Looking at my install options for mounting my recently received US HestiaPi I could easily mount the thermostat on or near the furnace and extend the cables to the temp sensor from six inches to eight feet to put just the sensor on the wall upstairs, and put the thermostat in the basement where I don’t care what the case looks like. The sensor would be in a small but ventilated case painted to match the wall for the best appearance package I can come up with. I would use CAT6 for the run, because I have it handy.
My furnace is directly below the thermostat and I would use my PC or phone to control it all cases anyway, so I don’t see a downside to this. The only question is if that line is sensitive to that length or interference in some way. Considering it is right next to the AC in terminals presently it must not be very sensitive to AC.
I had asked the chief about this and he pointed me to https://www.raspberrypi.org/forums/viewtopic.php?t=82049#p580616
and the general idea is, a few meters, but you can get fancy with circuits and what not to make it longer. I am inside a few meters so I expect to be able to get away with it. I will wire it up first before installing it all. As always with twisted pair it pays to get the right stuff together. The general guideline seems to be put SDL and SDA on two different twisted pairs, and run a ground on the twisted mate. Power on their own pair. So in our application I would use 6 of the 8 wires, pair 1 SDA GND, pair2 SDC GND, pair3 Vin Vin. That combination should give me the best range without extra circuits. Any thoughts?
I like to keep all the electronics down in the basement to create the most minimal feel upstairs. All of my computers are in the basement, but the monitors and keyboards are upstairs, with USB and HDMI run to wall plates upstairs. It looks nicer that way.