I have a reasonably complex set of behavior set up. I live in Bend, Oregon at 4000 feet. This is somewhere with hot sunny days, cold nights, and sometimes a lot of smoke. The effect on the interior of the house based on upcoming weather is highly predictable.
My rules look something like this:
If tomorrow's high temperature is above 85, then as today as soon as outdoor temperature drops below indoor temperature, open all the windows and turn on the attic fan until either interior temp is below 62 or outdoor temp rises above indoor, whichever comes first. Unless the AQI outside goes over 100 for more than 2 minutes, in which case close all the windows and turn on the fan/AC to clean the air. All the while, if the house rises above temperature X, cool it to temperature Y.
Additionally, the upstairs heatpump is located in an uninsulated attic, which in winter means that heating is somewhat inefficient and we want to run it a minimal count of times to avoid warmup periods. So we program in our own hysteresis, so that there's a 6-degree difference between when the heat kicks on, and when it turns off.
No one is going to make a dream thermostat that meets your needs out of the box for you, because no one else has your local needs. The best you can do is give yourself the tools to make your reality how you want it.
If you have heat pumps or a multiple zone system, or a few other things they can get more complex, but they still aren't super complicated.
I have run across a few open source thermostats, but either they are some hobbyist's one-off project, or they are geared toward very different systems from the common North American forced-air furnace and whole-house AC unit combo.
Before the but-what-about-ers start brigading in, yes it is possible to make this system safe against engineering or software defects. They make analog non-adjustable temperature sensors that "trip" below a certain temperature, you wire one in parallel with your heat wires near the furnace and you house will never go below freezing. (Very common in rentals.) If you're concerned about over-heating, add another temp sensor and relay and put it in series. Put time-out timers on every function. Have alerts sent to your phone. Add a SIM card and modem if you don't trust your wifi. And so on, tastefully adjusted to your own personal level of paranoia.
https://claude.ai/public/artifacts/4d0ddccc-e7fa-4589-94ca-f...
Here’s why I think the hardware is so great:
- Simple, high-contrast temperature display (7-segment led)
- Just a few buttons to override temp/mode manually
- Reliable sensor (it’s made by Honeywell)
- Nice industrial design (doesn’t get in the way)
- Low price point (got it for $49)
I really wish someone would find a way to unlock their firmware. I tried to mess with it and figured it did MQTT (yay!) but it does certificate pinning :-/ I even opened one up but couldn’t find an easy way to dump the firmware off of its i.MX chip.
The one I have in the apartment I’m renting only gives Cool vs Heat. With Texas being as finicky as it is this time of year, the temperature swings from frigidly cold to swelteringly hot day to day, and with my ADHD and lack of connection to my body, I find myself wondering why I’m feeling run down and mentally foggy, and then I realize it’s because I had to switch to Heat yesterday (because the house dropped down to 60F) and today the apartment is now at 85F, and I’ve been progressively overheating for the past 5 hours or so.
A thermostat is dead simple, 24VAC and 4 wires: heat, cool, fan, power. You’d need to experiment and figure out where the point will be best to switch from heating to cooling based on the outdoor air temp, the controller would control this.
There might be commercial products available that do this already, but if they don’t have an outdoor air temp sensor, it won’t work.
FWIW I bid and run commercial building automation installations.
This probably exists for the really basic 4 wire versions, but I have a more complicated 8-wire Lennox thermostat and I haven't been able to find something like this.
I have a relatively simple Honeywell Z-Wave thermostat, and it works great with Home Assistant.
https://support.ecobee.com/s/articles/What-do-my-thermostat-...
This was at latest mid-90s, so not exactly "high tech". I don't think it's uncommon, as both random rented student house and the first place I rented while working had pretty much the same thing (though I think the student house only had 2 timers and no "day" functionality?). They didn't have air conditioning though, as that's not common residentially. And unless the allowed variance is also really wide (probably too wide for comfort in some situations), I don't really want the "same" temperature to be used for heat and cooling.
I'm surprised how many people I know IRL in the states that think this sort of thing is some fundamental tech limitation - like you need the latest fancy "smart" thermostat when 99% of what you will actually use it for is just "not the cheapest-possible" dumb one.
