Moving from HAI OmniStat to Aprilaire 8800 on Elk M1


New Member

15 years ago we got 9 HAI OmniStat (1st-gen) thermostats, connected to an OmniPro II controller. The OmniPro II has since been replaced by an Elk M1. The thermostats have been slowly dying one by one (failing to turn the heating/cooling on and off correctly) and now that we 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 replace all of the thermostats with something new.

Looking at all the supported thermostat options in the Elk M1XSP documentation, I think it makes the most sense for me to switch to Aprilaire. Here’s why I rejected the other supported options:
  • 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. (There are 7 HVAC units, two of them with Zone Controllers, in four utility closets spread around the building.)
  • 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. The documentation shows the 8870 thermostats and the 8818 distribution panel. However, it also mentions the 8800 thermostats. As far as I can tell, this means it should work just fine with the 8819 panel and the 8800 thermostats. Is this correct?
  2. 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 8819 distribution panel? If not, why not?
  3. I've seen other posts mention using a generic RS232-RS485 converter with the Aprilaire thermostats. Does anyone know of any specific make/model that works?
  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? (The OmniStats were limited to four units per M1XSP, so I'd assume I can have Aprilaire thermostats #1-#8 on one M1XSP and OmniStat #9 on another M1XSP, but confirmation that this can be done would be nice.)
  5. Is there anything else I should know about making this switch?
  • I figured out the answer to question 2: Aprilaire uses full-duplex signaling (four wires for data) more like RS422, but the RS485 port on the M1XSP is only half-duplex (two wires for data, and the other two are power and ground). I also found references to other models of converter that work.
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