My wife and I moved into a new home in October 2020. As soon as it started getting cold, we realized some shortcomings of the home's older heating system (including one heating zone that was *always* on). We had Nest thermostats in our previous home, and the current setup was not nearly as convenient. There are multiple thermostats in our house, and some had programmed heating schedules, others had different schedules, some had none at all.
I wanted "smart" thermostat control for all of the heat zones (schedules, automations, home/away, etc.). I had several options if I wanted to buy something off the shelf, but all of them have drawbacks:
* It's expensive: No smart thermostat can handle multiple zones, so I would need one for each zone (~$200*6 = $1,200).
* It's difficult: I would have to rerun the thermostat wire to get the infamous [C wire][6], which enables continuous power to the thermostat. The wires are 20 to 100 feet each, in-wall, and might be stapled to the studs.
I knew that I wanted to use a Raspberry Pi. Since they've gotten so inexpensive, I decided to use a Raspberry Pi 4 Model B 2GB. I'm sure I could get by with a Raspberry Pi Zero W, but that will be for a future revision.
I began drawing out the hardware diagram on [draw.io][11] and realized I lacked some crucial knowledge about the furnace. I opened the side panel and found the step-down transformer that takes the 120V electrical line and makes it 24V for the heating system. If your heating system is anything like mine, you'll see a lot of jumper wires between the Taco zone valves. Terminal 3 on the Taco is jumped across all of my zone valves. This is because it doesn't matter how many valves are on/open—it just controls the circulator pump. If any combination of one to five valves is open, it should be on; if no valves are open, it should be off… simple!
At its core, a thermostat is just a type of switch. Once the thermistor (temp sensor) inside the thermostat detects a lower temperature, the switch closes and completes the 24V circuit. Instead of having a thermostat in every room, this project keeps all of them right next to the furnace so that all six-zone valves can be controlled by a relay module using six of the eight relays. The Raspberry Pi acts as the brains of the thermostat and controls each relay independently.
The next problem was how to get temperature readings from each room. I could have a wireless temperature sensor in each room running on an Arduino or Raspberry Pi, but that can get expensive and complicated. Instead, I wanted to reuse the existing thermostat wire in the walls but purely fortemperature sensors.
* Most importantly, the DS18B20 can use "[parasitic power][15]" mode where it needs just two wires for power and data. Just a heads up… almost all of the DS18B20s out there are [counterfeit][16]. I purchased a few (hoping they were genuine), but they wouldn't work when I tried to use parasitic power. I then bought real ones from [Mouser.com][17], and they worked like a charm!
Starting with a breadboard and all the components locally, I started writing code to interact with all of it. Once I proved out the concept, I added the existing in-wall thermostat wire into the mix. I got consistent readings with that setup, so I set out to make them a bit more polished. With help frommy [dad][19], the self-proclaimed "just good enough" solderer, we soldered leads to the three-pin screw terminals (to avoid overheating the sensor) and then attached the sensor into the terminals. Now the sensors can be attached with wire nuts to the existing in-wall wiring.
I'm still in the process of "prettifying" my temperature sensor wall mounts, but I've gone through a few 3D printing revisions, and I think I'm almost there.
As usual, writing the logic wasn't the hard part. However, deciding on the application architecture and framework was a confusing, multi-day process. I started out evaluating open source projects like [PiHome][22], but it relied on specific hardware *and* was written in PHP. I'm a Python fan and decided to start from scratch and write my own REST API.
Since HomeKit integration was so important, I figured I would eventually write a [HomeBridge][23] plugin to integrate it. I didn't realize that there was an entire Python HomeKit framework called [HAP-Python][24] that implements the accessory protocol. It helped me get a proof of concept running and controlled through my iPhone's Home app within 30 minutes.
The rest of the "temp" logic is relatively straightforward, but I do want to highlight a piece that I initially missed. My code was running for a few days, and I was working on the hardware, when I noticed that my relays were turning on and off every few seconds. This "short-cycling" isn't necessarily harmful, but it certainly isn't efficient. To avoid that, I added some thresholding to make sure the heat toggles only when it's +/- 0.5C°.
With the software and general hardware design in place, I started figuring out how to package all of the components in a more permanent and polished form. One of my main concerns for a permanent installation was to use a breadboard with DuPont jumperwires. I ordered some [solderable breadboards][31] and a [screw terminal breakout board][32] (thanks [@arduima][33] for the Raspberry Pi GPIO pins).
Now I just need to organize and label the wires, and then I can start swapping the remainder of the thermostats over to ThermOS. And I'll be on to my next project: ThermOS for my central air conditioning.
