Thermostat Wiring question

Discussion in 'HVAC Heating & Cooling' started by nola mike, Feb 14, 2014.

  1. nola mike

    nola mike Member

    Messages:
    31
    Location:
    Richmond, VA
    So I have a standard a/c system wired to one thermostat (lenox 51m34) and my 2 wire gas furnace wired to another. I'm trying to consolidate them onto one thermostat. Ran new wire to replace the knob and tube to the furnace, but now can't seem to get the furnace working correctly with the lenox stat. Using the old stat, it seems that I get 24v from the red wire from the furnace, which is just jumped to the white wire when there's a call for heat. For some reason it doesn't work when I connect the R from the a/c to the stat (leaving the R from the furnace disconnected). It doesn't work at all using the lenox stat (I haven't tried changing any settings on that though, so it may be set for a heat pump or something that behaves differently).

    Anyways...

    The plan is to use this stat. Not sure how the wiring should be though. Any thoughts? I don't see why I can't just use the R wire from the a/c unit and the W wire from the furnace, but I guess I'm missing something on how this thing works. Only weird thing is that seems that I get 28v from the a/c (between C and R wires)...Thought that maybe using both Rh and Rc terminals might be an answer, but the motison only has an R terminal.
  2. cacher_chick

    cacher_chick Test, Don't Guess!

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    With the Lennox stat, both the Rh (heat) and Rc (cool) terminals can be used since you have 24v coming from each unit's seperate transformer. With the new stat you will only use the 24v feed from one or the other.
  3. nola mike

    nola mike Member

    Messages:
    31
    Location:
    Richmond, VA
    So I should be able to run the R 24v feed from the a/c unit to the stat and power both units, and just tape off the feed from the boiler, right? I don't have the new stat yet (I'm taking it from another house), but the generic programmable I've been using for the heat isn't working using the feed from the a/c unit...
  4. cacher_chick

    cacher_chick Test, Don't Guess!

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    Location:
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    If the thermostat is powered by 24v, it will require both the hot and common wires from one of the transformers be connected.
  5. nola mike

    nola mike Member

    Messages:
    31
    Location:
    Richmond, VA
    No, the stat that I'm testing with is battery powered. The boiler is 2 wire without a common, only red and white wires.
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,313
    Location:
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    Think of the thermostat as a simple switch...it connects those two wires for each system together when it needs to, to turn on the respective device. Since they each have their own power supply (transformer), and the power needs to return back there after activating the device, you'd put one wire from the boiler on the Rh, and the other on the heat lead (usually white), then the pair from the a/c would go to the Rc and the cooling lead (usually yellow). If there is a jumper between the two Rx leads, remove it.
  7. nola mike

    nola mike Member

    Messages:
    31
    Location:
    Richmond, VA
    I'm hearing 2 different things here I think. I don't understand what actually happens at the boiler. I assumed that when stat calls for heat, the 24v red wire was connected to the W. Further assumed that 24v now went to a relay in the boiler, which then switched 120v to the pump and ignition. If that's the case, and the W wire is providing a path to ground, what does it matter where the 24v hot is coming from?
    @jadnashua: You're saying that I need separate Rh and Rc terminals on the stat, and that the R and W must make a circuit on the same system? Why? So the stat with only an R terminal won't work?
    @cacher_chick: You're saying that I can use the 24v R feed from either system to power the other? If that's the case, what's the point of separate Rh and Rc terminals?
  8. nola mike

    nola mike Member

    Messages:
    31
    Location:
    Richmond, VA
    And why am I getting 28v from my a/c system? Would that make a difference?
  9. cacher_chick

    cacher_chick Test, Don't Guess!

    Messages:
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    Location:
    Land of Cheese
    I admittedly don't know the first thing about a boiler, but I know that the on/off control of either device will require the circuit to be complete. As Jim stated, the thermostat is only an on/off switch, allowing or preventing 24v from the transformer to get to the control unit in the heating or cooling system.

    Sometimes a picture is worth a thousand words, so you might benefit greatly by Google searching "thermostat wiring" and looking at the pictures.
    Last edited: Feb 15, 2014
  10. ImOld

    ImOld New Member

    Messages:
    73
    Location:
    In the rumble seat
    I'm not going to get into the wiring points of the thermostat since I really don't want to look it up.

    Seems like a standard install, separate wires from A/C and furnace.

    You have two transformers, that are not in common, outputting to separate circuits and you cannot mix and match.

    Voltages are listed as nominal, so 24 or 28 is fine.

    The thermostat battery is actually a backup for the electronics/memory in case the power goes off, which normally powers it.

    The previous posters are correct that all a thermostat is, is a type of switching arrangement.

    May I suggest an inquiry to the thermostat manufacturer help line.
  11. DonL

    DonL Jack of all trades Master of one

    Messages:
    4,508
    Location:
    Houston, TX
    Jim explained it well.

    As long as the two 24 VAC sources don't use a common ground then you can hook them together, because the 24V transformers provide Isolation.

    That T-Stat does require external power. Looks like you can provide a separate Voltage for the Wi-Fi power.


