M1 Support Forum Questions about moving from HAI OmniStat to Aprilaire 88xx


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    Moshe Katz


      15 years ago, one of my customers had 9 HAI OmniStat (1st-gen) models. They’ve been dying one by one (failing to turn the heating/cooling on and off correctly) and now that they are down to only 3 working ones (one replaced with an OmniStat2 and the rest with normal non-communicating thermostats) I think it’s time to suggest that they replace all of the thermostats with something new.

      Looking at all the supported thermostat options, I think it makes the most sense for me to switch to Aprilaire. Here’s why I rejected the others:

      • I can’t use RCS because the existing wiring (data wires all the way to the thermostat but nowhere near the HVAC units) leaves no place to install the Control Units and it is not feasible to run new wires for this job.
      • We aren’t replacing all the HVAC units and zone controllers with Carrier Infinity compatible units.
      • Wireless communication is not reliable across the building, so we don’t want to rely on Z-Wave or RadioRA2.


      1. I see that the documentation for the M1XSP says that the Aprilaire 8811 RS232-to-RS485 converter is required. I’m curious why this is necessary, since the M1XSP has an RS485 connector on it. Is is possible to use the RS485 connector directly with the Aprilaire 8818 distribution panel? If not, why not?
      2. Alternatively, is there anything special about the Aprilaire 8811 vs any generic RS485-RS232 converter?
      3. Is there any practical difference between the 8870 thermostats (with the 8818 panel) vs. the 8800 thermostats (with the 8819 panel)?
      4. Would it be reasonable for me to replace thermostats 1-8 with Aprilaire on one M1XSP and keep the one newer OmniStat2 as thermostat #9 on another M1XSP?
      5. Is there anything else I should know about making this switch?


      Thank you

      Don Kolva

        I’ve got 14 of the Aprilaire 8800 thermostats going through the 8819 panel then through the 8811 converter.  I don’t know if the 8811 is required, but I assume it’s doing some RS485 “translations”.

        I updated from the 8870/8818 panel to the 8800/8819 panel prior to the 8800 going out of production.  The 8870’s don’t have the touch screen and look older and  they seem to be easier to find now what both the 8870 and 8800 are out of production.  The 8818 panel I though was better with the LEDs, and I didn’t like that the panels are cross compatible.

        I think that you should be able to run 2 different M1XSP, but that would be a question for Elk.


        Moshe Katz

          I don’t know if the 8811 is required, but I assume it’s doing some RS485 “translations”.

          I did some more research, and I found out why it’s needed. The RS485 terminals on the M1XSP are 2-wire half-duplex, but the Aprilaire system uses 4-wire full-duplex communication.

          A number of people on other forums say that any generic RS485/RS422 to RS232 converter will work, not just the Aprilaire 8811. I ordered a cheap generic one, and I’m going to test it out.

          Moshe Katz

            I finally got everything together to do this project, so I’m starting to report back in case anyone else can use this information:

            Yes, any generic RS485/RS422 to RS232 converter will work, not just the Aprilaire 8811.

            I used this one: https://www.amazon.com/dp/B0196AO1IG/


            • A+ (org) to T/R+
            • A- (org/wht) to T/R-
            • B+ (blu) to RXD+
            • B- (blu/wht) to RXD-
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