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Thread: Honeywell Lyric millivolt adaptability

  1. #1
    Junior Member

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    Honeywell Lyric millivolt adaptability

    Bought new fireplace, (Majestic Mercury MERC32IL) and new Honeywell Lyric Thermostat to control same. Both devices work but will not communicate with each other as the fireplace only has millivolts, (DC) on the R and W wires and the Lyric is looking for between 20 to 30 VAC to be able to control the fireplace. So after reading that the Nest system has the same issues with working with millivolt systems it appears some people have been able to make the Nest's work by either jumpering R and RH or RC (Lyric equivalent of Nest RH??) or by using a SPST relay to ensure that C and W never touch. Currently I have the R and W wires from the fireplace going to R and W on the Lyric backplane and the two leads from the external XFRMR going to C and RC. So has anyone successfully managed to get a Lyric thermostat successfully communicate and control a millivolt system and if so what have you done to facilitate this. any pointers or documentation is greatly appreciated.


  2. #2
    HVAC Thermostat Specialist

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    You will need to use a thermostat with a MECHANICAL relay to control the fireplace. Some thermotats have them internal, some you need to wire external. The thermostats that click when the heat comes on have mechanical relays. RC - C for power and RH to W is correct for the thermostat side.

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