In the middle of a multi-controller install and need help

@fresnoboy, good evening – jumping in to help with your wiring questions. Could you post a photo of your wiring for a quick review? Are you using all 23 zones? Does the 12 zone wire do the front yard and 5+6 zone wire go to the back yard or vice versa? How would you like to schedule waterings? All zones on the same day?

Sorry for the long list of questions; just gathering a better picture of your watering needs to outline the best solution moving forward.

Thanks, Emil

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Thanks everyone. I’ll try and get a photo tommorrow, but there are 3 separate cables. The 12 zone one is all the backyard, plus one zone in the front yard. The other 2 cables go to different sections of the front yard.

The cables come up to a wall in the garage, which is close to the middle of the front yard area, no doubt that’s why they used two cables, one going north from the garage to plants on that half, and the other one going south to the zones in the other half.

Normally, I do either the front yard or backyard one day, alternating between front and back. The drip systems normally run an hour in each zone, so you can’t run a full cycle of all zones during the night when they usually run. The backyard vegetable beds run after sunrise to avoid fostering mold growth.

I’m glad multi controller support is being worked on as it seems so obvious for a product like this one. But my more urgent issue is dealing with the wiring.

i gather the issue with the grounds is that each controller has its own built in ac power supply and isn’t chained across?


I understand all of the challenges here.

As already figured out, 23 zones requires a minimum of two Iro2s.

At this time they don’t ‘talk’ to each other. To do so, it will require the Iros2’s ‘engine’ (internal calculations) to be expanded from keeping track of 16 zones to keeping track of 32 zones. It’s just like ‘bit’ expansion in the computer world. Right now the Iro2 is an 4 bit processor (for example). You need a 5 bit processor to keep track of all 32 zones independently of each other. Roughly.

All is not lost, though, if, the maximum watering time for the first 16 zones is never more than 12 hours. If so, setting the Iro2 to start at midnight, and setting it to full Auto, will result in watering being done by Noon.

Then, a second 16 zones can be \set to start at Noon. It too must be finished with all watering by midnight in order for full Auto mode to work properly.

If you have a residential yard, even a large one, that the first 16 zones can be always be completed within 12 hours, and the second 16 zones can be likewise completed in 12 hours, then you’re done. The Iro2 will schedule perfectly without fail and without incorrect load on the source water connection or pump.

This is the challenge parks have and why in most cases city parks cannot use Iro2 units. Many times the parks are so large that a ‘day’s’ watering can’t be done in 24 hours. And thus a more sophisticated ‘engine’ that can keep track of the accumulations past a 24 hour boundary. This is what the commercial units indeed do.

I don’t know if Iro2’s ‘engine’ is being modified to go past 16 zones. And if it is, I further do not know if the ‘engine’ is being modified to allow for a ‘cycle’ to got more than 24 hours. If so, for both, along with an Ethernet connection, the Iro2 will appear very attractive to municipalities.

WRT to wiring:

Your easiest solution is to stay within your wiring’s constraints. I would purchase a 16 zone, an 8 zone, and another 8 zone unit. Three transformers, three return wires each in the correct cable. You’ll end up ‘wasting off’ 4 zones on the 1st unit, 2 zones on the 2nd unit, and 3 zones on the 3rd unit. I think this cost is less that the cost associated with an isolation system otherwise needed to accommodate your 3 cables.

The reference Rachio refers you to is correct and accurate. And if you don’t follow it you will probably burn up your transformers (not your Iro2 units though).

This project is within your ability if you implement 3 Iros2 units as I described above. And that you can do all watering needed of the 16 zone unit in 12 hours, and all watering of the 8 zone units in 6 hours.

If you stay with your 16 and 8 zone controller, you need a commercial electrician to guide you. This is not an obvious setup and most residential electricians will fail.

This is a problem that comes up in the HVAC world. Multiple transformers that end up being put in parallel to each other and thus burning themselves out.

Go get three Iros - 16 and 8 and 8 zones - and keep the project easy!

This recommendation also provides to you some expansion capability per cable: 4 zones for cable 1, 2 zones for cable 2, and 3 zones for cable 3.

And then when/if the ‘5 bit’ Iro3 ‘engine’ becomes available, you’ll discard the 3 controllers in favor of a fully automatic 24 or 32 zone unit. And maybe you’ll get an added bonus that allows for watering to go past 24 hours per controller cycle.

