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How to wire stat to trigger mechanical relay???

Motorapido Member Posts: 292
I'll be using a Honeywell T5 wifi thermostat at my vacation rental so I can control the heat while I'm away. Heat comes from an Empire wall-mounted vented gas furnace that uses a millivolt thermopile and mechanical heat-only millivolt thermostat. I bought an Emerson fan control center which is a 24v transformer sitting on and connected to a relay, both of which are attached to a plate cover for a four-inch electrical box. I plan to run the Honeywell thermostat in parallel with the mechanical millivolt thermostat to ensure continuous heat if the electricity goes off. I'll keep the millivolt thermostat set to 50 so the pipes don't freeze and if power is on and wifi is working, I can increase the heat remotely by using the Honeywell app to raise the temperature higher than the millivolt's 50 degree setting. Whether it is the mechanical millivolt thermostat creating continuity to the gas valve or the relay triggered by the Honeywell T5 creating continuity to the gas valve, I have the millivolt stat as my safety for power outages and the T5 as the primary thermostat.

The Honeywell T5 cannot communicate directly with a millivolt gas valve. But by using the fan control center, I have a constant 24v power source as a C wire to the Honeywell stat. I need to figure out which wires to connect from the stat base to the fan control center, so that when the Honeywell calls for heat, it trips the relay in the fan control center, closing the relay's circuit to the millivolt gas valve and starting the heating. See the attached picture of a drawing of the stat base and the connections on the fan control center.

So far I am close to victory. I connected the C terminal on the transformer portion of the fan control center to the C wire input on the stat. I also connected Aux W2 on the stat base to the W1 terminal on the transformer and R on the stat base to R on the transformer terminal. With that setup, when I bump up the temperature on the stat to create a call for heat, I hear no click on the fan center relay and the relay creates no continuity between its three pairs of output wires. But when I change the fan mode setting on the thermostat from AUTOMATIC to ON/Manual or to RECIRCULATE, that triggers the relay to make a loud click, and I get continuity between one pair (the red and black pair) of wires across from each other at the base of the relay.

So I have succeeded in translating a change in the fan mode setting on the stat from Auto to Manual or Recirculate into triggering the relay. But I want the thermostat's call for heat to trigger the relay and not a change in the fan mode on the stat. Where have I messed up in my wiring?


    EBEBRATT-Ed Member Posts: 12,352
    Doesn't your thermostat have a W or W1 terminal?
  • Motorapido
    Motorapido Member Posts: 292
    Yes, it does have a W terminal, directly beneath the E shown in my drawing. I messed up my drawing.
  • Motorapido
    Motorapido Member Posts: 292
    I suppose what I am asking is whether there is a standard convention in stat terminal letter names that would allow me to identify which two terminals on the stat create continuity when the temperature falls beneath the setpoint on the stat. From there, I need to figure out which two posts on the fan control center to attach that pair of wires from the stat to trigger the relay.
  • mattmia2
    mattmia2 Member Posts: 5,988
    R/Red closes to W/Heat on the t-stat on a call for heat. The T-stat expects to see 24vac between R and C so R and C are the 24VAC connections to the t-stat for power and the end of the relay coil that isn't connected to W is connected to C. Some of these connections are probably hard wired in the fan center. The end of the relay that connects to W on the t-stat is probably labeled G on the fan center because G is fan.
  • Motorapido
    Motorapido Member Posts: 292
    Excellent. It's working now. Funny thing, with all the zillions of millivolt fireplace inserts and gas burning stoves and millivolt gas heaters in properties in the middle of nowhere, where power goes out frequently and millivolt keeps the heat on, the wifi thermostat manufacturers are missing a modestly large potential market by now including instructions for doing what I just accomplished. Now I have millivolt reliability when the electricity or wifi go off, and when power is on I have remote control over the heat. Once again, Heating Help came through for me. Many thanks.