Nest loses power when call for heat

dgiangrave

I just installed the nest thermostat (2020 model) in my house. When installing it needed the C wire to provide power. The wall was wired for it but the previous thermostat did not use the C wire so it wasn't connected. I connected the C wires at the thermostat to the 24V transformer C terminal. When I installed the thermostats I was able to set them up fine, however whenever they make a call for heating, they immediately lose power and the furnace does not turn on.

My theory is that there is not enough power from the 24V transformer for both the thermostats and the furnace. The furnace has three Taco 571 control valves for three different zones. Do I need to add an additional 24V transformer to power the different zones? I'd like to avoid adding a control board if possible.

Here is a diagram of the new C wire connection that I made.

EdTheHeaterMan

I have has that same problem. you have Taco 571-2 zone valves. One of the things that happens during a call for heat on the Taco 571-2 actuator is that there is a heating element that causes a wax filled piston to expand by the hydraulic pressure of the wax in the piston. As the wax expands there are 2 switches on a cone shaped cam. The cam moves to open the valve and the cam turns on the end switch to fire up the boiler.

The cam continues and opens the second switch to keep the heating element from overheating the wax in the piston. Once the heating element is off, the wax cools and the cam starts to close. Before it closes too much the cam allows the second switch to close and the heating element will heat up the wax until the cam then moves to cause the second switch to open again. This on and off switching of the heating element in the actuator is normal, and will last until the call for heat is satisfied.

on any other thermostat this would not be a problem. The NEST however interprets this open circuit of the heating element as an error and will then shut down until someone determines why. Since there is no completed power circuit from the thermostat R to the thermostat W and back to the zone valve #1 to the zone valve #2 and back the the C on the transformer. That open circuit which is normal for the valve actuator is the problem.

There are 2 ways to fix this. You can use an isolation relay for each thermostat. or you can purchase a Taco ZVC403 zone control and the problem is solved (because each zone has an isolation relay in the control)

I would recommend the second choice (ZVC Control) because there are more features with the ZVC controls to help with trouble shooting.

EdTheHeaterMan

Here is your wiring diagram with isolation relays.EDIT After closer inspection there are some important changes to your diagram
. This is because when using a C wire on a thermostat, the R wire MUST come from the R on the transformer and connect to the R on the thermostat in order for the C to work properly.

This is not important when using only 2 wires from a thermostat.

The W from the thermostat must go to the component it is operating (in this case the isolation relay coil) and the other wire from the component MUST go to C on the transformer to complete the circuit return path.

dgiangrave

Thank you Ed for the extremely informative response, this is so helpful. I will look into getting the Taco ZVC403, though in either approach, do you think it might be necessary to install a second 24V transformer anyway to supply the power for both the valve heads and the thermostats? As I understand, the Taco 571-2 is rated for 3 heads before needing another transformer. With the thermostats also pulling from the same transformer, I'm worried there won't be enough power for everything.

EdTheHeaterMan

This is the ZVC 403 wiring diagram if you happen to get the older control without the C terminal (3 wire) at the thermostat

The R and the W are not designated on the older ZVC controls because with only 2 wires it does not matter. BUT, when you add the C wire it does matter, so the left T = W and the right T = R, and C can be found on the corresponding zone valve #1 at the bottom.

kevink1955

You could try without the relays by correcting the original diagram you posted

First swap the Rh and W wires on the thermostat. As Edward said the w1 must connect to the load, in your case h1 on the zone valves

Then connect your new C wire (shown as proposed) to the R terminal on the transformer

I know it sounds strange to be putting your new C wire on the R terminal of the transformer but your wire colors are already screwed up

EdTheHeaterMan

The newer model has the C terminal at the top where you wire up the thermostat, to relieve the confusion
https://www.supplyhouse.com/Taco-ZVC403-4-3-Zone-Valve-Control-Module-with-Priority

dgiangrave

kevink1955's suggestion has everything working! I know the valve head thing could still present a problem, but maybe the nest thermostats have gotten updates to not detect it as a fault, so it was just a wiring issue. In any case, thank you both for the help, you got everything working in a matter of hours!

SteveSan

@dgiangrave If you do decide to go with our ZVC403-4, you won't need an external transformer with the Nest t-stats. Just make sure to pre charge the Nest if needed before wiring to the ZVC control and if you need the resistors, ours are not compatible ( according to Nest ) with the Nest. They suggest a 220ohm up to 5 watt across the W-C terminals on the ZVC t-stat terminals.

pecmsg

Why are these questions always about the NEST? All the other manufactures don't have these issues.