Closed is the way to go. I have not put a pressure gage at the end of my mainline but with my slope and when my laterals are full of sap I bet I have good natural vac. One time I removed my stuppy at the very end tree of one my laterals and I could hear the vac suck as soon as I did. I was actually startled by it!
FIRST GENERATION SUGARMAKER
First boil 2/22/2012! Went Pefect!
3,500' of laterals
1,000' of mainline
2012 - 105 taps on gravity, 12 sap sacks.
2013 - 175 taps on gravity, 25 on sacks = 200 taps for 2013! Second year.
2014 - 250 taps on gravity, 25 on sacks
Tapped on February 16, 2014
2015 - adding vac sap puller no more gravity for me!
275 gallon holding tank for 2014
20'x30' Sugarhouse