Y1 not detected unless RH and W1 are disconnected

[njja25]

The motor in our AC unit died, and needed to be replaced. Our system was offline for 3 days, and after the motor was replaced, we had issues with our Nest Gen 3 thermostat. I got the Y1 wire not detected message, and the Nest unit needed to be recharged via USB. After recharging the Nest via USB, we reconnected it and again got the Y1 wire not detected message. Our wiring configuration is for separate cooling and heating units, so uses the G, Y1 and RC for cooling, and the W1 and RH for heating. The technician confirmed with a voltage tester and jumpers that everything is fine with the AC wiring (both the unit in the attic, and the compressor outside). The technician got our AC unit working by disconnecting the W1 and RH wires, and leaving them disconnected. However, now that the temperature is dropping, we need to get our heat reconnected. Any ideas what the problem might be? Thanks!

[CoolingWizard]

@njja25 ,

I am sure your nest system is set to dual fuel correct? When you say you have separate systems do you have say a boiler for baseboard heat and an air-conditioning for cooling?

AC Cooling Wizard 

[njja25]

That's correct @CoolingWizard . We have central AC and a boiler for baseboard hot water heating. I generally use either the heat mode or cool mode setting for the Nest depending on the time of the year, rather than the heat/cool combined mode. But I may occasionally have used the dual/combined mode. Thanks for asking!

[CoolingWizard]

@njja25 , you need to verify the two separate power sources to ensure that both are producing approximately 24VAC independently.  You need to ensure that the Rc and Rw are in correct positions.  Make sure that the Rc wire is going to the cooling system and that the Rh wire going to the heating system and that you haven’t reversed them by accident.  The nest will not see the Y1  wire if it has the wrong R wire connected to the Rc 

You cannot damage the thermostat so just go ahead and swap the Rc and the Rw wires and see if this makes your Y1 not seen problem go away. 

AC Cooling Wizard

[njja25]

@CoolingWizard @Given that the AC is working now with only the Rc connected on the left, and there is no error message about the Y1 not detected, is that still a possibility that the wrong R wire is connected to the Rc? Currently, the heating wires are not connected at all. That was the only way that the technician was able to get the AC to cool: by disconnecting the heating entirely from the thermostat.

Initially, there was no change in wiring configuration from before the AC blower motor was replaced. The "Y1 not connected" message started displaying as soon as he took out the old motor. Then after he replaced it, I had to charge the Nest with a USB cable, and when I reattached the control unit, the Y1 disconnected message continued to display.

[CoolingWizard]

@njja25 , which motor did he replace? The condenser fan motor or the compressor?  
 Either of those motors should not have caused a change in your Y1 wire being detected. 

The Y1 wire goes from your thermostat to the air handler and is cross-connected to the wire going to the outdoor unit to start the compressor and condensor fan.  

The AC Cooling Wizard

[njja25]

@CoolingWizard He replaced the condenser fan motor. The Y1 not detected message started displaying when he removed the old motor, and then remained until he disconnected and tested all the wires, including the ones running to the compressor. The only way he was able to get the Y1 message to stop displaying and the AC to work was to leave the W1 and Rh wires disconnected from the Nest. I'll also add that we have another Nest gen 3 downstairs that is connected to the heat only, and that one is working fine.

Is there a way to test the Nest unit itself to see if that's the problem?

[CoolingWizard]

@njja25 ,

it’s hard to understand why it might be a problem, technically without doing some actual testing on site. But what I was trying to suggest is that if the power from the secondary system used L2 instead of L1 and the air conditioner is using L1 as the primary there would be a possibility that the problem would show up. If however, they are both using L1 from the power panel then it should still work. What my suggestion was is to hook, the W1 the Y1 the G and then reverse which wires in RH and which wires in RC and see if the problem goes away.  

The AC Cooling Wizard