I just confirmed that Irritrol does have one side of the 24VAC tied to common as you indicated that OpenSprinkler has. Thus a DPDT switch switching the 24VAC line connected to the commons and switching the common line to the solenoids should be all that is required. There will be a AC signal on the active station triac of the unit that is not powered up but that AC signal will have nothing to reference to since the common line will be floating. While I will get everything ready, the Chicago weather will not permit an active test as I have no spare solenoids sitting around in the basement. I will advise of how this configuration works as soon as testing is possible. I will only keep this configuration until a local manual control becomes available on the OpenSprinker. 😀