So last night I fired the boiler expecting to take measurements of boiler temp, stack temp, tank temp, o2 content, etc at hourly intervals... but guess who fell asleep at about 11:30 and failed to wake up to the hourly alarm? I did get some very impressive numbers on delivered btu's to the tanks however.
Notes on this run:
1) I did not clean the heat exchanger prior to the test, though the tubes appeared to be quite clean except for a very thin coating of fly ash.
2) I did clean all of the ash out of the upper and lower chambers, but left a couple handfuls of dead coals in the upper chamber for easy lighting.
3) The wood load in the 7.4 cu-ft firebox weighed 173 lbs... mostly oak, hickory, and beech at around 16%-20% moisture content.
4) All of the heating zones were switched off, including dhw, for the entire burn.
5) Total system volume is around 1070 gallons... if you take into consderation the thermal mass of the steel in the tanks you can add a few more gallons... perhaps 35 gallons from the tanks... another 15 or so from the boiler... (converting lbs of steel into gallons of water with respect to specific heat)
At the beginning of the burn, the tanks were reading (top to bottom) 91/91/91/90, and the boiler was reading 73°. 40 minutes after lighting, the boiler reached 149°, at which point it energized the loading unit circuit and began to charge the tanks. By the time I drug my tired bones out of bed this morning, the fire was gone, and the tanks were reading 187/187/186/184!! That's between 95° and 96° temperature rise. I was expecting about 80°, so you can imagine my suprise. This was the first time I had ever run the system with absolutely no load whatsoever.
Now I realize that we're not really measuring boiler efficiency here... but rather the overall efficiency of my system to transfer Btu's from the firewood into the storage tanks. Remember that we have "loss" from the hundred and some feet of 1.5" copper that connects the boiler to the tanks, and also some minimal "loss" through the insulation on the tanks. "Loss" is in quotations since during the heating season, nothing is really lost as the entire system is indoors.
Now depending on what you're take is on how many btu's there are in lb of wood, the overall efficiency numbers will vary quite a bit.
at 7000 btu's/lb system efficiency = 72%
at 7500 btu's/lb system efficiency = 67.5%
at 8000 btu's/lb system efficiency = 63%
I have seen ratings for wood as low as 6400 btu's/lb at 20% moisture content. see here:
http://bioenergy.ornl.gov/papers/misc/energy_conv.html In which case system efficiency would be around 79%.
My gut tells me we are somewhere around 70%... which is very good. Very very good. This is an increase of 20% in system efficiency over the previous gasser we were using. Remarkable.
That's all for today. Happy Easter.
Cheers