Using master value output during blowout


I’m trying to understand better how the M terminal on the Rachio 3 works. Background is this: I’m a new Rachio user this year going through my first winter blowout with the new controller after buying a house with a small yard and a Rain Bird controller a couple of years ago. The blowout service we’ve hired says that in order to minimize the chance of damaging the fittings in the irrigation system from the pressure of their air compressor, they prefer to keep one zone open throughout the blowout while they sequentially open the remaining zones. (At least that’s my understanding of what they’re doing.) With the Rain Bird, I believe they jumpered one of the zones open at the controller and then used the Rain Bird’s manual controls to open other zones sequentially.

When they blew out my Rachio 3 recently, they said they used the voltage from the M terminal (master valve) on the Rachio to hold one zone open while using the Rachio’s manual controls to cycle through the other zones. This wasn’t how I envisioned them doing it, so I read up on the M terminal later and saw that in order for the M terminal to function the Advanced Wiring setting needs to be configured in the Rachio app. So I imagined the M terminal didn’t hold a zone open as the contractor thought it would. But when I tested with a voltmeter I measured 25-28 VAC between terminals M and C all the time, regardless of whether the Rachio was watering or not, and regardless of whether I had the Advanced Wiring setting set to Nothing, Master or ther Pump Relay. I’ve read up on how M is supposed to work but am kind of mystified by what I’m seeing on the meter. Additionally, I didn’t see exactly how they jumpered it, but based on the wire he used it looked like they were jumpering between M and a zone, rather than unplugging a zone and plugging it into M. If in fact that’s what they did, I wonder if there’s danger to the controller from applying a voltage to a zone.

I’m also questioning whether there are other ways to accomplish what the contractor wants to accomplish. I thought that perhaps the water hammer reducer feature would meet the need, but by testing I determined that this feature has no impact on the Rachio’s behavior when you run zones from the manual controls on the unit as they’ve been doing (while another guy in the yard radios to say when to move to the next zone).

Any help understanding how M works or thinking through this would be appreciated!