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Thread: Thermostat Wiring question

  1. #1
    DIY Junior Member nola mike's Avatar
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    Default Thermostat Wiring question

    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. #2
    Test, Don't Guess! cacher_chick's Avatar
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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.

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    DIY Junior Member nola mike's Avatar
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    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...

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    Test, Don't Guess! cacher_chick's Avatar
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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.

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    DIY Junior Member nola mike's Avatar
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    No, the stat that I'm testing with is battery powered. The boiler is 2 wire without a common, only red and white wires.

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    Retired Defense Industry Engineer jadnashua's Avatar
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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.
    Jim DeBruycker
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    DIY Junior Member nola mike's Avatar
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    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. #8
    DIY Junior Member nola mike's Avatar
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    And why am I getting 28v from my a/c system? Would that make a difference?

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    Test, Don't Guess! cacher_chick's Avatar
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    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 by cacher_chick; 02-15-2014 at 09:45 AM.

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    DIY Member ImOld's Avatar
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    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.

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    Jack of all trades DonL's Avatar
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    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 by DonL; 02-15-2014 at 10:31 AM.
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    Retired Defense Industry Engineer jadnashua's Avatar
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    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.
    Jim DeBruycker
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    DIY Member ImOld's Avatar
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    Quote Originally Posted by jadnashua View Post
    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.
    A more detailed description of one of my points that can be the difference between success and failure.

  14. #14
    DIY Junior Member nola mike's Avatar
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    Quote Originally Posted by ImOld View Post
    I'm not going to get into the wiring points of the thermostat since I really don't want to look it up.
    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.


    Quote Originally Posted by ImOld View Post
    You have two transformers, that are not in common, outputting to separate circuits and you cannot mix and match.
    Still seems to be some disagreement on this still, see below.

    Quote Originally Posted by ImOld View Post
    The thermostat battery is actually a backup for the electronics/memory in case the power goes off, which normally powers it.
    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).
    Quote Originally Posted by ImOld View Post
    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.
    I understand that. May try to give them a call on monday.

    Quote Originally Posted by DonL View Post

    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.
    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?)

    Quote Originally Posted by jadnashua View Post
    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.
    Does it? Or does it just want to go home to the neutral bus on the panel (no sarcasm intended)?

    Quote Originally Posted by jadnashua View Post
    While using the power from one, MIGHT power the other, it isn't necessarily going to work.
    Yeah, that's where I'm at, but I still don't get why.
    Quote Originally Posted by jadnashua View Post
    Also, if you do power each properly from their respective devices, you could turn the other entirely off when not needed.
    Not much of an issue for me.

    Quote Originally Posted by jadnashua View Post
    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.
    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. #15
    DIY Junior Member nola mike's Avatar
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    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.

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