Do I have to build my own touchscreen thermostat?

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Those are all very nice things to have in an improved thermostat, I agree completely

The point I was replying to was making the erroneous claim that basic operation of an HVAC isn't possible with an RPI thermostat because of things the thermostat doesn't handle

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For one, UI/UX is actually hugely important for a consumer device and definitely nontrivial

Hugely agreed.
I'm a huge proponent of DIY / open-source / self-hosting / repurposing etc...but also I realize if I duct-tape-engineer something that "requires a little fiddling until it works" and I'm the only one who can competently use it, I'm setting up the rest of my household for failure when (not if), for whatever reason, I'm not there to babysit it or walk them through it.

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I'm a programmer. I miss my 70s thermostat. Hot/cold/off, temperature, and fan on/auto. Very simple. Just worked.

The smart thermostat is nice, but it's something I'll need to revisit when I have time.

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This made me think:

What about thermal runaway protection? I'm betting that might be easily overlooked in a custom "smart device" if, for whatever reason, the temperature sensor were to fail and keep reporting "Hey it's still only (below_target_F_degrees) in here! Keep that heat on full blast!"

This was an issue that made jank 3D printers catch fire and burn houses down until it was mitigated with open source firmware.

Point being, unless there's a "custom smart thermostat project" that's vetted and trusted, stuff like this might be overlooked, wherein it's bog standard in consumer devices.

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My Ecobee not only has some fairly sophisticated software, a touch screen, and remote sensors, but enough CPU to run Alexa

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I think what you're failing to understand is that newer HVAC units are designed to be run in a specific way. if you disregard that, and run it like a unit from the 1980s you will halve the life of the unit.

yes, most of the "brains" are on the unit itself but without proper "eyes" the brains are useless.

I'm not even an HVAC tech but I've worked on them enough to know it's a dumb idea to just replace your thermostat with "a raspberry pi and a bunch of 24v relays". there's just too much risk of things to go terribly wrong and multiple people literally die.

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I think what you're failing to understand is that newer HVAC units are designed to be run in a specific way

Not missing that, as I and yourself said it's done via the actual HVAC unit and not the thermostat

but without proper "eyes" the brains are useless.

No, the nicer features just don't get used, it doesn't make the entire unit not work

there's just too much risk of things to go terribly wrong and multiple people literally die.

Lol, people aren't going to die if you replace your thermostat with an RPI unless you wired the thermostat so poorly that you somehow manage to schock someone touching the thermostat

Again: thermostats just tell the HVAC to run and which part to run, any other advanced features are not required to be catered to in order to work. My brand new fancy schmancy heat pump with all those cool features listed runs off the same $2 Honeywell thermostat my old heater did, if I want the advanced features like zone sensing I can upgrade my thermostat, but my heat pump works without them just fine

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I have smart thermostats. I block 100% of their traffic.

Why?

One winter I woke up to my house being not 70, but 50. BOTH of my units had stalled after a botched firmware update.

I can't trust a multi-billion to fuck my house up while I sleep. not once, not twice.

now they are nice glowing thermostats I can see in the dark that my power company paid for.

couldn't be happier since cutting them off.

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I'd be curious to see what system resources are required for Alexa

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I’m sure not much: Amazon is not likely taking a loss on all those $50 Echo Dots, but that’s a lot more than a thermostat needs. It’s also on WiFi and can play streaming audio, so that’s not nothing

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That will get you a really shitty thermostat. Sure, even modern boilers can be controlled with a simple on/off signal but you really don’t want that, because it sucks. At the very least you need to make something that speaks OpenTherm. That allows you to modulate the boiler. With a simple on/off style thermostate you get relatively large temperature swings, with a modulating boiler/thermostat you can achieve very constant temperatures, which is way more comfortable, but requires both a more complicated protocol as well as more complicated logic.

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Ah sorry yea agreed, at least for the units I know about