Nest Randomly Turns on Zone Valve/Heat

[sdebruin]

Hello. We have a Nest that controls a White Rogers zone valve (which controls our boiler) and a separate AC system. Periodically even when the Nest is set to OFF or COOL the zone valve cycles, turns on the boiler, and then turns off. The problem goes away when I remove the Nest from the wall. The total cycle time is about 20 seconds. I have confirmed that this is the behavior of the zone valve if you momentarily short Rh and W1 so it seems like the Nest is just momentarily shorting Rh and W1. 

In case it is relevant, on the Equipment page on the Nest display it says that the Rh wire is not connected even though it is. I assume this is because the C wire connects to the air handler (separate from the heat system) and Nest is therefore not detecting a valid AC voltage between Rh and C. But the Nest is nevertheless able to turn the heat on so I know Rh is connected (and I can measure a voltage there with a multimeter).  

Do you have any recommendations for how to get this to stop happening? Its definitely short-cycling the boiler which is not good for the equipment. 

Also I have already replaced the Nest and the base plate and the issue continues. 

Thanks! 

[sdebruin]

Sorry I should have added, this is a 3rd gen Nest learning thermostat. All of the learning features are turned off and the safety temp and eco temp are significantly below the current temperature. 

[sdebruin]

Hello. I did check all the connections at the thermostat, zone valve, and boiler. All are correctly wired (I can post a wiring diagram if that would be helpful) and all are secure. The system does have a C wire but it comes from the air handler not from the boiler (which as I understand is the correct wiring). The power supply to the thermostat is stable as measured by a multimeter. I also believe the Nest is compatible with the zone valve because we have a second Nest controlling a second WR zone valve and don't have any problems on that side. 

And unfortunately I have had two HVAC technicians out to try to help with this. The second said that the issue was probably with the Nest itself and I should replace it, but as I said after replacing the Nest the problem still exists. 

[Patrick_Caezza]

@sdebruin 

Can you post a picture of the original thermostat's wiring and how you wired the Nest here?

What is the model number for the valve? If it is a Type 1311 you will need a simple relay and possibly an external 24vac transformer.

[sdebruin]

I am attaching a picture of when we first moved in before we installed the Nest. But it is not very representative because at that time we didn't have the zone valves (the entire house was one zone) and we also didn't have air conditioning. I'm also attaching a picture of the Nest connections now (W1 and Rh go to the zone valve/boiler and Y1, G, Rc, and C go up to the air handler). There is no AC in the downstairs zone (the zone that does not have this issue). In case it helps I'm also attaching a wiring diagram of the system showing both zone valves. 

The model number for the zone valves is White Rodgers 1361-103. Can you elaborate what you mean about the relay and external transformer? 

One other thing, on the equipment display of the main floor nest the Rh wire is greyed out. I've measured it with a multimeter and it has the 24V, and if you unplug all of the air handler wires it goes to black (normal). So it seems like the Nest is just trying to compare it to the C, which comes from the air handler, and isn't measuring 24V (because they are separate sources).