw a Garn brochure that said 97% burn efficiency and 87% energy stored. That last statement might mean that a WHS 2000 rated at 425,000 BTU/H burn rate = 425,000 x 0.97 x 0.87 = 358,660 BTU/H output to storage. Or it might mean 425,000 burn rate x 0.87 = 370,000 BTU/H output to storage. A similar statement, maybe with different percentages, could be made about any wood boiler.
I have to say that on every Garn I have installed or been in contact with, the burn rate listed is actually the btu delivered to storage.
I guess I'm looking for either real engineering science or at least pretty good user science to get some hard data and data for an apples/apples comparison with gasification boilers. I have been deceived by anecdotal observations with my Tarm, which is why some real measurements are important. I'm not an engineer, but I have done what I think is pretty good user science with my Tarm. As to the factors below, I have measured BTU/H output and in doing so have measured factors 1 - 7 and 10 or near equivalent. [b
]I would fall into the user generated science category. Read my post below and see what you think. [/b]
Several things jump out at me: 1) what species of wood is being burned, 2) what is the M.C. of that wood; 3) what was the weight of the wood in the test burn; 4) what is the split size/mixture in the burn; 5) is the burn a beginning to end (single load) or is it near continuous feeding to maintain highest burn conditions; 6) how many BTU's/lb for the wood at the specified M.C. and stack temperature; 7) what is the stack temperature; 8) with the Garn, what is the starting tank temperature, and is it uniform top to bottom (no stratification); 9) with the Garn, what is the ending tank temperature and is it uniform top to bottom (no stratification); 10) what is the flow rate through the tank during the test; 11) what is the ambient air temperature and R-rating for the Garn insulation; 12) what is the combustion air input temperature; 13) what is the air relative humidity; 14) how may CFM of combustion air from the draft blower.
in the final analysis, it seems to me that the most important factors for a specified BUT/H output rating are: A) what is the heating system return temperature to the boiler; B) what is the supply temperature needed to meet system demand; C) required GPM flow rate at boiler output rating then = supply temp - return temp x 500; D) and to get an accurate rating, since the Garn supplies directly from storage, it would seem that starting uniform, non-stratified tank temperature must equal the heating system return temperature.
If a WHS 2000 (425,000 BTU/H burn rate) is to be accepted as the output rating, then if the heating system demand is 180F (like for baseboard) and return is 160F (typical standard), then GPM = 42.5. I use the 180F example because that is needed by many of the older heating systems with baseboard, so it meets the real world. The same GPM holds true with a radiant system with 120F supply and 100F return, but I think I can say with confidence that the lower the required supply temperature the easier it is to get a higher BTU/H rating due to increased efficiency in the boiler heat exchanger in transferring heat to the surrounding water, so a boiler at rating X for supply temperature of Y almost certainly has a lower rating if the required supply temperature with the same delta-T is higher than Y.
Required supply pipe size for 42.5 GPM is 2", and at this flow rate assuming 10 feet of system pump head (which likely is low), a Grundfos 40-50 or larger circulator (or equivalent) is required to provide the flow in the middle of the pump curve.
Here's where the rubber meet the road. Who has such a setup or equivalent, what is the continuous output performance of the Garn, and what data has been collected to determine the output performance?
With my Tarm, I have on several occasions logged supply temp, return temp, gpm, and stack temp. I determine gpm by a flowmeter on my system. I find that I can meet or exceed the BTU/H output rating at lower return temperatures over a larger portion of the burn (high delta-T), but that at return temperatures of 160F and higher (lower delta-T), it becomes increasingly difficult to obtain the output rating and I have to maintain a high burn. Also, at lower return temperatures and stack temperature in the 450-470F range, I can obtain the rated output, but as return temperature goes up, I need to increase the burn rate, resulting in stack temperature in the 500-525F range, to meet the rated output. I have some ability to do this by frequent wood loading and also by increasing the draft fan CFM. I have a fixed output circulator, and it should be possible to increase output at higher return temperatures and delta-T less than 20F by moving more water, but I am unable to do that in my system.
What do you think? Who has a system like that which I described? What data has been collected? Am I off-base