any recommendations for a cheap, simple, but "smart" relay+thermometer like this that can be added to a apple home/zigbee network in plug-and-play fashion[0]? i also have a dumb heater that would be nice to be automated. the existing dumb thermometer/switch is too close to the heat, making that thermostat unreliable, and i can't move it without tearing up walls.

[0]: without having to run home assistant if possible

There "Blakadder's Smarthome Shenanigans" website is a good reference to identify if a Zigbee device might work with your system. If a device is compatible with many, it is almost certain it is also compatible with your solution.

https://zigbee.blakadder.com/index.html

i went down the rabbit hole a bit and there doesn't seem to be a homekit-enabled millivolt (not low voltage, which is 24V apparently) thermostat available. however, i did find this little project: https://zekesnider.com/using-an-ecobee-thermostat-with-a-mil... it seems you could patch any low-voltage thermostat to support millivolt in the same way (with a 24V transformer and a relay switch), but the ecobee3 lite has homekit support built-in.

Having a mechanical fallback is a really good idea. I always get a little worried when I see people patching together mission critical systems on a raspberry pi with little thought to failure modes. Learned this is the hard way.

Dealing ~99% reliable smart bulbs and guests not understanding the fancy buttons is death by a thousand cuts. Even a 0.1% failure to respond will add up fast if you have multiple bulbs per fixture.

Eventually, I switched to using Lutron Caseta products as the base layer. Now my light switches will still act like light switches if the hub is down and all the remotes are dead. And the remotes don't need the hub online.

Yeah there is very little thought in resilience in most such systems.

Ideally I'd see some generic rule-based system (maybe just WASM VM fed with some code and triggers?) that does just that - feeds code some data and pushes output to controllers, maybe with standarized methods to ask for historical/other sensor data, then have that just be a blob you can deploy on few devices, share a password and it would pick a master and keep rest in standby.

Then just have HA-like Web UI for configuration so the big fat of UI can run really wherever on anything, but the core that is needed for everything to actually work can sit on some cheap device in the closet.

I’m not sure if you are using the qualifier “good” to exclude the nest, but it can work on a two wire system (it can siphon enough power to work without making the relay close)

Unfortunately not in the case of my furnace...

It was upgraded with a rather new controller board at some point despite the machine being rather old, and is very unhappy about any extra loads on the heating control wire. In fact the controller supports all the things (even potentially controlling AC) but there are only 2 thermostat wires in the wall...

So why not run a common off a plug in transformer?

The old thermostat is placed quite far from any place I could plug in a transformer to... (otherwise, yes, that would have been the solution)

My current setup has the relay right next to the furnace, where there's plenty of mains lines to tap into for continuous power and this is fine since it doesn't measure anything and just gets commands from hass.

Similar here in California, except the original thermostat is in series and set to an upper limit. Pipes freezing aren't a concern, but a $3,000 heating bill caused by a corrupt Raspberry Pi SD card is.

That's also the reason why boards that depend on SD card rather than for example eMMC are despised in the industrial world. SD cards are highly unreliable, whatever their brand or price: it's just a matter of when they will fail; I've lost count of how many of them I had to pull out and check/recover on my formerly Raspberry PI based media center before I ditched it for a Chromebox with reflashed firmware. Now that's not a big hassle in a home media center, but I wouldn't want for example to travel 300 Km to a mountain and climb a 20 meter pole to service a sensor up there that stopped working because its SBC SD card failed.

I think you meant to say:

> I also kept the old dumb* thermostat connected in parallel and just set it low as a failsafe in case my utility wants to think I should freeze for my life choices