I just acquired a Honeywell VisionPro TH9421C1004 thermostat but I didn't get the Equipment Interface Module (EIM) with it. Can I still wire this directly from my furnace? It only has 3 wire connections: Data, R, and C.

1 Answer 1


No, you can't

Unfortunately, the Honeywell thermostats that use an EIM rely on it to translate either RedLink (for wireless versions) or a proprietary protocol (for the wired ones) to the 24VAC signals that a furnace understands. So, since Honeywell discontinued the wired versions in favor of their RedLink counterparts, you'll need to hunt down a secondhand EIM for your thermostat.

  • since I only need it for a heat call, I can't just hook up the C & R?
    – MilkyTech
    Commented Aug 23, 2020 at 18:39
  • @ChrisL -- you can't just hook up the C and R, the data line does complicated things instead of being a simple 24VAC on/off signal Commented Aug 23, 2020 at 19:23

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