Replacing Chronotherm TS882 with Nest Learning

[Cisfinest]

Hello,

I want to replace my existing Chronotherm and can't figure out the wiring situation. Looked up previous posts about this thermostat but cant find a 1 to 1 comparison. 

I have 2 wires (green and yellow) going to the left side labeled "Clock Terminals." Based on other posts, I can just cap/tape and put in the wall (thanks @CoolingWizard ). 

I also see 3 other wires, a red wire which isn't connected to anything, a blue wire going into the R and a white wire going into the W.  My house only has heating, no AC. 

The red going nowhere has me confused. Is the blue into R normal?

Thanks in advance!

[CoolingWizard · Accepted solution]

@Cisfinest , From the look of you you only have a heating system attached.  The clock wires can be tapped off and tucked into  the wall. Keep in mind the clock wires are also 24VAC Power and Common. So do not let them touch each other and make sure you turn the power to the furnace off before making any changes. 
Thermostat’s  have no ability to know the color of the insulation of the copper conductor wire.  Since the R has a blue wire, you can count on it being the 24 VAC Power and it will be attached to the R or Rh terminal of the Nest Thermostat. 

The AC Cooling Wizard

[Cisfinest]

Thanks! So just sticking with this set up is the move, ignore the red and just connect those 2 up to the R and W on the Nest? 

[CoolingWizard]

Yes, blue on R and white on W.

AC Cooling Wizard

[Cisfinest]

So I finally got around to this today and it's working - but not charging enough to connect to my Nest account (that tile says low battery) and not enough juice to do an update.  

I set this up around 6 hours ago and it's still saying low battery.

I tried putting it on the USB charger but it dies immediately. 

I did buy this thermostat in November 2021 so could the battery have failed being unused?

Any thoughts? Thanks!

[CoolingWizard]

@Cisfinest , you need to verify the AC Current between the Blue Wire and the Yellow wire and then the Black wire.  One of those two wires is the Power Common. You should see approximately 24 VAC +/- 5VAC.  If there is no or very little voltage, then your R (blue wire) and the W (white wire) are likely millivolt system. If so,  check the AC Voltage between Yellow and Black Wire. If you see the 24VAC, this can become your power source for the Nest Thermostat. 

AC Cooling Wizard

[Cisfinest]

So all's been good the last 4 weeks. I just put it on a charger for a few hours and it's been fine in that regards since. 

I did just get an E74 No Power error in my apps. Went to check on the actual thermostat and it was fine. Back to the phones, no error. Fluke? Does this happen sometimes?

Also, I've been noticing a buzzing sound for a 1-2 seconds after the heat kicks on or off. Is this a concern?

[CoolingWizard]

I’m not sure what the buzzing sound might be other than current flowing through a relay

AC Cooling Wizard