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Possible to wire furnace fan to thermostat controlling a boiler?

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cm6230
cm6230 Member Posts: 5
in Thermostats and Controls

I’m new to the trade and still learning control wiring. I have a navien combi on an ecobee thermostat, is it possible to wire a furnace fan to that thermostat? I want to use just the furnace fan to cycle air throughout the house. I just swapped from oil furnace to propane boiler. Yes, my place has both baseboard and ductwork. No ac installed.

Comments

  • HomerJSmith
    HomerJSmith Member Posts: 2,652
    edited July 14

    All things are possible. Do you want the fan to operate in union with the boiler or separately? That is the key question before a solution is possible. Other questions abide in my mind, what make of furnace, is the only feature of the furnace that you want operating is the fan, is the ecobee thermostat only connected to the boiler, presently, etc?

    Ed, is the resident expert on appliance wiring on this site. His insight is valued.

  • cm6230
    cm6230 Member Posts: 5

    I just want the fan on the furnace to run independently through my ecobee. Oil Furnace is a boyertown regal 750. Only the boiler is connected to the ecobee.
    would like to use the fan for the time being until I put a small heat pump in the furnaces place.

  • ratio
    ratio Member Posts: 4,061

    Use a relay attached to the C & G terminals on the thermostat to make R to G on the furnace to turn on the fan

  • mattmia2
    mattmia2 Member Posts: 13,446
    edited July 15

    Or Rc and G on the t-stat to R and G on the furnace if the t-stat has separate Rh and Rc

    cm6230
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 18,216
    edited July 15

    Agree with @ratio get a RIB relay from Supply House .com relay # is RIBU1C.

    once you get the relay come back for wiring help if needed.

  • HomerJSmith
    HomerJSmith Member Posts: 2,652
    edited July 15

    Got it! A cheap fan control, off hand, I think, would do it. There may be a direct wire method that others know. The ecobee gets its power from the transformer on the fan control which plugs into 110V outlet and the ecobee turns on the 2 pole relay (2 pole N/O). One pole turns on the boiler and the other pole turns on the furnace fan. A single pole would probably work, but I like separating the two appliances, for safety reasons. It has to do with a possibility that the two appliances may be connected to opposite buss bars on the circuit breaker panel which then, may pass 220 Volts. This would be a situation where the boiler and fan would operate together.

    image.png
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 10,755
    https://forum.heatinghelp.com/discussion/201735/possible-to-wire-furnace-fan-to-thermostat-controlling-a-boiler

    Easy to do. I believe you want to use the Ecobee that is connected to a boiler. R and W on the Ecobee is already connected to the boiler Central Heat (CH) terminals. With the boiler diagram I can offer you the best wiring diagram for your needs. This will integrate the thermostat's R to W for the boiler and the thermostat's R to G for fan on the furnace. Give me a minute to look up your diagrams.

    Edward Young Retired

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

  • mattmia2
    mattmia2 Member Posts: 13,446

    does Rc close to G or does Rh close to G?

  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 10,755
    edited July 15

    After reviewing the Regal furnace (i don't know your model number but this is the standard oil furnace control) and the Navien combi (i don't know your model number but this is the standard typical Navien control) and the Ecobee thermostat (i don't know your model number but this is the standard wiring for many of the Ecobee thermostats.). this is what I have come up with.

    Screenshot 2025-07-14 at 8.48.04 PM.png

    There is no need to purchase or add a relay. The Boyertown will need to be powered the entire time you want the thermostat to operate, because the Boyertown Regal is the 24 VAC source for the Ecobee.

    If these wiring connections do not look familiar, then I will need the model number of the unfamiliar item, or a picture of the control board on the equipment or thermostat.

    Edward Young Retired

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

    109A_5Alan (California Radiant) Forbescm6230HomerJSmith
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 10,755
    https://forum.heatinghelp.com/discussion/comment/1860216#Comment_1860216

    I believe that the Ecobee needs to have the Rc to C power for the thermostat to operate. I also don't believe that @cm6230 would know that answer. I hope this info helps and the above post helps more.

    Edward Young Retired

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

  • mattmia2
    mattmia2 Member Posts: 13,446
    https://forum.heatinghelp.com/discussion/comment/1860224#Comment_1860224

    What you showed assumes Rc closes to G. Obviously Red Cool goes to Y but I would assume it makes the fan part of cooling as well.

  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 10,755
    edited July 15
    https://forum.heatinghelp.com/discussion/comment/1860227#Comment_1860227

    I believe that @cm6230 mentioned that there is no air conditioning presently. I also used the Ecobee wiring diagram that shows the Rc and G are used for the cooling side and Rh and W are used for a heating only boiler when the Ecobee is connected to both systems.