    Have Fun.
    Last edited: Feb 15, 2014
  12. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,313
    Location:
    New England
    Look at it this way...ground isn't necessarily ground, especially on an ac circuit. You have two separate systems, each has their own transformer making 24vac (well, it could be anywhere from about 22-30 and still work). The power to switch each respective system needs to both come and return to their respective system to ensure that you get a proper complete path for the current. IOW, power from the boiler wants to return to the boiler, and power from the a/c unit wants to return to the a/c unit. While using the power from one, MIGHT power the other, it isn't necessarily going to work. Also, if you do power each properly from their respective devices, you could turn the other entirely off when not needed. Keep in mind that when you power things with 240vac, the ground wire you send to it is there for safety, not intended for current except in a fault situation, and all of the electronics in there may never have a ground connected to them at all.
  13. ImOld

    ImOld New Member

    Messages:
    73
    Location:
    In the rumble seat
    A more detailed description of one of my points that can be the difference between success and failure. :cool:
  14. nola mike

    nola mike Member

    Messages:
    31
    Location:
    Richmond, VA
    Fair enough. The specific stat isn't the issue here, it's more general as to whether I need a stat with separate Rh and Rc terminals. That's actually the only practical question that I need answered. Everything else is just for my own learning.


    Still seems to be some disagreement on this still, see below.

    Not on the boiler stat as currently wired. My understanding is that's what the C wire is for (which doesn't exist on the boiler).
    I understand that. May try to give them a call on monday.

    Sounds different from what ImOld said above? Are you saying that both 24v inputs can go to the same R terminal on the stat? That seems wrong, but I don't know why. Under what circumstances would both sources use a common ground (are you saying sharing a neutral on the same 120v house circuit?)

    Does it? Or does it just want to go home to the neutral bus on the panel (no sarcasm intended)?

    Yeah, that's where I'm at, but I still don't get why.
    Not much of an issue for me.

    Forgot that my a/c unit is 240v. I guess "neutral" is a better term than ground in this situation, which is still present in a 240v.

    Thanks for all the help guys. I'm probably making this much more complicated than it needs to be.
  15. nola mike

    nola mike Member

    Messages:
    31
    Location:
    Richmond, VA
    I may have my answer. This article was very helpful, particularly the diagram on page 8. While I was correct in my assumption that the 24v powers a relay, I had thought that the negative terminal of the relay was connected somehow to the 120v circuit. It appears that it is instead connected to the other end of the secondary winding on the transformer, which makes sense I guess. So there isn't in fact a path to ground when using the R from one transformer to power the other system.

    Bottom line, I'm still not sure if I can connect the 2 R wires to the R terminal of the stat (still seems like that wouldn't work, but I still don't know why). If not, then I'll probably just put a microswitch in there to switch between heat and cool.
    EDIT: Aw, crap, that won't work either. The stat wouldn't have a C wire from the furnace. I'll need a stat with both Rh and Rc terminals.
  16. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,313
    Location:
    New England
    If you get ONE 24vac transformer that can provide enough power to run both systems, you can then use one to do what you need. That may or may not be easy. The a/c, running off 240vac, may not have or need a neutral wire attached to it.
  17. DonL

    DonL Jack of all trades Master of one

    Messages:
    4,508
    Location:
    Houston, TX

    The primary of the transformer does not mater. If it runs on 120V it will have a neutral, But the transformer Isolates the neutral from its secondary 24V winding. So ground is not in the picture, yet.

    The problem you have to deal with is IF your controller Grounds one side of the Secondary winding of the 24V transformer. Then you could have problems running two, Because one transformer could be shorting out the secondary of the other, when connected together.

    Another thing that can happen if mis-wired, is that you could end up with the transformers in series and apply 48VAC where you do not want it to be.

    You can use a VOM to check for these things.


    Are You confused Yet ? It is simple really.


    Good Luck on your project.
    Last edited: Feb 15, 2014
  18. houptee

    houptee Member

    Messages:
    186
    Location:
    Monmouth County, NJ
    Sorry posted this to wrong thread but cant delete it.

    You said the circulator keeps running after both t-stats are no longer calling for heat?
    And the zone valves are both closed I assume if neither t-stat is calling for heat?
    If so, where is the circulator moving the water too when zone vales are closed and its still running?
    Last edited: Feb 15, 2014
  19. nola mike

    nola mike Member

    Messages:
    31
    Location:
    Richmond, VA
    Right, although I'm thinking of ground in this case as just a return path for current.
    How do I find that out?

    Well, I hooked up both 24v wires to the R terminal of the Lennox stat. From R to C I get 28v (which was the voltage from the a/c unit). From R to W I get 24v (the voltage I was getting from the furnace). So presumably if I had the transformers in series I would see 48v there? Anyway, seems that a) the stat is being powered by the a/c transformer correctly, b) the furnace is working correctly. can't test the a/c unit yet.
  20. DonL

    DonL Jack of all trades Master of one

    Messages:
    4,508
    Location:
    Houston, TX

    You do not want return path current on the ground, That is what creates problems. The return is the other wire for a two wire circuit.

    You can measure from your transformer secondary to Ground (or neutral, but the system should be grounded) and the reading should be 0 VAC and 0 VDC. The controller may use a bridge rectifier, So DC could be there. If your meter picks up stray readings, You can use a 100K ohm resistor across your teat leads.

    If you check using resistance with power removed, on the same terminals, the reading should be Open, or > 100K ohm.

    If not, you do not have complete isolation.

    The way you have it now, there could be more than 48V between terminal C and W. That is enough to give you a nice shock.

    A single 24V source would be ideal like Jim said, But it needs to carry both loads.


    Have fun.
    Last edited: Feb 17, 2014
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