You curently need a 24VAC NO DPST relay as the Rachio 3 does sadly not support concurrent zones, see: Run Multiple Zones At Once
Hope to get support for native concurrent zones setup in near future but yet support didn’t give a statement on the suggestion.
Here is my take on concurrent zones, specifically utilizing Flex Daily…if anyone cares.
Flex is an awesome, dynamic, always changing schedule, and if Rachio allowed multiple zones to run (thru programing), some days you might have 1 zone run, another day 2 zones, and another day 3 zones depending on how the fluid schedule ran that day. Issue I see is, when I combined 3 of my grass zones (thru wiring), I had to adjust the spray patterns of the nozzles to account for the change in pressure thru the system. Combining the zones actually brought my zone pressure into the sweet spot for my particular nozzles, so it worked out well for me. But, if you are dialed in with 1 zone running, and Rachio decided to run 2 or 3 at the same time, you can have a drastic drop in pressure and volume, thus reducing the PR of your sprinklers and making the Rachio algorithms inaccurate. I know it sounds miniscule, but depending on the GPM of a zone, it could make a significant difference in how the zone performs.
Just my $.02.
Exactly: Thats why I feel a simple “system gpm limit” would allow users who experience pressure drop, to define a limit to avoid this, while other users who do not experience a drop can set a higher bar.
If you had one or two heads that this was a problem, and others that did not - you could exclude the zone from concurrent operation in theory with a check box.
I ran 1" pipe, and found that even running 3 heads at a time, there was no change in how far each head would throw water.
I’m currently only using 3.0gpm heads, but plan to try swapping out a few nozels to 8.0gpm, then re-test.
Personally, I think that is asking a lot of a system. Not saying Rachio can’t get there in the future, but now you are talking about adding the need for flow/pressure sensors to monitor the system as it is running in real time. Hey, I’m on board if it ever comes, but people complain about the complexity of the system as it is…
You are still on the design/instal phase of your system, right? If you are building a system that can run 10 rotors at a time, why not just pipe it that way from the start?
To add my 2cents, Rachio flow meter is rated up to 25 GPM, about 8 of the 3GPM heads, or only 3 of the 8GPM type.
Besides that, running multiple zones at the same time would hinder Rachio’s ability to quickly identify zone issues, especially near the max limit of the meter.
What I am saying is that it is not as easy to just turn on the ability to run multiple zones, designers have to accommodate extreme cases.
Well, it certainly depends on how many nozzles per zone you have. In my case it is a maximum of 2 nozzles. 
I set up 10 zones on 180sqm yard and wanted to take control of single nozzles for child entertainment on hot summer days. 
The problem now is that a single schedule lasts for about 14 hours… 
I think I have to go with this solution to combine at least 4 nozzles per zone:
Can your piping support more than two? I have 2 “zones” that I have wired 3 solenoids to each terminal. I was lazy and didn’t want to dig up and re-pipe just yet, so this was a quick fix. I went from 18 zones to 12. My main motivation was the same as yours. I have Hunter MP-Rotator nozzles, and each zone would run for over an hour, leaving little time for anything else to run in a day.
Yes of course! I use 1" piping. I have anough pressure and water throughput for at least 5 nozzles per solenoid.
As far as I know, the Rachio support stated that 4 solenoid per terminal will work.
If concurrent zone setup will not become available in the near future, I will follow the suggestion of DLane using 3 relays to drive all my 10 solenoids.
Relay 1 on terminal 11: 4 solenoids
Relay 2 on terminal 12: 3 solenoids
Relay 3 on terminal 13: 3 solenoids
This leaves me the freedom to still control all my 10 zones independently, as I said for child entertainment or whatever. 
I don’t want to loose this freedom of control by repiping the 10 zones to just 3.
lol… so true! That’s why I hope to see concurrent zones setup in near future. 
I disagree, its not that complicated, nor are any extra sensors needed.
You simply enter a (gpm) number you feel is the max, and the system automatically limits itself to only run as many units as it can below that number.
Hard Pairing the heads limits your options.
Just curious, why are you trying to run more than one zone at a time? Most residential systems are generally 15gpm or less per zone (or at once, maybe even less if going uphill). If you tell me your meter size and wayer pressure I’ll give you the recommened gpm (the five gallon bucket is not accurate). Your nozzles may “look” like they’re running properly, but they are probably wasting water and money if the system is overloaded. It can be difficult to tell visually. 1" pipe may not be large enough if you’re trying to run 25 or 30gpm nozzles.
Gpm refers flow (volume) if you want to put more water down it’s best to look at the precipitation rate (inches) in the manual for the nozzle.
Gpm = dollars
Precipitation rate = time needed for watering
I want to run more than one at a time for a bunch of reasons.
75psi 1.049" ID pipe (per head)
If I turn a valve on w/o a sprinkler attached, I suspect I would fill a 5 gal pail in about 3 or 4 seconds. It flows pretty good, and has some good velocity yet for coming out such a large pipe.
