FYI, there is a user nest integration available.
We have a small warehouse facility in Scottsdale, Arizona, divided between some office space and a warehouse space. The warehouse space has both an evaporative cooler and a heat pump for cooling or heating. This time of year, we are in cooling mode.
The evaporative cooler has a simple on/off control. I linked a ST multi-sensor to Rule Machine, and a z-wave relay wired into the low voltage control for the cooler. RM easily handles this automation. Once the temps here got to above 40° C, the evaporative cooler no longer could keep up. So, I installed a Zen Thermostat to replace the existing thermostat that controls the heat pump.
Now, using Rule Machine I have a two stage cooling system. When the temperature is in the range that the evaporative cooler works, the heat pump is disabled by setting the Zen thermostat to off. Once the temperature rises above that range, the evaporative cooler is turned off, and the Zen set to cooling mode. This is done with a single rule, so it can move back and forth between the two stages. I suppose there is some loss of efficiency due to the evaporative cooler introducing humidity, and the heat pump extracting that humidity. However, I'm sure that the overall energy usage is reduced by using the evaporative cooler as much as possible, and only resorting to the heat pump when it's very hot.
Rule Machine is fine for this because I'm using a single multi-sensor. To use several multi-sensors one would need to write a small app to average their temperatures, and present that through a child virtual device. Hardly seems worth the effort to write an entire app devoted to this application -- it is just a cobbled together thermostat: a sensor and a switch.
Here is the two threads, the first shows where to find the code, the second has all the development and instructions.
Perfect option of switching between the two cooling sources for efficiency @bravenel. I will look at similar between my split system heat pumps (electric) and the gas heating once I get a whole house energy monitoring device. This will be aimed at reducing my electricity consumption from the grid and rely further on the produced solar power. At the moment Aeotec don't supply an AU version (921.45mhz) smart energy meter.
Agree that standard temp control is far too much work for no added benefit.
Thanks @napalmcsr for the threads. I will look into theses.
Cheers for all your comments and opinions on this subject, most helpful.
here is an app that allows creating rules for temperature management. turning on a thermostat to cool or heat a house is already built-in to the app. but it does not currently handle a two stage solution like @bravenel described using evaporative cooler and heat pump or heat pump and gas heating in your case.
if you are interested in giving the app a try i would be happy to extend the app to support two conditional cooling/heating sources based on temperature (and power usage) range.
I ported my average temp device driver and little app to Hubitat a long time ago if you need it.
I wrote it to average out a couple of indoor/outdoor temp devices in my conservatory to help control my windows.
Has worked reliably for some months
Let me know if you would like a copy of the code and I’ll post it on github
Andy
Thanks for the offer. Not sure which way I will go now. Ordered a Zen thermostat 2 days ago then yesterday the NEST integration was released 
I ended up getting a Zen Thermostat that has been working perfectly. I have a RM rule for mode change (Heat/Off) based on presence with a virtual button to override off. I also then have a rule setup for each of the day modes which sets temperature. ie day mode (20degC) evening mode (22degC) and night (16degC). This has been working very well with no issues.
I updated to the latest 1.1.1.117 which seams to have caused issues with the communication to the Zen. Thus far it appears that RM is doing it thing correctly but the Zen is not receiving the communication. I have had to manually set the temp or Mode a number of times now on the Zen. Would this be an issue with the latest update or a coincidence @patrick . I have not yet removed and repaired the Zen to see if this helps but certainly don't want to do this on every update.
I checked my Zen and it also was not responding to change in setpoint automations. Only after I enabled debug logging and saved did it respond. A voice command of "I'm hot" or "I'm cold" should throw a virtual switch which triggers a setpoint adjustment of + or- 1 degree. The virtual switch turned on but I do not have any events logged for the actual rule that changes setpoint. FWIW, that rule was built with the old version of RM and I'm on .118.
I also turned on debug logging and it seamed to work then. Will monitor the performance and advise of any changes. Cheers for the response.
I am a google home user so would have to use IFTTT, but how did you handle the rules for increase in temp by voice command?
I created two routines within the Amazon Echo app (I'm hot and I'm cold) that turn on corresponding virtual switches in Hubitat("cooler" and "warmer"). Those in turn trigger one of the following:
I keep my Zen in Auto mode with a six degree F difference between heating and cooling setpoints, so I adjust both to take all seasons into account.
Is your Zen still be responsive? I changed my rules to the 2.5 version and my Zen isn't consistent about responding to the new temperature settings. I turned on logging but that didn't seem to help.
So far it is working. I didn’t really get time to investigate the increase/decrease in temp.
My temp changes on mode and previously it was very responsive. Now it does seem to have a delay. I have not had enough time to investigate logs on this one yet but will as soon as time permits.
I just checked and all my zen thermostat rules are already 2.5’s
Also upgraded to 118 last night.
This is interesting. I tried my voice command adjustment again and only the heatingSetpoint was changed. It took four attempts to manually adjust the coolingSetpoint from the device page. I'm going to play with this a little more, but right now I'd say something is wrong.
I would appreciate your input on this @patrick incase this is an issue with update 117 or 118. I am now running 118 but issue started on 117.
OK so here goes.......
Came home tonight and Zen did not turn on. It is initiated by change of hubitat mode from virtual presence:
This was taken after i returned home but the Zen did not change to heat as is should:
It also should have changed to 21 deg C temperature when the hubitat mode changes to Evening which it did not:
Other than logs that I will place at the end of this post which I may not have set up correctly, the below rule works from the same mode change and this worked as it always does:
Also I have a bypass to manually turn off the Zen if not required. I have this virtual button on the dashboard and the below shows it was on which should not bypass the rule (this virtual button is in the restrictions of the Thermostat mode change rule above):
The logs I had going are below:
So what I am seeing here is its not a communication issue on zigbee, its a rule machine issue that is not initiating this particular rule.
Thoughts/comments/suggestions to fault find????
Nothing regarding zigbee or zen was changed between 117 and 118.
Regarding Rule Machine. I'd start by duplicating the simple parts of the rule that doesn't seem to be working and see if you can recreate the issue.
See if a very simple rule off a virtual switch can control the zen. Then try a simple mode change test to see if it works as well.
First step is to see if we can find a repeatable pattern and something we can reproduce 100% of the time.
It could be that the zen dropped the packet (or some point in between) and it never processed the command. You might be able to reproduce this, but if there was interference or just a device hiccup, we might not know.
If you can reproduce it, let me know.
We've determined there is some problem with Zen in this release. Chasing it down.
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Thanks guys. Happy to assist where I can.
Look forward to hearing if you find anything.
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