Look at Johnny Cuervo's pictures. He made a pre heater that works real well, out of copper and in contact with the stack. He even regulates the temperature.
On my evaporator I must shut down when I have 18 gal of sap in the head tank. Then when the fire burns out I have ended at the perfect level. It will just take time to find where you need to shut down.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.