Legacy HVAC controls (what the industry calls "24 volt") are relatively
> trivial -- battery is provided to some number of control signals that
> interact with the plant.
>
> Note that the interface doesn't even provide a complete closed circuit;
> the "return" is implied in the plant and not carried in the control
> signals from the thermostat.
>
> I'd *assumed* (bane of all designs!) that keeping track of how *I*
> was driving those control signals would allow me to know the state of
> the plant; if I've applied battery to the Green lead, then the fan
> should be running (RSN), etc.
>
> But, newer systems apparently leverage this interface and piggyback
> their own controls on it. I.e., even if *I* don't call for the
> fan to be running, another system can power the Green lead and cause > the fan to run.
>
> So, I now need to be able to sense the states of these signals, in addition
> to controlling them.
>
> And, do so in a legacy-compatible manner.
>
> I'd not like to have to decouple those piggy-backed systems as that
> adds to the installation cost AND puts me in a bigger liability position.
> And, would like to limit my interface to the "legacy" control cable
> from the thermostat (replacing the thermostat, if you will).
>
> If I assume that I can't tell if anything else is driving a control
> signal WHILE that signal is also being driven by myself, would the
> safest way to sense just be (AC) voltage mode referenced to the power
> lead(s) -- no returns -- available at the thermostat?
>
> [I don't see how to determine the state of those signals while also
> trying to drive them, myself -- not without climbing deeper into
> the controls and exposing myself to the vagaries of their different > applications]
R wire usually red 24VAC (for some reason *you* call it a battery?) C wire usually blue 0VAC often grounded at the plant often absent at the thermostat. W wire usually white connect to R to turn on heating G wire usually green connect to R to turn on circulation fan Y wire usually yellow connect to R to turn on cooling These signals are sensed by 24VAC relay coils (or equivalent) between the W/G/Y signal terminal and the C terminal of the plant. So you can sense if a different station has called for a service by measuring the voltage on the W G or Y wire either WRT R or C
If you want to sense at the same time as calling you could perhaps drive them through a resistance suficient to de-pritoritse this station and measure current that you are massing, it will recude if another station is calling.
Alternativly you could measure resistance between R and W at some other frequency If the other thermostats are all open you see the impedance of the relay and 24V transformer, if one of the others is closed you see the impedance of the wire to the other thermostat in parallel (typically much less), the splitter needed would probably be quite bulky, because it would need to pass 60Hz at AC relay currents. perhaps you can measure during zero crossing instead?