It constantly tried to infer schedules and change the temperature on its own. I would set the temperature, come back an hour later to find that it changed itself back to what it thought it should be.
Also, there was no way to just activate the fan. I live in a very temperate climate and I generally like to keep a few windows open but run the fan to circulate air through the house.
I sold the nest and now a $15 dumb thermostat from the local hardware store now lets me set a temperature and it won't randomly change it when it feels like it. And it has a switch to turn on the fan.
While temperature is important, if you have a well-seal and well-insulated house (i.e., up to modern code), your AC won't be running that much, and so the incidental dehumidification you get with it won't happen, which will lead to an RH that creeps up.
A stand-alone whole house dehumidifier is thus often needed to deal with RH.
The last apartment I lived in installed new "smart" thermostats throughout the building. It always seemed to change the target temperature at random and I got so frustrated that I swapped it out with a dumb Honeywell one (and swapped it back when I moved out). I never found having to make the occasional manual adjustment to be an issue.
https://www.amazon.com/Honeywell-Intrusion-TH6320ZW2003/dp/B...
The killer feature of Home Assistant vs. my previous Nest setup came when I added door and window sensors, and programmed HA to pause heating/AC whenever doors or windows are open. I no longer turn into my father, who I remember shouting at us kids, "close the door, I'm not paying to air condition the outside!"
One oddity with my ecobee - if it's not connected to wifi, the clock runs fast. Sometimes gaining a whole minute every few days.
Apparently that’s touch to ask in 2025.
I've changed and rebuilt it a few times since then. The version in that blog post expects the microcontroller to have external USB power (I had an outlet with a USB port right next to the thermostat mount, so I didn't care), but you can trivially wire in any voltage converter capable of 24VAC->5VDC/3.3VDC to remove that requirement. I plugged an automotive voltage converter module I had lying around into the first design at some point, and then the next design had a proper rectifier and buck converter to accomplish the same thing.
[1] https://blog.benhaney.com/2019/03/26/building-a-thermostat
The difficult part is opening up all of the plumbing fixtures and taps automatically, I haven’t tried draining water out of a home’s plumbing system without opening all of the taps and am unsure if it works.
If you have a well, you’ll need to drain the expansion tank and turn off the water pump, the latter can be achieved with a relay that breaks power to the pump but I’m unsure of how to handle the former.
You’ll also need to automate filling the traps with antifreeze, unless you’re cool with replacing sink traps and toilets after a freeze event.
Draining the pipes isn't that simple, and even if you did, you still have things like a water heater, toilets, softeners, filters, and expansion tanks. You also wouldn't get the water sitting in drain traps; winterizing a house or RV involves pouring a little RV antifreeze down each drain.
Perhaps next time I’ll make something like that, using home assistant you can easily turn any switch with any temp sensor into a thermostat.
> OpenTherm (OT) is a standard communications protocol used in central heating systems for the communication between central heating appliances and a thermostatic controllers.[1] As a standard, OpenTherm is independent of any single manufacturer. A controller from one manufacturer can in principle be used to control a boiler from another. However, OpenTherm controllers and boilers do not always work properly together. The OpenTherm standard comprises a number of optional features and some devices may include manufacturer-specific features. The presence or absence of such features may impair compatibility with other OpenTherm devices.
* https://en.wikipedia.org/wiki/OpenTherm
* 2.2 spec PDF: http://files.domoticaforum.eu/uploads/Manuals/Opentherm/Open...
Industrial Design: [Drops a brick with a photo of Dieter Rams' original Braun radio taped to it onto your head from 3 storeys up]
Edit: non-joke feedback- this is great from a techie perspective but it's too much actual hardware complexity and visual presence for a task that will be done maybe twice per season. You have added about 50% to the cost of goods sold to do this - is it worth it? Will people buy it? Yes I understand this isn't a totally serious device proposal, but I would love people to try to understand why things end up looking like they do!
- I want a button to make it a little colder for a period of 30-45 minutes
- I want a toggle to alternate for days where I WFH
- I want a 3rd time range: noon to 2PM
https://www.honeywellhome.com/us/en/products/air/thermostats...
I feel that while Nest changed the whole scene, it never lived up to its “smart” thermostat promise.
And the schedule editor is way too cumbersome. Something like the above is what most people would use.
For a given day it’s easy.