Best regards,


Bill, things are a little more complicated than being able to segment by time of day. For example, I don’t want the lawns being run during most of the day, because I don’t want them to spray on the kids or adults when we are outside playing. I don’t want the vegetable beds being run at night, but only in the AM and before it gets hot.

I thought I could segment the zones by day, and have to do it by hours of the day. Is this not possible?

And which transformers are you talking about? The Iro2’s have built in 24VAC transformers, so if they fry, I assume the IRO is useless. Is there a way to hook up an external transformer to the Iro? If so, I could feed them all from a common transformer and then I don’t think there is a problem.


Mike, you’re going to have to decide to invest in a sprinkler controller more sophisticated than what’s offered by the Iro2 or you’re going to have to ‘make do’ with the capabilities you have with 16 and 8 zone Iro2s. And if you decide to proceed with the former then IMO it’s going to need to be a high end commercial unit that allows for watering a controller cycle past 24 hours and provides the scheduling front end you want.

You’ve got a yard and landscape that warrants a 23 zone controller and you have scheduling desires that probably means you won’t always be able to accomplish getting the water down in a 24 hour day during the summer.

IMO the Iro2 is too much of a residential product to satisfy your needs. In fact, noting you’re out to 23 zones and you have some significant restrictions on time of day for watering, IMO small commercial units won’t satisfy your needs either.

You’ve got many requirements to ease up if you want to use Iro2 product for your situation. Wish I had your yard and landscape.

You can convert your need to one within residential controller requirements if you can increase the capacity of your water supply to where two zone valves, each on a separate controller, can simultaneously be open. This means you need two residential connections to the city water supply, or a commercial connection, or a larger pump. All assuming correct size piping is implemented in and around the water supply connections.

I’m not familiar with Iro2s with built in transformers. I believe all Iro2s, for inside or for outside in an enclosed box, have an external transformer. This is how all the controller manufacturers avoid otherwise significant UL testing expenses.

You are correct that if the Iro2 transformers ‘fry’ the Iro2 is useless.

You bring up another technique that I didn’t think of earlier. Yes, you can rewire such that you run both (or all 3 Iro2s) from one transformer. The key is there’s only one transformer present.

You will still need to be exceptional careful with your scheduling such that you don’t have two zone valves turn on at the same time.

Best regards,


@fresnoboy, correct. More technically, there’s a polarity issue if you share power adaptors across the same common.

@a0128958, thanks for jumping in! This would be the easiest solution. Alternatively, swapping out the 8 zone for another 16 zone controller and routing the 5+6 zone wires to the second 16 zone controller would do the trick.

@fresnoboy, where did you purchase your controllers?

Thanks everyone. I had a system before that supported 24 zones easily, and had good capabilities, but was not user friendly to operate, and no app platform etc…, had poor WAF (she wanted something the gardeners would be able to easily control), and not have to ask me to do something or that required her to learn something new. That’s why I am switching to the Rachio in the first place. :slight_smile:

I don’t understand the polarity issue. Can someone explain this to me? The Rachios are all of a common design, so I don’t understand how the polarity would be different between the different units. You are right in that there is an external transformer that powers the Rachio - now I get what you guys are saying. It appears the transformer that feeds the Rachio main unit is 24VAC output, which makes perfect sense.

I hadn’t thought about this before, but if I got a bigger external transformer (2X the current capacity of the current external transformer), could I feed both Rachio units with a common feed, since presumably this would support everyone being on a “common” common? I am an engineer, so wiring the coax power jacks to feed the rachios is pretty easy. This seems a much cleaner approach that using the isolation boxes.

Emil, I got them both via amazon.

Given the lack of scheduling integration, it seems a better idea would be to replace the 8 zone unit with another 16 zone unit, as that would mean only having to divide things up between 2 day or time blocks instead of 3. I wish I had known about the issues with a common common before, as it may have changed my selection of products. The rainmachine boxes support jumpering the common of one unit to the common of the other unit: , so there is no need for isolation boxes in their design.


Any updates for support? My irrigation has been down for 3 days… Rain coming at least.


@fresnoboy, I understand your logic with this solution; however I wouldn’t recommend doing this as we haven’t tested this configuration internally. The polarity issue arises when you split one of the three wire strands between two controllers operating on different power adaptors.

Agreed; three controllers would be overkill and require more manual scheduling logic between the controllers.

