I just have one question, if you have a spitfire evaporator, 2x6 with a 2x2 front pan, why are you batch boiling? That rig or any rig with a divided syrup pan and a flue pan is designed to continuous flow. You add sap on one end, in the flue pan and draw off finished syrup from the syrup pan. Or are you just saying you draw a larger amount and finish it on a different heat source?
Back when I was in my early days of maple production, I attended a seminar given by Glen Goodrich. He said he makes a constant draw or at least tries to. He was running a large evaporator. He encouraged producers to try to match the draw to the evaporator. At that time I was boiling on a 2x6 on a rig that only go 25 gph evaporation on a good day. I tried to throttle my draw down and I was able to draw off finished syrup in quite long draws. Back then I just used 219.3 as my draw temperature. When I got to that on my thermometer near the draw off I opened the valve slightly. Then as the draw progressed I often had to tweak it slightly faster or slower as the draw temperature changed. I was able to get quite a few draws that ran 12-15 minutes and a few time almost 30 minutes before I had to close the draw off valve.
Later I got a 3x8 and on that I did the same, this time I often got 20-25 minute draws and a few times I got almost 60 minute draws but not quite.
It is a slow steady flow that matches the evaporator that is the key.
I now run an RO and a higher performance 3x8 and until 2017 I still ran the longest draws I could. Then for 2017 thru current, I started using an auto draw. I still regulate the flow rate by having a ball valve in line with the auto draw. However I do not get the real long draws, but I will often get draws that run 3-7 minutes, and in that time I might draw 2-8 gal. The 8 gal will be if I recently reversed the direction of flow, my first draw afer that is generally longer, the others will be in a 2-4 or 5 gal draws with 2-3 gal most common. Since it is controlled by a ball valve and is reset each time there is no constant.
I think you should try the same thing. Practice starting a draw and watch the thermometer, if the temperature climbs open the valve slightly more, if it drops too soon, close it some. It takes practice but you will get it in time.
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.