Thermostat Wiring question

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

  1. nola mike

    nola mike Member

    Nov 13, 2012
    Richmond, VA
    Done. The voltage between the R wires is now 3vac. I get 24v between the TV and Z/T terminals. However, I get 24v between Z/T and ground--Doesn't this mean that the system isn't isolated and I have a problem?

    So, aside from being careful to turn of both units when servicing, any other safety issues/concerns?

    Lastly, could someone explain to me exactly how changing the phase resulted in a decrease in voltage? Were/are my transformers essentially wired in series, so that when the 2 ac currents are out of phase, the max amplitude of the sine wave (and therefore voltage) is equal to only one xformer's output, and when they're in phase, the waves stack on top of each other and become additive? And would switching the breakers to different legs on the main electrical panel have the same effect?
  2. DonL

    DonL Jack of all trades Master of one

    Mar 30, 2011
    Rocket Scientist
    Houston, TX
    It is all about the angle of the dangle. And the phase will make the voltage add or subtract.

    And Yes switching the breakers to different legs on the main electrical panel would have the same effect. But the other way was easier, switching at the transformer.

    Have Fun.
  3. Reach4

    Reach4 Well-Known Member

    Sep 25, 2013
    I don't know how your transformers are wired. The Honeywell link does not work for me. The best description of how a typical thermostat works, that I have found, is "Thermostatic Wiring Principles: by Bob Scaringe Ph.D., P.E.
    Let's assume your original system was wired as shown on page 8. huge2.png This is slightly modified version of the page 8 schematic.

    The thermostat's job is to connect either the Rc or Rh, as appropriate, to whatever is being controlled, or to provide an open circuit to turn the thing off. The thing being controlled , such as a relay, would be connected between the controlled terminal and C. Thus power provided = on. Open provided = off.

    The two transformers in the page 8 2-transformer situation don't have either terminal of either transformer connected together.

    In your thermostat, there is only an R terminal. Now the thermostat cannot provide power from the just the appropriate supply. Instead it can only provide power from the one terminal, if I understood the discussions here. I think that Rh and Rc could be connected to R since the non R legs of the transformer are not connected together.

    Transformers should not normally have secondary windings in parallel. So connecting two transformers to a single R terminal and also connecting the other side of each transformer to a C terminal would be bad.

    Suppose Rh and RC are connected together in your situation. Let's call the other secondary lead of the transformer that connects to Rh to be Ch. Let's call the other transformer's C line Cc.
    Anyway, are transformer secondaries that have one wire in common in series? You could think of them that way. But since there is no real load AFAIK that is between Ch and Cc, there doesn't seem to be a reason to consider them to be in series or not.

    That is until you put your voltmeter where you put it. Now you see the effects of the transformers, for this purpose, being in series. Depending on the phase, you will see more or less voltage between the terminals that you are measuring between. Yet that voltage is only academic.

    If you reverse the primary lines on one transformer, you should get the same effect as reversing the secondary lines on that transformer. The phase changes. So yes, if you wire one of the transformers to a different side of the AC line, the phase changes.

    To see a description of how you can get less voltage from two transformers in series, look up "bucking transformer". In an example of a typical case, a 120 vac transformer is put in series with a 12 vac transformer. Depending on how the wires are phased, you get 132 (boost) or 108 vac (buck).

    So yes the voltages add, but they add as vectors or complex numbers. When we say 120 vac, that is only the magnitude, and magnitude is sufficient for most purposes. For computing the effects of combining voltage sources in AC, we need to include the phase in the math.

    In the kind of thing you are doing here, considering only 0 degrees and 180 degrees is enough for most simple calculations. You can make a simple model by thinking of a 120 volt DC battery in series with a 12 volt battery. In 3-phase, it gets harder. The supply voltages are 120 degrees apart; the battery analogies don't work in that case.

    Attached Files:

    Last edited: Feb 22, 2014
  4. DonL

    DonL Jack of all trades Master of one

    Mar 30, 2011
    Rocket Scientist
    Houston, TX
    That was nice Reach4. What is the name of your book ?

    Are we having fun yet ?
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    Another way to look at it is, between L1 and L2, when one is going positive, the other is going negative. Depending on how each system is installed, if both units have L1 on the same terminal, they'd be in phase, but if they were swapped, both L1's on different units, would be going in opposite directions, and when measuring the difference between the two, when they're in phase, you'd have zero volts, but when they're opposite, you have the sum of the two. Works the same when it goes through a step-down transformer. Note, it really doesn't matter between two systems what phase they call the 'R' lead relative to the incoming or supply voltage.

    The fact that you have 3vac between the two could be related to the output of each transformer may not be exactly the same or they were not careful about the phase angle when designing and winding the coils (not particularly critical on this type of device). It also means that there's a constant current running between the two - now, whether in the long term, that might create problems, I do not know. It would be much better if it was zero.
Similar Threads: Thermostat Wiring
Forum Title Date
HVAC Heating & Cooling Furnace Thermostat wiring help Oct 2, 2019
HVAC Heating & Cooling Wiring help with thermostat and condensor Jul 24, 2019
HVAC Heating & Cooling Help wiring thermostat Sep 28, 2018
HVAC Heating & Cooling Wiring thermostat Apr 8, 2018
HVAC Heating & Cooling Wiring diagram for furnace relay & thermostat common wire May 26, 2016

Share This Page