I believe your use case is a bit different than the solution outlined by Rainmachine as you have three different common wires. If all of your zones were running on the same “shared” common, then you could leverage this configuration for your system.

I hope this helps.

Best, Emil

Alternative 1: 16 + 8 + 8 zone Iro2 controllers to accommodate existing wiring requiring no wiring changes (3 transformers, 3 return lines). You’re correct in that it’s more work to manage the scheduling. You’d have to make double/triple sure the first controller can finish in 12 hours, regardless of day of year. Likewise, you’d have to make absolute sure the 2nd controller can finish in 6 hours and the 3rd controller likewise. If you have time periods that you want to avoid watering then designing your schedule will get to be complicated.

Alternative 2: 16 + 16 zone Iro2 controllers. You’ve got some rewiring work to do to get this to work. Two controllers, 2 transformers, 2 return lines. You still have to be maniacal about no more than 12 hours per controller 1. You have increased flexibility with zones 17-23. Same requirement - have to finish in 12 hours.

Alternative 3: 16 + 16 zone Iro2 controllers on one transformer. Probably doable, but, not a certified implementation per manufacturer and thus you may have warranty pushback if something goes wrong (ever). Still requires maniacal attention to watering all zones within 24 hours.

Alternative 4: Change the plumbing such that one zone on one controller can operate at same time as another zone on another controller (and maybe even a third zone on a third controller if you have 3 controllers). Can’t tell at this point if this is a small or big effort for you. Certainly it simplifies scheduling dramatically. Still have to choose alt 1, 2 or 3 above with respect to wiring. Still have to finish all watering in 24 hours but it’s now easier to do so since you can run two zones at same time.

All four alternatives above assume you’re putting in Iro2 controllers with current capability. There’s no way around the alternatives with present capability Iro2 controllers.

You may get lucky in that the new Iro3 capabilities to be announced allow for watering a complete cycle across the 24 hour boundary. If so, then you can choose from Alt 1 through 3 and ignore the plumbing related alternative.

Hope this helps.

Best regards,


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I can think of a much easier way to use just the 16 and 8 controllers. If either the 5 station or 6 station bundle has an extra lead unused, simply split that bundle and run two Commons. Rewire and group at the solenoids for the correct Commons.

Actually, you could split the 12 station bundle too pulling 2 or 3 circuits over to the eight station, and the spare common lead, with one of the other bundles.

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@Lars, great ideas. I believe Mike is using all of the wires in each bundle. Just to clarify, your idea is to use one of the unused zone wires as an isolated common to avoid sharing a common between two controllers.

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Yes easy enough to rewire at the ends. Seems a 12 station bundle would be 13 wires. An odd number for sprinkler wires. Might be easy to run another common.

I don’t follow your whole post, but I have 2 16 controllers for 23 zones. Rachio support guided me. I named one Waterbox 1 and the other one Waterbox 2. I have to go back and forth from one to th other, which is quite easy. Wires for zones go to each separate controller, no integrating the two. I guess that is the way until Rachio gets controllers beyond 16?

@camasguy, correct – this support article provides a good overview of how to manage multiple controllers for anyone that’s in a similar boat.

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Hi I’m doing the same and would like to avoid having to use an isolator, I only have one MV , would you be able to share a pic of your setup ? Thanks !

@Ninjago - If you’re trying to have two different Rachio’s control one Master Valve see this post for a simple solution using a SPDT or DPDT 24 VAC relay -> 2nd Gen sharing a pump start relay with a Hunter Controller?

thanks Dlane , I guess no way around it without having to add a relay ? I really don’t have to use both controllers at the same time I have Zone 1-15 on one ( front yard ) and Zone 17-23 on another ( backyard ) I barely water the backyard since is mostly weeds for now. So the question is , if I’m using the controllers at different times do I still need to do this ?? also is the Relay going to give me the same functionality as the Isolators will or is the Isolators a better way to go ?? sorry for all the question’s I’m don’t know much about electrical.

@ninjago - Yes, you’ll need either the isolator or a relay (cheaper). The relay will cost about $15 (including shipping), or less, here is an example ->

The one Gene sourced in the linked post is ~$7 and has a light when one side is active.

You don’t want to connect two electrical sourcing (i.e. Rachios) devices together, the current flow can get strange.

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Got it ! Thanks DLane I appreciate your help, I will give the relay a try since is cheaper and would look cleaner if hidden somewhere , Thank you !

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