Thermostat Wiring question

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

  1. houptee

    houptee Member

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    You cannot have both transformers Red + wires connected to the same terminal on the Tstat.

    But if you have a T-stat with separate Rh and Rc terminals then it will work with two separate 24vac transformers, but only if the jumper is removed between Rh and Rc on the Tstat base.

    The manual for that smart web based Tstat you want to use says it will not work on systems with two 24vac sources with Rh Rc terminals because it needs a single R + and single Common wire.
     
  2. DonL

    DonL Jack of all trades Master of one

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    That Wi-Fi T-Stat can also be powered by a separate External supply, So it must have all Dry relay contacts.

    They just all need to be connected properly.
     
    Last edited: Feb 17, 2014
  3. houptee

    houptee Member

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    Yes he needs to look at the devices in the low voltage loop that would need 24v ac, such as the gas valve on heat furnace, the contactor in the outside a/c condenser, and the fan relay in the indoor a/c air handler. Or does the a/c system use the hot air furnace as the air handler?

    Since it will only need the maximum VA (volt amps) when two devices are drawing amperage, i.e. the gas valve and fan relay in heat mode, or the a/c contactor and fan relay in A/C mode, the 24v source has to supply the amount of current of whichever mode draws the most VA, not both modes at same time.

    I would think a standard 24v 40va HVAC transformer would be more than adequate to power either mode.
     
  4. DonL

    DonL Jack of all trades Master of one

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    40 VA should work just fine.

    It would not hurt to fuse each feed with a inline fuse.

    I am a big fan of fuses, if something is not right, You will know it before you smell smoke.


    Have Fun.
     
  5. nola mike

    nola mike Member

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    Heard back from customer service at motison. They said that if I have no voltage between the 2 R wires, then I can connect them both to the R terminal on the stat. Unfortunately, I have 50v between them. I also checked voltage from the secondaries of the transformer in the furnace--with both R's connected at the stat I get a voltage of 50v at one of the secondary wires to ground. With the R's disconnected I get 0VAC/0VDC.
    @DonL: what are "dry relay contacts"?
    @houptee: The heat is a boiler/radiator system, no air handler.
    Either way, a new stat with separate Rh/Rc contacts would be much easier than a new transformer...
    Guess that means I'm SOL with this stat.
     
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Not necessarily...one of the transformers in one of the units may have enough extra capacity to run both. If that is true, while it might not meet UL, you could use the transformer of one, disconnect the other, and then attach the lead from the secondary of the working one to the other system.

    If one side of the secondary is not connected to ground on either system, you might be able to swap the leads which would reverse the phase...that should get the two into phase and reduce the voltage (it should have been double the 24-28vac, or the sum of the outputs of both 24vac transformer's secondary windings).

    Don't try it then blame me, though...proceed at your own risk! This is where an o-scope would come in handy.
     
  7. DonL

    DonL Jack of all trades Master of one

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    Dry relay contacts is a set of contacts that have no voltage on them because they are not used by another circuit. They are isolated.

    Like Jim says you can phase the transformers.

    Are both transformers on the same leg of your 120V feed ?


    Good Luck.
     
  8. houptee

    houptee Member

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    So you have hot water heat with one 24v transformer (xfmr) in or near the boiler correct?
    Do you also have controls on or near the boiler like zone valves that use this same 24v to power them?
    Or do you have switching relays that use the 24v to trigger the circulator pump or pumps?

    On the a/c side, do you have an air handler and is that where the 24v xfmr is located, or is the xfmr outside in the condensing unit?

    There is a way to wire it to use one xfmr but we need to know everything you have now that needs 24v to draw a schematic how you can do it.

    Also the 24v is AC voltage not DC, so keep your meter on AC setting.

    Maybe take some pics of the boiler and all controls that have tstat wire going to them so we can see what you got then the guys on here can guide you better.

    Its totally doable with one xfmr and that new tstat don't give up yet!
     
