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Honeywell RTH6580 WiFi stat with millivolt heater

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Burnthillian
Burnthillian Member Posts: 4
in THE MAIN WALL

I'd like to operate a natural gas heater (currently controlled by a battery powered Honeywell thermostat) with a Honeywell RTH6580WF1003 WiFi thermostat. I have a 24 volt transformer to supply power to the thermostat, but may need a relay or control device to reliably operate the millivolt gas valve? I know it's basically an off-label use of the thermostat, but we have the same model thermostat serving the main furnace and I'd like to use the same app to control both. Would also like to have WiFi control of the heater while away from the house. Any suggestions for how to wire properly would be greatly appreciated.

Comments

  • Jamie Hall
    Jamie Hall Member Posts: 27,112

    I'd be very much inclined to use a relay — control the coil with the new thermostat, and use the normally open dry contacts to operate the millivolt heater. Most smaller relays have low enough resistance in the contacts that they should work, but you could also use a reed relay.

    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    Burnthillian
  • pecmsg
    pecmsg Member Posts: 6,976

    That Millivolt heater will operate without electric. By installing a transformer and Wi-Fi now it won't.

    EdTheHeaterMan
  • Burnthillian
    Burnthillian Member Posts: 4

    Thanks Jamie Hall. Would you be able to provide some guidance on wiring for the input of the transformer and output of the thermostat? I can get the thermostat to power up by wiring the transformer to C and R. What output goes to the relay? Also, if you have any suggestions on a relay, that's appreciated. I have done some basic thermostat installations in the past, but not with this level of detail. Thanks!

  • 109A_5
    109A_5 Member Posts: 3,765
    https://forum.heatinghelp.com/discussion/comment/1887235#Comment_1887235

    With careful wiring and the use of the old thermostat wired in parallel with the new one you could have the best of both worlds.

    Wow,

    image.png
    National - U.S. Gas Boiler 45+ Years Old
    Steam 300 SQ. FT. - EDR 347
    One Pipe System
    EdTheHeaterMan
  • pecmsg
    pecmsg Member Posts: 6,976
    https://forum.heatinghelp.com/discussion/comment/1887252#Comment_1887252

    Loose the power and you loose the T-Stat / Relay. Unless there's a second control no heat.

  • 109A_5
    109A_5 Member Posts: 3,765

    @pecmsg The old thermostat apparently worked. leave it in play. Parallel the new thermostat system with the old thermostat. Best of both worlds. Turn the old one down to 55 unless needed in a power failure situation.

    National - U.S. Gas Boiler 45+ Years Old
    Steam 300 SQ. FT. - EDR 347
    One Pipe System
  • Burnthillian
    Burnthillian Member Posts: 4

    @109A_5 Any suggestions on the wiring layout to leave the old t-stat in parallel? I like the idea of having it as a backup, though we do have a generator that can be used for an extended outage. And any thoughts on a type of relay to install between the wifi t-stat and the heater? I'm a newbie at control wiring so as explicit as possible would be helpful for me! Thanks.

  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 11,840

    I got this: wiring diagram to follow

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 11,840
    edited 7:43PM

    sorry for the delay. I needed to be sure the NO relay contacts would be compatable with a millivolt gas valve circuit.

    Screenshot 2026-02-10 at 1.51.30 PM.png

    Since the general purpose relays that we normally use in the HVAC trade are not rated for Millivolts I did some research to find that the normal RIB1UC contacts are not rated for 250 to 750 millivolts so that rules out that diagram I already have on file. Those contacts may work most of the time may not be reliable ALL of the time.

    I did find that the White Rogers 90-340 relay contacts are more reliable for this purpose. Although they are also not specifically rated for millivolt operation, they have proven over time that the relay works for this purpose and therefore I have the following parts list for you.  

    You may find that the Fan center already had the proper White Rodgers 90-340 relay included, So I would hold off on the relay if you are on a budget, but that relay is not that expensive so I included it in the list.

    EDIT:

    The wiring diagram shows that millivolt thermostat connected parallel to the relay contacts. This is to allow for power outage operation. I would use an inexpensive millivolt thermostat for this backup thermostat and forget about the battery operated fancy thermostat. It is just a backup and you want to set it to 60° so it never actually calls for heat unless there is a power problem.

    More Research:

    The White Rodgers 90-112 Fan center does in fact come with the White Rodgers 90-340 relay and was often used on millivolt equipped boilers to connect the thermostat in order to bring on the gas valve and the circulator pump on several different boiler manufacturers when standing pilot boilers were still available.  So you do not need the additional White Rodgers 90-340 relay

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • Burnthillian
    Burnthillian Member Posts: 4

    @EdTheHeaterMan

    Thank you so much for the detailed response! I really appreciate it. I'll get the WR 90-112 fan center ordered.

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