Hello! Our old heat-only thermostat had exactly two wires running to it from the boiler room: red and white (connected to the R and W terminals respectively). It was powered by batteries, which needed replacing every two years or so. The zone consists of two rooms with different temperatures in them, so we wanted to replace it with a thermostat capable of taking a reading from a remote sensor.
I picked the Honeywell TH8110R1008 Vision Pro 8000. These radio-using devices require wired-power -- 24VAC -- to talk to sensors so I decided to reuse the existing two wires to supply that, with the signaling done wirelessly. I kept the red wire where it was, but switched the white wire from the W to C-terminal on the thermostat and to the "common" terminal in the boiler room (inside a TACO 406 ZVC). My new thermostat booted and I was able to pair it with an in-house and an outside sensors with ease.
However, it, obviously, could no longer actually turn on the heat. Honeywell's Equipment Interface Module (EIM) THM5421R1021 IAQ was supposed to help there -- hooked up to the TACO 406 ZVC in the boiler room instead of the thermostat, and talking to the thermostat via Honeywell's proprietary RedLink-protocol.
And that's where I am stuck... Though connecting the wireless sensors was easy, doing the same with the EIM is not working. When the thermostat tells me to "press Connect on the wireless device", I do press the sole button on the EIM, but nothing happens. The EIM's little status light flashes amber, then goes red, and then turns off. Repeatedly... Thinking, I may have a defective unit, I exchanged it -- but now the replacement is doing the same thing...
Once, when I pressed Connect on the EIM first, the thermostat would not go into "Add a Device" mode complaining of another wireless devices listening -- perhaps, the EIMs are meant to be connected differently from sensors for some reason?