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Taco ZVC-405 C terminals not working with Nest

We have a Taco ZVC-405-4 with 4 zones, hooked up to Nest thermostats. It's been running for some years in a 2-wire configuration but there are actually 3 wires going to each thermostat, with R, W, and C all hooked up to the terminals in the ZVC but the green (C) wires left unconnected at the thermostats.

We just tried connecting the C wires at the Nests to provide constant power, but all 4 thermostats do the same thing - the Nest senses the C wire and reboots, but then says "not enough power available on C" (showing the C terminal with a red color on the equipment diagram) and refuses to work. If you disconnect the C wire at the Nest backplate it's fine again.

I found an official wiring diagram from Taco for Nests with Taco ZVCs and it implies that this configuration should just work - R, W, and C on the Taco connected to R, W, and C on the Nest. But it doesn't.

I checked the terminals with a meter. R to W gives ~25 VAC, but when I try R to C it only gives ~12 VAC (and similarly C to W gives ~12 VAC). Should R to C give ~25VAC like the R to W does? Is there something wrong with the ZVC? Or am I missing something else?

Would this be a case of adding the 220 ohm 5w resistor between W and C? Would you do that in a 3-wire configuration or is that just for 2-wire configurations? (the Taco wiring diagram for the 2-wire configuration shows the resistor, but the 3-wire configuration doesn't)


  • Jamie Hall
    Jamie Hall Member Posts: 23,421
    R to C should be about 24 volts, more or less. There is something wrong -- but not with either the Taco or the Nests. Double check all the wireing.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    HVACNUT Member Posts: 5,860
    Check R and C at the transformer where it plugs into the board. Should be 24-28v there. 
    Try removing one C wire at a time at the thermostat terminals on the board and test power after each time.
  • HomerJSmith
    HomerJSmith Member Posts: 2,467
    edited April 2022
    So, you have 4 Nests, 4 zone valves, relays in the 405. Guessa what? You dona hava no power.

    Disconnect all Nests except one and see if that works and add one at a time back until failure occurs.

    The 405 probably has a 15 VA( Volt Amp) transformer in it and you're drawing more current than that with everything hooked up.

    I just replaced the 15 VA transformer in a Taco SR506 with a 30 VA transformer because the owner had 3 WIFI thermostats and several relays. He had 7 zones and who knows if he will add more WIFI thermostats at a later date.

    I got the 30 VA transformer from Supplyhouse. It was the largest that would fit in the case with the cover closed. It worked great.

    You can add an auxiliary transformer to the one in the 405 if you know how, to boost your power.

    That's my take on the sitchation. Often wrong, but never in doubt.

    At the Nest Rh to W1 should give you 24+ Vac. An idling transformer is 26-27 Vac.

    A Rh to C should give you 24+ Vac. These are at the wire connections with the Nest pulled off the back plate. Voltage is different than current. You can have enough voltage at each thermostat, but not enough current for everything to operate properly.

    Think of voltage as pressure and current as flow. At a given pressure, flow gets used up by resistance. Every current using device adds resistance to the circuit.

  • Alan (California Radiant) Forbes
    Have we ever figured out the VA draw of a Nest or Ecobee or any other power stealing thermostat? This would be good in determining exactly how many thermostats we can install on a 30 VA transformer, no?

    Same with zone valves. Could we have that information all in one place as a resource here on HH?
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • HomerJSmith
    HomerJSmith Member Posts: 2,467
    Sounds like a good project for you, Alan.
    Alan (California Radiant) Forbesrick in AlaskaGGrossBCApollo