Rachio 3 18 zones, two wire bundles/commons, two Rachio install


Hello. I have a total of 18 watering zones that used to be hooked up to a single old controller. (see pictures) There were 2 wire bundles of 9 wires each and each bundle had a common wire. A local sprinkler installer saw the old setup and recommended that if one Rachio could not accommodate all 18 zones, that I should install each 9 wire bundle + common wire to a single Rachio controller each. I have purchased a 16 zone Rachio 3 and an 8 zone Rachio 3. Going through the support forums, it isn’t clear if this is even an option or if I would need to purchase 2 isolators and use a single common wire split to the isolators.

Thank you for your help.


@WaterWorks - I think there are four options. I listed them in what I thought was the best first.

  1. Return the 8 zone Rachio 3 for a 16 zone Rachio 3 and install as suggested - 9 zones plus common wire in each Rachio. The common wires sound like they are unique to the wiring bundle they’re in.

  2. Buy a 24 VAC NO SPST (Normally Open, Single Pole Single Throw) relay (probably about $7.00 US plus shipping and connect one side of the relay coil to an unused terminal on the 16 zone Rachio, with the other side of the terminal going to the 16 zone Rachio C(ommon) terminal. Hook the NO (Normally Open) terminal on the relay to the zone from the 8 zone Rachio that isn’t connected to a terminal and the common terminal on the relay to the 8 zone Rachio Gen 3’s +24 terminal.

  3. Use two isolators to share the common wire between the two controllers.

  1. Find two zones that can be run at the same time, in the same 9 wire bundle and put them together in one terminal on the 8 zone Rachio. A little sub-optimal as that one terminal will need to be set for the most sensitive zone, so one will be over or under-watered.


Thank you DLane. Lots of great suggestions.

When I asked Rachio company support the same question, they pointed me to the isolator option on the site and it wasn’t clear that the other suggestions were options (plus as I mentioned the sprinkler installer made a different recommendation). I don’t want to fry anything and I don’t know much about controllers or electricity.

If I go with option 1, is there any way to verify that it is ok to run each bundle to a separate controller without causing interference or problems?
For option 2, are you suggesting to limit the first controller to 15 zones and put the other 3 on the 8 zone Rachio? (trying to understand where the common wires go and what the unused terminal you are referring to is).

For additional context, I believe zones 16-18 are mostly plant zones, but they have lots of sprinklers and pressure does become an issue.


@WaterWorks - all the terminals on the Rachio are low voltage - 24 VAC. Plus the Rachio protects itself from a zone drawing too much current. Odds of “frying anything” are remote (one should never be absolute).

For option 1 - An ohm meter (normally combined with a volt meter) (and I realize I’m skating on thin ice here as in the prior post there is the comment about not much electricity knowledge) could be used to test connectivity between the common wire and each wire in the bundle. There should be some resistivity (not much) between the common and a zone wire in the same bundle. Between wires from different bundles (e.g. common from bundle 1, a zone wire (say zone 3) from bundle 2) there should be infinite resistivity (i.e. no continuity). If there is resistivity (continuity) across wires from different bundles then the common wires are joined somewhere out in the field. Now that I think about it, a quicker test is to see if there is resistivity (continuity) between the two common wires from the separate bundles. My guess is no, and if you don’t have an ohm meter I might just roll the dice and hook them up. If for some reason one set doesn’t work, then we’ll start trouble shooting the wiring.

For option 2 - No. Put all 9 wires from bundle 1 in the 16 port Rachio. Put the first eight wires from bundle 2 in the 8 port Rachio. Hook the relay up to wire 9 in bundle 2 and port 10 (or 11, 12, 13, 14, 15 or 16) on the 16 port Rachio. Connect the common for bundle 1 to the 16 port Rachio and the common for bundle 2 to the 8 port Rachio.

Option 2 is the next to least cost option (a relay is cheaper than the 8 - 16 port delta and cheaper than two isolators) as option 4 is a no cost option, but uses the Rachio intelligence sub-optimally on the two zones that are tied together. But option 2 will also have a relay (small) and wires running between the two Rachios, so it may not look as “pretty”/neat and tidy, etc.