The interface commonly uses a line voltage connector or a specialized low-voltage connector, designed to allow “first make/last break” for safety (ground-first). [1]
The reddit thread ended abruptly, so not clear if the solution listed there was real or not. If it was real, there doesn’t appear to be anything written down about it.
I did find some documentation on the API the Wi-Fi module provides:
If you have the thermostat and the Wi-Fi module it should be possible to reverse engineer the protocol it uses to talk to the thermostat.
Since the thermostat is designed to work standalone, I would expect that the Wi-Fi module sends commands to the thermostat to change its behavior. That might be JSON or it might be a more compact binary blob that you would need to understand (or perhaps just copy).
But, I have to ask why?
I remember when these thermostats were popular. I thought about buying one, but never did. The industry has moved on. A lot changes in a decade.
I’d like to have the fleet standardized on the CT80 because it measures humidity AND now relatively inexpensive, but they do not come with Wifi modules.
Reliable DIY wifi modules would be useful twofold due to their scarcity. The existing wifi modules frequently disconnect to the network, despite have an AP within 5 feet line of sight. A reliable DIY wifi module would persistently try to attach to an AP when it is powered on.
It sounds like you have both the thermostat and at least one Wi-Fi module.
You have some data on the interface. This is about the amount of information I had on my previous HPWH. I used my voltmeter to measure the voltages on the various pins to make sure I didn’t destroy my logic analyzer.
Once I confirmed the voltages were within range of my logic analyzer, I hooked it up to see if I could make sense of the communication. It didn’t make sense at first. But, then I realized it was using 9-bit serial. With that, I was able to use a terminal emulator to capture the very chatty protocol. It then took some patience to figure out the interesting bytes.
I didn’t buy the Wi-Fi controller, so it took longer to understand what the data meant. Since you have a controller, you should be able to figure it out quicker. Unless, it is using some kind of encryption (but probably not, since it is so old).
I don’t have the thermostat or the Wi-Fi module. The work I did was for my prior water heater, so not applicable to your thermostat. The process I used might work. It typically requires patience and the ability to solve puzzles and/or see patterns. I suspect AI might be able to help. AI didn’t exist yet, when I did it.
As for #4, I can report that the wifi module doesn’t store anything, it’s just a network interface. (Well, ya, it remembers its MAC . . .)
I’ve also found the easiest way get get a usnap wifi module for the ct80 is to buy a ct30 (or the Filtrete branded version) with one installed and cannibalize it.
The module has a 10 pin connector conforming to the USNAP standard with the
following functions:
Pin Signal
1 SELECT*
2 ATTENTION*
3 SCLK
4 MOSI
5 MISO
6 RESET*
7 +3.3V Power
8 Signal Ground
I plan to connect an esp32 to tap into the communication between the module and the thermostat. That is, as soon as i find the best way to do this.
The other night, I was able to pull raw data from between the two devices.
Now the hard and/or hopefully fun part, making sense the of data(framing and whatnot..)