When I was still collecting and boiling sap, I used a flue brush to clean the bottom every day. A few times over the years i missed it and i could notice a difference.
For those who ask how when the pan is full, to clean the underside you only use a brush on a long (pipe) handle to get rid of the soot and fly ash that is stuck on the bottom. On my last evaporator I had to use a 2 piece handle, because the evaporator was only 4.5' from the back wall, so I couldn't get a 7' handle in thru the rear cleanout door, I had a handle made up of 2 lengths of 3/4" black pipe, one 4' and one 3' with a coupling to join them. I never found I could get a good cleaning working thru the front door.
I never found it necessary to clean under the syrup pan. Anything on there seemed to burn off with a proper fire under it.
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.