I leave it sweetened from first boil until the end of the season with some exceptions. 1. To clean the pan I drain it into my draw off tank, then clean the pan, then pump the concentrate back into the pan. 2. If I did not have enough sap to boil hard at least 3- 4 hrs and temperatures are forecast to be above 40F or below about 15F I drain it and store it in a cool but not warm location.
Once boiled 3-4 hrs you should have enough sugar in the concentrate to keep it from freezing, it may form slush on top, but not freeze solid to hurt the pan. In my early days if I doubted the safety of letting it set, I used to put a 40 watt bulb in the firebox. The heat from that even kept slush from forming, I have not done that in likely 15 years. The main thing is to have enough sap on your first boil to last at least 3 or 4 hrs. Your 2x3 should evaporate 5-7 gal/hr thus you want 15+ gal of sap.
I always cover all of my pans when I shut down, to prevent anything from getting into the pan with the concentrate.
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.