Change from nest 2 to nest 4 (learning) with heating bug

[Flessard]

Hi,

my nest 2 work very well. I change for nest 4 learning, and now… the heat is always ON.  I don’t change wire. It’s low tension with W1 and Rh.  The probleme start immediately when I put nest 4 into socket. If I go back with my nest 2… no bug… it’s working.  The probleme is my nest 4 ?

[CoolingWizard]

@Flessard ,

Since both thermostats use different bases, are you setting the temperature set point above the ambient room temperature?

The Cooling Wizard

[Flessard]

@CoolingWizard 

Yes, the chosen temperature was lower than the room temperature. Every time I placed my Nest4 on the base, it started the heating. Every time I unplugged it, the heating stopped. As soon as I put my Nest2 back on, everything goes back to normal, and I can control and start the heating whenever I want.

[CoolingWizard]

@Flessard ,

Do you have a Multimeter?  With the Nest Gen4 attached go to the furnace and check for AC Voltage between W1 and Common.  

The Cooling Wizard

[Patrick_Caezza · Accepted solution]

@Flessard 

It sounds like you have a 2-wire heat-only system. Is that correct?

You will need to use the Nest Power Connector with the 4th-gen thermostat and an external 24VAC transformer if your system doesn't have a C terminal.

Wiring would be as follows.

If your system does have a C terminal, you would wire it like this.

[Seymore]

I have a two-wire (W and RH) thermostat that's connected to my electric boiler furnace. For the past 12 years I have been using the Nest 2nd generation thermostat without any problems.

This summer I purchased the Nest 4th Generation Learning Thermostat. I checked beforehand to know whether it was compatible with my system or whether I needed a C-wire or power connector. Google's online verification system advised me that my system was compatible.

However, when I installed the new thermostat, my furnace turned on and heated my radiators continuously. I received no error messages. When I removed the 4th gen thermostat from the new base, the furnace turned off.

From your last email on the subject, must I purchase a Google Power Connector to make my system work, or will Google issue a firmware update to make the 4th Gen compatible?

Thanks!

[Flessard]

@Patrick_Caezza 

I don't have a Nest Power Connector in my Nest4 box. I'm thinking of changing my wiring to a 2-wire cable for 18/3 solid copper. We'll see if it works. But I have my doubts.

[CoolingWizard]

@Flessard ,

If you replace the thermostat cable. You should install a 7-conductor cable for each thermostat.  

The Cooling Wizard. 

[Seymore]

I had the same problem. My Nest 2 worked perfectly. I switched to the Nest 4 and my furnace was heating all the time. I had to go back to my Nest 2. I have a two-wire (W1 and Rh) thermostat that's connected to my electric boiler furnace.

Were you able to fix your problem?

[Flessard]

@Seymore  Yep! I  find solution....I returned my Nest4 and put my Nest2 back in place. I have no time to waste.Maybe Google Power Connector will be a solution ? ..., but I don't want to try.