I setup boiler control via comparators on tank valves today. Was really scratching my head until I noticed that valves only seem to work on the back of comparators, not as side inputs. So I've got a comparator on two valves, a comparator sideways on one of those original comparators, and then the RS latch. Seems to be working. Need to compact and hide it now.
This is the messy original version:
http://i.imgur.com/1gqGA6G.jpg
If the valve on the left falls beneath a value of 13 it turns on one side of the RS latch, turning on the boiler.
If the valve on the right raises above a value of 14 it turns on the other side, turning off the boiler.
Output side of latch away from boiler leads to boiler.
Test for a few cycles, seems to work as intended. Might drop the turn on value down to 7 or 8 now that it works.
Funny side note: I built everything to use sensor cards and a range trigger except for the last two wires which I thought were electrum, but were platinum of which I have none. doh.