Do you think the Opensprinkler would be ok to run without removing the existing manual sprinkler transformer? The existing manual control is still useful at times, instead of having to rely on the web access to control. Especially while testing.
Was thinking of wiring the pump controller and solenoids in parallel between the existing manual controller and Opensprinkler. Can you see an issue with this? Both are 24VAC controllers.
What would happen if both controllers opened the pump and a solenoid at the same time? Being in parallel the voltage would be the same but being AC is that an issue? Would they have to be powered from same 24VAC power supply?
Also if one controller has opened a solenoid, would the 24vac feeding back through the other controller’s solenoid port cause an issue?
Without knowing the internal circuitry of your manual controller, it’s hard to tell. For example, on some controllers the COM wire is the logic ground of the circuit, while on OpenSprinkler it’s not. If you try to connect the two in parallel without knowing these details, it may directly short the sprinkler transformer. So unless if you are confident about how the manual controller works internally, I don’t recommend connecting them in parallel.