    Screenshot 2025-07-14 at 8.54.24 PM.png

    Diagram from Ecobee information available online.

    Once @cm6230 look at the diagram, if there are unfamiliar illustrations/diagrams, they can post the appropriate model numbers so I can fine tune the info provided.  I really did not want to include this diagram as not to make it confusing for a DIY thermostat install. The first illustration with the 3 components should be all that @cm6230 needs, unless they have different model numbers on the boiler, furnace and thermostat.

    Thanks for keeping me on my toes @mattmia2, I often look over a post and see an error that needs correcting. I try to do that before you see it, that is how I keep my stellar wiring diagram reputation. LOL

    Edward Young Retired

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

  • cm6230
    cm6230 Member Posts: 5

    thanks everyone for the fast replies!!!! Really appreciate it.

    I’m coming from a plumbing background and getting more into hvac/controls.

    Ed, the ecobee does need the rc and c wire to power thermostat. If I connect just the rh and c wire the tstat won’t turn on. Right now I have the boiler (navien ncb 190-80H)

    R from boiler to rc on t stat, w wire from boiler to w1 on tstat, c wire from boiler to c on t stat.
    from furnace I have r to rh on tstat. And g from furnace to g on tstat.

    I’m thinking your diagram is right. I’ll try r from furnace to rc on tstat and take the c wire from furnace instead of from boiler. G from furnace to tstat. R from boiler to rh on tstat and w from boiler to tstat.
    also the thermostat will ask what appliance I’m using for heat, either furnace or boiler. I believe either way I can still use the fan.

    I don’t have time to mess around with the wiring today, but will let you guys know how I do on Wednesday.

    Thanks guys!

  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 10,755
    edited July 17

    According to what I have read about most smart thermostats and specifically the Ecobee information online, Rc and C must be connected to the same transformer to power the thermostat. If you connect Rh and C the thermostat will not get power. If you connect Rc from on the Ecobee to the transformer of the duct system control transformer and connect C from the boiler control transformer, you will not get a completed circuit.

    Screenshot 2025-07-14 at 8.54.24 PM.png

    See how this R and C connections are going to two different transformers? That not going to power anything. The source 24 v. transformer is different from the Return Path transformer. No completed circuit here

    The smart thermostat's (CPU) central processing unit (computer) gets power from Rc and C. So this next diagram will also not operate the thermostat.

    Screenshot 2025-07-15 at 10.49.55 AM.png

    The reason this wiring configuration will not work is the Rc does not have power to operate the thermostat's CPU.

    The correct heating only wiring for most smart thermostats is this diagram

    Screenshot 2025-07-14 at 8.54.24 PM.png

    With this wiring configuration there is power to the smart thermostat's CPU. Now the brains of the thermostat can figure out that the thermostat is connected to a heating system that needs Rc to W in order to make the boiler or heating only furnace to come on.

    The only time you need Rh in most smart thermostats is when you have an air handler system that might be cooling only and a separate heating system like a boiler for hot water or steam heat. Then you need to isolate the two different transformers. That is the function of the Rh terminal on any thermostat. Make sure the Air Conditioner's 24v. control transformer does not connect to the heater's 24v. control transformer. This diagram illustrates that point.

    Screenshot 2025-07-14 at 8.54.24 PM.png

    The Blue and White areas represent the air handler or furnace side of the thermostat. Once you connect a wire to the Rh terminal on a smart thermostat the thermostat's CPU recognizes that and creates a kind of a firewall to separates the heating only section in the red area from the rest of the system in the blue area within the thermostat.

    Without the Rh wire connected then the thermostat's CPU does not form the firewall and the Rc will be able to operate the heat burner thru the W (and W2 if needed) like you would find on a regular Gas furnace.

    Screenshot 2025-07-14 at 8.54.24 PM.png

    Edward Young Retired

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

    cm6230
  • cm6230
    cm6230 Member Posts: 5

    great explanation Ed, thank you for all the help! I will let you know how I make out when I dive into this tomorrow.

    Really appreciate it!

  • cm6230
    cm6230 Member Posts: 5

    Ed, had a few minutes and re wired everything.
    r from furnace to rc tstat

    G from furnace to g tstat

    C from furnace to c tstat

    R from boiler to rh tstat

    W from boiler to w tstat

    I can now turn on the fan and control boiler separately through my ecobee.

    Thanks again for your help!!

    Chris

    mattmia2ratioEdTheHeaterMan

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