I'm shocked at how a clean heat exchanger improves stove efficiency. I've been keeping track of the stoker runtime lately, by adding an hour meter hooked to the stoker motor that reads to 1/100th of an hour (similar to an hour meter on a tractor). Similar to this:
I built a spreadsheet that I can enter the hour meter reading and current date/time, and it spits out the % runtime since the last reading.
During the cold weather here in Wyoming, the stove was stoking a steady 55%, based on a few days of data. I then took some time to clean the heat exchanger (3 pipes that run through the firebox) as well as the inside walls of the firebox. With outside temps roughly identical, my runtime as dropped to 40% over the past 24 hours. The blower washes the heat off the stove quicker, and the stack temp has dropped some.
Nothing else changed during this period, so I'm guessing a dirty heat exchanger was the culprit. I burn bit. coal, and am having some trouble getting a real clean burn, so it gets soot and ash on the heat exchanger. They did not build the stove with easy cleaning in mind. I'm contemplating creatively wrapping a chain around the heat tubes. I can then grab a chain link with a hook and slide it along the pipes to easily remove ash and soot. Something like the Harman Magnum does. As for the firebox walls, I'll just have to continue to scrape them with a tool that came with the stove. But I think the tubes get the dirtiest.
Just thought I'd share that for the pondering.