TCaldwell said:Jim, i am not familiar with a tarm's internal hx/ transfer to water setup, when you mention flue temps/ to rated output, isn't the flue temp dependant in part on the to the water temp the hx tubes are seeing, for example given the same process gas temp entering the hx tubes, with cooler water the tubes wil shed more heat , resulting in a lower flue temp and a higher combustion effiency calc. If the entering water temp is higher the hx tubes shed less heat and show higher flue temp and a lower combustion effiency, even though the boiler is delivering the same process gas temp ,ie same fire size to the hx tubes the flue temp will differ, creating a different rated output. I am not sure flue temp alone is a indicator of rated output. My experience is with a garn, i can watch with the same secondary burn temp[process gastemp] that with starting water temp of 140 , my flue will be 265, high effiency calc, as the boiler water temp rises to 200, my flue temp will top off at 300. Basically for me a lower flue temp = higher effiency for the same output. i know you have some thoughts!
I would like to answer "yes, no, maybe," and probably I generalized too much and this likely is way more complicated than what I see from observation. And since I didn't write down data showing flue temp, supply temp, return temp, and gpm flow at variations of each of these, I'm going to reply at this time from a rough memory.
At low water supply temp, say 100F, I can quite easily obtain approximate rated output of 140,000 btuh with a flue temp of 465F and supply temp of 175F; flow = 3.5 gpm +/-. To accomplish rated output at return of 160F, supply temp of 188F, I will need flue temp of about 515F; flow = 9+ gpm. Incidentally, my typical burn usually will hover right around 465-480F and may start to creep up as system return water temp rises above 160F. I think we pretty much agree.
Clearly, your Garn flue temps are lower and the Tarm are higher. Lots of potential reasons for this, including different secondary combustion/gasification temperatures; different heat extraction design; one being more and the other being less efficient in converting wood to heat energy, and probably many more. How this relates to overall efficiency is an interesting question. It would be fun to have a gasification boiler and Garn with the same output rating (need to be based on the same method of calculation), same amount of water storage and insulation, same lbs. and types of wood, same water starting temperature, and do a side-by-side burn to see how many btu's each extracts to its storage. That would seem to give an overall efficiency result. Maybe this has been done? At the same time data could be collected on flue temp, etc.
What's funny to me is that with my wood stove in the house I just load it with wood, burn, get heat, and pay attention to keep the stack at 300-400F when I need heat. No measurement, no desire to measure, only a desire to stay warm. What is it with one of these boilers that drives some of us to learn more and more, experiment, pretend like we know something, and tell others about our experiences? Beats me.