What do you know about frozen pipes? 
I didn't always live in Texas. 
Also, I also realize the only example I used is the hub crashing.... What about if you get your user logic wrong?? That has been known to happen, too.
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I guess knowing about frozen pipes could be a reason to move to Texas. Why am I still in the frozen north??
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Great answers, thanks everybody!
So I need to select from thermostats that are compatible, but don't require any cloud services.
Jack
I'm pretty happy having Hubitat completely run my thermostat I've been fine tuning the details and so far has worked flawlessly. Goodbye Ecobee
Then I deploy my backup hub, restore the database from my current hub, and I'm back up and running withing minutes.
But you still have a thermostat in your setup, correct? The OP is asking about having a system directly controlled by HE via relays and eliminating the thermostat all together.
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Yes I do. Not sure why one would consider going that route unless maybe it's a brand new HVAC install with no existing wiring. If the existing thermostat is already there and wired why not use it. I'm literally accomplishing the same thing as what the OP is asking, but just using existing equipment wiring and the thermostat itself as the relay.
Exactly.
I read a post somewhere about a guy wanting to eliminate all his light switches and using motion sensors only. One thing people don't think about is building codes. I'm pretty sure codes require a thermostat. Not that a lot of people care but in the sale of the house, it might become an issue.
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That's the main reason I live outside of a city, because I will never care about building codes. That is how the majority of outside of city houses are built and routinely added on to is by DIY'ers. Building codes are solely aimed at protecting contractors from competition of DIY'ers willing to charge less $$$.
I think you will find building codes arose from unscrupulous contractors and incompetent DIY'ers.
If I choose to hire one of those it was my decision and thereby my fault. I should always be given that choice to hire them if I that is who I want doing the work. Preventing them from doing so takes away the low cost choice and institutes a one size fits all minimum standard, which has no choice.
I'm done ranting now, time for coffee
Not when a fire in your house could also destroy your neighbor's house and also costs the taxpayers money for the fire department to come and put it out. We don't live in a completely free society. This isn't Ayn Randville.
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Not if you're on vacation or not home you don't... Last hub crash I had, I was in the UK.
But I agree that it is up the the end user to decide what is ok/not ok for them - up to, but not violating building codes.
My original question was answered quickly and answers were really helpful. I'm encouraged to see such an active support community for Hubitat. [quote="waynespringer79, post:12, topic:24978, full:true"]
Yes I do. Not sure why one would consider going that route unless maybe it's a brand new HVAC install with no existing wiring. If the existing thermostat is already there and wired why not use it. I'm literally accomplishing the same thing as what the OP is asking, but just using existing equipment wiring and the thermostat itself as the relay.
[/quote]
I have existing wired thermostats, but they're old and dumb. I assumed I'd have to replace it with something, and since I'm not yet familiar with HE's capabilities, I was afraid "smart" thermostats might be overkill. But you (all) cleared that up for me.
Some of the discussion has wandered well beyond my question, but no worries there. I'm learning from it.
Thanks.
Jack
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And I guess I'll need to learn how this forum software works. I see I broke the quote feature, and can't seem to edit my posts after publishing. I'll figure that out eventually.
Jack
One thing to put into perspective is most all (smart) "devices" (thermostats/sensors/switches/etc) the word "smart" is really only referring to their ability to communicate with other things.
The actual "smarts" is being done with what they are communicating with (HE hub) and the automations the hub tells it to do.
My Zen (smart) thermostat pretty much the only difference between it and your dumb thermostat is that the Zen has the ability to talk to other devices with the zigbee wireless protocol. Other than that it's really identical to a dumb thermostat (besides that it looks aesthetically better)
This is also not required I could still use the Zen thermostat without even joining it to a zigbee network and would operate just as your dumb thermostat does.
There are several reasons, but the most obvious to me is what low-voltage switches and/or relays would you connect to your furnace and air-conditioner control terminals. Here is a typical picture of the terminals in a furnace (where your thermostat wires connect to) there are also terminals in your air conditioning unit. 
By the time you bought the zwave or zigbee switches/controllers for these low voltage terminals, it probably would have been less expensive just to buy a thermostat. A device designed for that exact purpose.
Furthermore, some home automation enthusiasts like to call Zwave or Zigbee thermostats "Smart-dumb thermostats". The thinking is they do no logical processing themselves but rather connect to an automation hub to do all the logical processing. This is NOT exactly true, thermostats have background logical processing that I have not seen in an automation hub. For example, my zwave thermostat will only allow for so many heating cycles per hour, it will also not allow the air-conditioner to turn back on within the first 10 or so minutes of shutting off. This logic is to protect the equipment and no setting in Hubitat can change that.
If you where to by-pass a thermostat and manage to control the HVAC through a variety of switches/relays connected to the terminals of the equipment, you better know all the safety logic that is built into thermostats (a lot more than just turn on/off heat, turn on/off cooling, turn on/off fan) and be able to replicate them in Rule Machine.
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@Stephan.J will agree I suppose
Also, thermostats are not just only relays switches. Inside a thermostat, there are rules that determine when the relay ("heat") will be triggered. And those rules are not simply on/off. There is parameters like deadbands, hysteresis and temperature curves.
Also, on HVAC system, thermostat will prevent some errors (like going from cool to heat immediately without delays). You can destroy a compressor, especially with reversing valves, if you switch too fast from one setting to another...
Ok, you can probably program this on very complicated rules (and for the temperature curves, I really doubt). But keep it simple, let the thermostat do its job and just command it through HE with the end user needs: higher, lower, heat, cool, fan auto or on.
Far easier than build complicated rules.
Black box concept !