In summary, it just seems like a simple thing to offer.
and should be easy to implement.
Now… having the sprinkler heads go to your music, that might be a bit more difficult 
In my case this was a conscious decision to have 10 solenoids with a max of 2 nozzles per solenoid as I said for child entertainment on hot summer days.
Why so complicated? If you plan a sprinkler system you can do the faults as well and connect too much nozzles per pipe.
And the next question with your suggestion is, how should the Rachio 3 know what zones you would like to pair if it does this automatically based on the total gpm value?
Let’s go further… If you would like this feature, you would also need to have an throughput gpm setting per zone. In my case this differs from zone to zone. Some zones have only 1 nozzle, others have 2 (twice as much throughput on that piping.)
I my case it would help for the start to simply pair zones in the app.
I totally agree!
As I and @tmcgahey already said. Watering my yard currently takes about 14 hours for 10 zones with 15 nozzles in total.
I’m not talking about huge nozzles to water golf yards or so. I use Hunter prs40 with rotary nozzles and these have a max throughput of 0,484 gpm. With a max of 2 nozzles per pipe I’m at a max of 0,968 gpm.
My water main pipe delivers 7,925 gpm at 3.4 bar. So any more apprehension I cannot pair up to 4 zones?
My yard is just 180sqm and totally flat!
I totally agree, but it should be enough for the max of 4 solenoids per terminal with a max of 5 Hunter prs40 nozzles having a total throughput of 2,42gpm on that setup with the paired zones.
In a nutshell… that’s it!
I think there is some confusion on what I’m saying, as it is the simpelest thing ever.
Lets pretend you had 16 zones
some zones 3gpm (single head)
some zones 1gpm (drip systems)
some zones 9gpm (3 sprinklers on one pipe/solenoid)
If you set your limit to 5gpm, the system would never run another zone at the same time a zone that exceed this number. e.g. the 9gpm zone would always run solo.
as for the 3gpm zones, it will never run two of them because (2) * 3gpm again exceeds the value of 5 you entered
The 3gpm zone WOULD however run at the same time as the 1gpm zone IF* (and only if) needed, otherwise it would only run the zone it needed.
If you had (5) 1gpm zones, the system could run up to all 5 of those at the same time (based on watering needs)
If you were to raise your limit from 5 to 6gpm, well now the system could run (2) of the 3gpm zones at the same time (if needed) and could even switch out one for up to (3) of the 1gpm zones on top of the one 3gpm zone.
Does that make perfect simple elegant sense now?
If you never wanted more than one zone tonrun at a time, set the system limit to 0.
This is WAY better than plumbing a bunch of sprinklers to one zone because once you do that, they are linked.
All the people that dont want to use this can set the value at 0
If you had a fire hydrant feeding your system, you could raise the number to 50gpm, and the system could then run as many sprinklers as it wanted without having to que up a wait list. i.e. it could be running (3) of your 3gpm zones, for 22 minutes then drop to (2) for the next 8 minutes, and if a drip system were needed, it could just come on without affecting anything else, etc…
Just seems so stupid simple that Im surprised it isnt already like this.
Another reason for concurrent zones as an OPTION for the software/controller: winter blow-outs. I’ve asked about this a couple of years ago with no response. For those of us in winter climates, while we run only one zone at a time for irrigation, the huge compressors used by system blow-out services before winter create enough pressure to blow out at least 2 zones at a time. Everyone I’ve hired has ended up manually opening valves in the yard just to get the job done quickly enough (I have 15 zones).
Those companies have been really frustrated with the Rachio… as older controllers let them just jumper the cables at the controller, but not enough wire is exposed with the Rachio for that option. They said they’d love it if I could have a schedule that rotated through two zones at a time for a few minutes each, giving time for water to settle in the pipes before the compressed air returned to the pairs of zones.
Feature Request: Enable the Generation 4 controller and app to run 3 zones concurrently. Your competitors have been doing it for years. Don’t make it complicated. My old Irritrol TotalControl did it easily. https://www.irritrol.com/en/controllers/total-control-r-series
The problem is, you start letting people purposefully, or accidentally, run multiple zones at once, they start having pressure issues which results in poor watering, and they come back and complain that Rachio isn’t watering correctly.
While asking about max load per terminal, Rachio support told me I should not pull more than 300mA from any given terminal.
Agreed … 
I managed to build a custom PCB with 4 relays that fits behind my rachio 3.
Relay 1 controls zone 1 (up to 4 channels)
Relay 2 controls zone 2 (up to 4 channels)
Relay 3 controls zone 3 (up to 4 channels)
Relay 4 controls all zones (good for draining the pipes before winter)
I needed to replace the power source with a new 5Ah that also fits in the Rachio 3 housing after doing some grinding work on the housing.
I also put a jumper connector to the PCB so it is possible to change the valves to zones assignment.
Works like a charm!
Anybody interested in further informations?