Ya you need below zero at night but not just warmer in the day, you also need sunshine. Has to do with photosynthesis too. If you have freezing at night and thawing during the day combined with good sun those will be the best sap flow days but if the trees are in really shady areas they will usually produce less flow. And it also depends on the canopy of the maple tree. If there is a good spread of branches and a wide canopy those trees will produce better than the sparsely branched, narrower canopy maples.
I've got no experience tapping box elders as we only have Big Leaf Maples on our property and the surrounding area. And depending on the time of season the sugar percentage in the sap will range from 1% up to 3% sugar content in the sap. And you will yield a lighter colored syrup earlier season and darker syrup later in the season but that also does vary a bit with the conditions the trees are growing in. With the Big Leaf's depending on the sugar content of the sap you will need anywhere from 40 to 60 liters of sap to make one litre of syrup at 66 brix. If you are tapping just a few trees it's best to freeze the sap as you get it and save it up until you have enough to boil down to get at least a litre of syrup from cause as you get closer to finished syrup the syrup will burn very fast and if you only have a little in a pan boiling down the whole batch can get ruined real fast if you don't have much in the pan. It takes a long time to boil the sap down to finished syrup.
We used to sell ours but don't any more as we just make enough for our family now and for trading with neighbors for other foods. It's fun making the syrup for yourself and our Big Leaf syrup here on the west coast of BC tastes much better than the Eastern Sugar maples in my opinion. It's got more complex flavours with caramel, butterscotch, toffee type of flavour notes. The darker it is the more flavours develop as well.
Some pics from the syrup we've made over the years. And we progressed from small pans over outdoor wood stoves, to a custom made hybrid tig welded evaporator pan. The big one we can go right from sap to finished syrup on that pan with a continual drip from our bigger main tank of reverse osmosis sap. When reverse osmosising the sap it concentrates the sap a lot before boiling down. Saves a lot of fuel and time to get to the syrup stage.
View attachment 61286
View attachment 61287
View attachment 61288
View attachment 61289
View attachment 61290
View attachment 61291
View attachment 61293
View attachment 61294
View attachment 61295