I have push pull pumps on two buildings that are piped with primary/secondary, what I am looking for is a controller that only looks at temperature difference,[between push & pull] the temperature would need to adjustable maybe 2 -20 degree range or something like that. What I want to do is cycle one pump off when the system is not loaded, then when it senses the temperature difference of the cycle on load, that the other pump turns on. every thing I have looked at has a set point also, [solar stuff]can't use that because of large swings in operating temps.