I've been running homemade HVAC control for 12 years now. A few points:<p>1. Have "can't fail" safeties. In my case, that's the existing thermostat in parallel with the relay (so that the house can't freeze up if my system fails) and a second extra thermostat in series with it (so that if it fails stuck on, things won't overheat too badly). The second safety thermostat can be out of the way, like in the basement.<p>2. Assuming you live in a northern area and have a furnace that's less than a decade old, it may well be a two-stage (i.e. high and low heat mode) one set up for autostaging (run on low for 5 minutes, then fire up full blast) because the installers were too lazy to fish the extra wire to run a proper 2-stage thermostat. My homemade system has a 0.3 degree Celsius difference between the low and high stage cut-in, plus some hysteresis to avoid short cycling, and it keeps the temperature wonderfully stable (within 0.2 degrees of the set point) with the furnace only ever switching to high if you turn up the temperature (on the very most brutally cold winter days it might run almost continuously on low). At least here in Canada, installers have to leave the technical manual with the furnace, so you know which DIP switch or jumper to change to disable autostaging.<p>3. Mechanical thermostats have something called an "anticipator" which slightly warms the sensing element as the furnace runs, so it tends to turn off below the set point. This is to avoid overshoot. Digital thermostats do it in software by pulse width modulating the furnace enable as the temperature nears the set point. In my case it's completely unnecessary to code this up - I guess the thermostat location is just perfect - but if you do get overshoot, just simulate it in software.<p>4. AC units can be damaged by short-cycling and sure enough an early version of my system had a bug that caused this. Luckily I noticed right away. Now it's full of safeties like minimum run and rest times and probably much more conservative than a real thermostat. Longer AC run times are better for dehumidification anyway. I also enforce minimum run times on the furnace.
For anyone looking to tinker with automating their thermostat, I highly recommend looking at the OpenTherm Gateway[1] (OTGW). It uses the OpenTherm protocol which is supported by a lot of heating systems.<p>Though the protocol is backwards compatible with the old type of thermostat which would simply turn on and off for heating, OpenTherm works with modulation as well. Not simply turning the heating on at 100% capacity and turning it off again when the target temperature was reached, but taking a bit more of a gradual approach.<p>The OTGW fits in here beautifully. It sits in the middle between your heating system and your existing (OpenTherm-speaking) thermostat. Listening to all the messages being sent between your heating system and thermostat. And if the onboard firmware fails, it should even fall back to that behavior completely.<p>For me, it allows to get a ton of information from my thermostat and heater, and automate heating and cooling down my home. All in a much better way than a simple relay with an ESP8266/ESP32 could do. It also retains my (non-smart) home thermostat functionality, including the ability to change setpoints on it. Which is much appreciated by my partner.<p>Third party support for the OTGW is relatively good as well. Home Assistant and Domoticz both support the OTGW, and the serial protocol it speaks is quite simple as well. Best of all, its opensource and has no need for any cloud connection you don't want.<p>[1]: <a href="https://otgw.tclcode.com/" rel="nofollow">https://otgw.tclcode.com/</a>
I have to say that HomeKit really does a good job in unifying the Home IoT experience. I used to use Philips Hue exclusively, but the HomeKit app is just so much nicer. And since then I've branched out considerably, and the only unifying factor is HomeKit. It's great.
It would be nice if more hardware vendors got out of their own way and supported HomeKit. It’s not perfect, at all, but it is far ahead of the typical vendor provided software experience.
I’ve been quite frustrated that both of the houses I’ve lived in in the past decade have used “communicating” systems, meaning that the products exploring this space are out of reach. HVAC professionals describe these systems “Ferraris” of thermostat control, but their remote features, API, etc are awful - I’m stuck with what I can only describe as a touch thermostat made by the lowest bidder.
I’ve had a very good experience using one of these to provide HomeKit compatibility to Nest devices: <a href="https://www.starlinghome.io" rel="nofollow">https://www.starlinghome.io</a>
A related question regarding Wi-Fi line voltage thermostats:<p><a href="https://iot.stackexchange.com/questions/2895/wi-fi-line-voltage-thermostat-for-baseboard-heaters" rel="nofollow">https://iot.stackexchange.com/questions/2895/wi-fi-line-volt...</a><p>The question lists a number of commercial thermostats that may be of interest, as well as a nod to WebThings:<p><a href="https://iot.mozilla.org/gateway/" rel="nofollow">https://iot.mozilla.org/gateway/</a>
Complains about not having a common wire for an ecobee, cobbles together a separate box with a USB cable draping down around the door jamb - love it :)<p>As others pointed out, put a plain old bi-metallic thermostat in parallel with these confounded computerized thermostat as a fail safe. Frozen pipes are no joke! I have one in parallel with my Nest and would probably do it with a "normal" thermostat anyway as I've had those fail in weird ways in the past too. Redundancy on critical systems is never a bad thing.
Neat, this dovetails into a recent comment of mine quite nicely..<p><a href="https://news.ycombinator.com/item?id=25465667" rel="nofollow">https://news.ycombinator.com/item?id=25465667</a>
It doesn’t render on my iPad but which esp32 HomeKit library is the author using?<p>I have an esp32 sensor system in my garage I’d like to add HomeKit functionality to.
Honestly, if HomeKit was something that we could use in an operating system that wasn't owned by Apple (~70% of devices on the planet ride Android, to be clear), the title here would feel a little bit more reasonable, but alas, Apple is the truest of true closed ecosystems. Open doesn't look like HomeKit and those boycotting walled gardens should know better than to find themselves trapped inside Apple.