  9. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Keep in mind that if you do manage to combine the 24vac controls, while the high voltage stuff won't come on, you'll still have voltage on the other unit when you turn the power off to it and things like relays can be turning on and off. That can be particularly scary if you're working on it when that happens.

    You could also do it with a couple of relays, but that could get messy, and then, try to explain it to someone when you sell the place or need service you can't do, could be a major pain. You might be able to use the Heat/Cool switch to control a latching relay to control which power (R-wire) was connected to the thermostat, but that would mean messing with the thermostat and probably voiding the warranty. Or, simply put a break before make auxiliary switch, and just switch the R leads for the requisite mode on the thermostat - but, unless you swap phases of the secondary of one of them, failing to swap both at the same time could cause damage - a on-off-on switch might work so you could disconnect both, switch the thermostat, then flip the switch to the appropriate mode.

    None of these are ideal - I think I'd try swapping the phase of the secondary on one unit, then, it should be safe to combine them at the thermostat. Note, there may be minor variances in the winding of the transformers, and you still might have a slight difference in voltage between the two - IOW, it may not be zero...this would mean a constant, but small, amount of current, and the transformer might just eventually overheat.

    Lots of things you could do in a laboratory situation, but not a great idea in the real world for everyday users - you want it idiot proof, and primarily, safe.

    Best solution...get a new thermostat that has separate inputs!
     
  10. DonL

    DonL Jack of all trades Master of one

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  11. nola mike

    nola mike Member

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    Most of the Wifi stats that I've seen have some sort of web interface or proprietary app. This one is really cheap, and would work very well for most installations. I've had it for 2 years controlling a heat pump. I'm only replacing it because I can't fine tune the aux heat. BTW, transformers are on different circuits but on the same leg of the panel.

    @jdnashua:
    I can't switch R wires, since I only have a C wire from the a/c unit, and that is needed to power the stat.

    Yes, it was double the voltage. When I disconnected everything, I got 0 VAC from (what I think were) both sides of the secondary windings at the boiler. So I assume that that means that neither of those are grounded? I did get 4 VAC to ground from both terminals when I had the stat connected, except for the R from the A/c unit (28v from a/c - 24v from furnace? Not sure what's going on there).

    Heh heh. Ok, but at this point it's purely academic. I can just use the stat that I was going to use at the other house here--it has separate Rh/Rc, but then I still wouldn't have control over my aux heat using the motison. My wife of course insists that none of this is worth my time :D


    @houptee:
    Yes

    No zone valves. Only electrics used by the boiler is the circulator pump and the pilot, AFAIK.

    Yes, there's an air handler. It's not that easily accessible, so I haven't looked too closely at it. Not sure where the xfmr is. Condenser is on the roof.


    So can I switch the phase either by moving the breaker to the other side of the service panel, or reversing my secondary leads, or is it more involved than that?

    I'm going to try to upload a pic of my boiler controls--lmk if this is what you need to see. 20140220_152307.jpg
     

    Attached Files:

  12. DonL

    DonL Jack of all trades Master of one

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    You do not need to get C from either units. That T-stat will let you apply power for it from its own separate power supply.

    I think you are over thinking things and your
    getting confused with something simple. I could be wrong.


    Have Fun.
     
    Last edited: Feb 21, 2014
  13. nola mike

    nola mike Member

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    OK, so looking back there a bit more, it would appear that I'm measuring from the wrong terminals. Not sure what the box is posted above, but took another pic of what I think is relevant. So on the below picture, I get 24v to ground from the blue and black wires, and 24v from blue to yellow. Nothing from yellow to ground. The blue and yellow connect to the red and white tstat wires, respectively. On the upper left of the pic, the terminals are listed as "burner" (B1/B2) connected to blue and white wires. Beneath that is "circulator" (C1/C2, red/yellow). To the left of that is my 120v line input. Not sure what that is above the relay, or where the blue/red wires lead--looks like a timer switch or something?

