OK, I think I get it. Floating actuators take a continuous control signal 0-n V, where 0 means closed and n means all the way open. Turn off the current, and the valve closes.
Then there's modulating actuators, can't figure out what that means or how they're controlled.
The third are the ones controlled with a relay - probably two control wires, one ground, and depending on which side the +24V are connected to, the valve rotates towards open or closed as long as the current is on. From what I see, most of them also deliver a proportional voltage as feedback that I could read the current valve opening position from.
Am I understanding correctly?
What I want is definitely one of the third kind described above - use DT relay to put voltage on either lead to open or close as long as current flows. This will be the easiest to control.
Based on that, do you think something like this would work?
VC8711ZZ11 - Honeywell VC8711ZZ11 - Two position, Low Volt Actuator for VC Series Valves, w/ end switch
I'm just looking to see if I can find something that fits the bill, but doesn't end up costing me $600
Thanks for all the information!