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Posted By rockwall,
Dec 13, 2018 at 7:58 AM
Is that a normal flue temp, seems high?
It's near the upper max set point (see my previous posts). Desired working point is 374. Tubes could probably stand to be brushed.
Hopefully this isn't a thread hijacking...
That does seem high. I have the same boiler (FHG 20/30, setup as a 30). I went back and took a look in my logs. If I averaged my high temperatures for each burn, it would come out at 390 or so. Without looking at each chart, my all time max is around 412 or so.
I've also tried experimenting with cleaning the heat exchange tubes, and that doesn't really correlate to lower max temperature. In fact, I had back-to-back runs where I logged data for a "dirty" boiler, then cleaned it, and logged a run with the "clean" boiler. Guess what? The clean boiler was hotter! Same wood, same settings, same everything, just a cleaned boiler. My heat exchange tubes only really collect fly ash, from what I can tell.
I've only logged about 45 burns so far and what exactly causes the highest temperature is somewhat elusive. I see a strong correlation to how wide open the boiler is burning, say, if the secondaries are open to 90%, then you will have a higher flue gas temp than when the secondaries are open to, say 20%.
I wonder how much the natural draft of the chimney has to do with the maximum flue gas temperature. Mine measures out on the low end - around 0.03" WC.
I'm finding the variability of the wood which goes into the machine has a lot to do with the max flue gas temp. Lots of small pieces = lots of wood gas = lots of secondary action = high flue gas temp. Also a factor seems to be just how the wood is packed into the firebox. My observations: After the wood has been turned into charcoal via pyrolysis, the charcoal pile collapses in the firebox in stages. What I see in the data logs are jumps in secondary opening percentages to handle the wood gas released by the collapse of the charcoal pile. It is a gas boiler, after all, and it is going to try and process that wood gas to maintain setpoint.
The S3 is heating like a champ here and I realize I may have been over thinking things a bit. I’ve learned that pump speed has a lot to do with how the boiler performs and have slowed the pump by turning the speed control switch down which is on the pump into the middle range. Everything is set to factory defaults now and I’m keeping an eye on it. I have been told to increase and to decrease the pump speed so I’m at the decrease stage. Also the guys at Tarm said that the flue gas temps stated in the “boiler planning guide” are from a boiler running at laboratory conditions. So (my results WILL vary). I have also learned that if my pump speed is too low, I’ll have 95% secondary and primary 31%, boiler 187 degrees, fan at 66%. Thats when the fun starts- the boiler begins to “furnace" at these settings because the gas flame tries to come up into the firebox from the nozzle area. The sheet metal covering the boiler starts to vibrate and shake due to the rapid pressure changes in the intakes due to the furnacing. I don’t plan to make any changes in the settings from here on out and just let it go the way it was set from the install.