    So...
    I'm guessing that the blue wire is one terminal of the secondary. The yellow wire (connected to W) is the back side of the relay (which is bottom middle of the pic). Unless in use, that wire should be isolated. Correct so far? The black wire on the right is connected to a terminal marked "W" under the tag.

    Further guessing that the first pic I posted is a power distribution box. Red wire on the left is coming from what appear to be some temperature limit fuses/switches. White goes to the B(urner)2 terminal in the transformer box.

    Ok, so that first pic is fed by 24v and feeds the gas valves and ignitor. It should get power directly from the transformer, right? That would be the red wire? And the white goes to the second secondary terminal? One of them must be connected to the W wire from the stat?

    In the transformer box, one side of the relay should be fed by the transformer (which wire?). The other side should be the W wire from the stat (yellow in the box). The stat connects the red (blue in the box) wire to the white wire, trips the relay, and supplies 120vac to the circulator.


    So putting it all together, my understanding is:
    Line voltage to transformer primary and relay. Transformer secondary (blue wire) goes to R in stat, which connects to W, which powers relay. The other side of the relay is connected to the other side of the transformer secondary (which wire, the black one on the right?). Somehow the red and white wires on the box in the first pic are energized when there's a call for heat, and then each is connected to one of the secondary terminals (?).

    Sorry for the long post, just trying to learn something here :)

    20140221_123049.jpg
     
  14. nola mike

    nola mike Member

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    Oh, I know I'm overthinking things (see last post). Which is causing me to get confused over multiple simple things. But I don't have a separate power supply for the stat. I could add one, but I'd like to KISS.
     
  15. DonL

    DonL Jack of all trades Master of one

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    What is that jumper with the warning tag ?

    What does it say ? And what does it say behind it ?

    Does that transformer have a center tap ?


    Hard to tell by that one picture.
     
    Last edited: Feb 21, 2014
  16. nola mike

    nola mike Member

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    Not sure what's a center tap?
    Behind the tag there's a screw terminal labeled "W". Underneath all 3 terminals is "thermostat". The black wire goes to a terminal "Z". 20140221_140938.jpg
     
  17. DonL

    DonL Jack of all trades Master of one

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    That may be a jumper that needs to be changed, So that It should be set to run the zone valves only. Maybe you could power the T-stat from the other unit. Or Run the T-stat on a small wall wart.

    It may be saying you need a bigger transformer to run more stuff. That AT72D may be for running a T-Stat and more zones, and it is externally mounted.


    Maybe Jim or another Pro has the correct answer.
     
    Last edited: Feb 21, 2014
  18. nola mike

    nola mike Member

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    Found it! Here's the setup for my boiler. I think I've figured out all the components in my boiler system. My initial pic was of the ignition control module. Second pic is the transformer--that switch above the relay is the aquastat. Fig. 11 is my setup, except that my ignition module doesn't have a TH-W terminal. Per another honeywell manual, the wire that goes to TH-W in that schematic should connect to 24v(2) on my module. And I don't see the relay there anywhere. And with that wiring, it seems that the ignition module is powered regardless of the tstat? I need to look at this some more to digest it...
     
  19. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    There are at least a few interlocks in the system...running 24vac to it does not mean it will run...it needs a complete circuit - IOW, a return path back to the 24vac transformer that powers it. That could include a path through the thermostat to make the complete circuit, or safety interlocks, over-temp sensor switch(es), and maybe vacuum switches or pressure switches if it is a closed combustion device to verify that the pipes are intact and not plugged or just open.

    Often, somewhere in there, there will be a unit schematic. Or, if you have the documentation or can find it on-line, you can find it as well.

    A 240vac device will probably not change anything by moving the CB, but you could swap the L1 and L2 leads on the CB, which should change the phase of the input, and thus the output of the 24vac transformer.
     
  20. DonL

    DonL Jack of all trades Master of one

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    It looks like you can just swap the the L1 and L2 input to change the transformer phase, If needed.
    Everything appears to be Isolated by the transformer.


    Good Luck.
     
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