Nest thermostat malfunctioning.

[dsm]

I have 2 systems for which I have replaced old thermostats with new Nest Thermostats. I have followed installation instructions and wired both exactly the same. Downstairs works fine. Upstairs blower turns on and off repeatedly regardless of whether it has reached selected temp. Also, if it's blowing and I walk by so the farsight triggers (dial and temp light up), it shuts the blower off. Prior thermostats had a C-wire (between RH and RC), however Nest instructions say it is not needed. The Nest thermostat has Rc on the left and Rh on the right. The old thermostat had them right next to each other. Will connecting an additional wire between Rc and Rh correct this problem??? If so, why is the downstairs thermostat operating properly without a C-wire?

[Patrick_Caezza]

It sounds like they are wired wrong.

Can you post a picture of each original thermostat's wiring?

[dsm]

@Patrick_Caezza thank you for your reply. 

I'm having no issues with my downstairs Nest Learning Thermostat.

The old upstairs thermostat was wired like this...

I replaced it with the Nest Learning Thermostat and wired it like this...

That is exactly the same as I wired the downstairs thermostat, which is functioning properly.

Your thoughts???

[Patrick_Caezza]

@dsm 

That looks correct.

You said you have two systems. Is that to completely separate systems or one HVAC unit with a zone controller?

[dsm]

@Patrick_Caezza , I have 2 separate systems. What’s strange, is that I have now reconnected the old thermostat and I’m not having the same issue. Is it possible that it’s the actual Nest Learning Thermostat device that is not working properly?

[dsm]

Yes @Patrick_Caezza, two separate HVAC systems with individual thermostats... one upstairs and one downstairs. I initially tried using two room temperature sensors in bedrooms upstairs, along with the Nest Learning Thermostat in the upstairs hallway. However, when I started to hear repeated on/off issues, I eliminated them thinking that could be the problem.

[Patrick_Caezza]

@dsm 

The only other thing I can think of is installing a C wire.

My personal recommendation is to always install a C wire as with out it strange power issues can happen.

[dsm]

@Patrick_Caezza, please help me understand. A C-wire is not the same as a jumper, like in the old thermostat wiring? I actually attempted to install a jumper, but I couldn't fit 2 wires into the Rc wire hole. Please advise. I installing a C-wire something I can do, or do I need to hire a HVAC service company? Does this require pulling a new 5-wire cable from the furnace to the thermostat? I have read the community info on C-wires, but am still unclear on how to accomplish this. Also, I am uncertain exactly where the wires should go on the Nest Thermostat. If this is too difficult, I may just return the Nest and keep using the original... although I would prefer not because it is not wifi or app controlled. Thanks!

[Patrick_Caezza]

Correct, The C wire is not a jumper. It is another wire, like the Y wire, that goes to the furnace/air handler and provides a constant power source for the thermostat.

You can create a C wire if there isn't a spare wire by using the Google Nest Power Connector, which is installed in the furnace.

This is how you would connect the power connector to the system.

[dsm]

@Patrick_CaezzaI appreciate your trying to help, but this is still very confusing to me. You say to wire the power connector to Y, but I see 4 things with a Y label. I gather that the blue colored wires in the diagram, are the 4 wires coming from the thermostat to the furnace. I am not certain what these wires (specifically the Y and the R) go through, before going to the power connector? Are these some sort of wire connectors? Why aren't the thermostat wires connected directly to the power connector? Do I need to add wires from the power connector to the furnace. Do I need to purchase 6" of 4-wire to make these connections? And wouldn't the 2 wires from the AC compressor (dotted lines) already be connected. I'm sorry to be asking so many questions, but I don't want to make a costly mistake. Frankly, I still don't understand why the downstairs Nest works and the upstairs creates this problem?

[zoeuvre]

Hi dsm,

I wanted to check and see if you are still in need of any help. Please let me know, as I would be happy to assist, and answer any questions you may have.

Thanks for your help, @Patrick_Caezza. 

Best,

Zoe

[dsm]

Yes, I do! Thank you so much for following up with me. I’m very frustrated. I still do not understand why the Nest works fine on the downstairs unit, but starts and stops repeatedly on the upstairs unit. They are the same size units so I wouldn’t think one would require different wiring. Installing a C-wire seem to be a big job. I understand there is some sort of c-wire adapter, but it appears it needs to be plugged into an outlet. I really don’t want to see a wire hanging down from the thermostat. I reinstalled the old (non-wifi) thermostat and it has operated properly ever since. Keeping that installed is not a great option because we are only in this house 6 months a year and I like having remote access. I don’t know if a different manufacturer’s wifi thermostat would work without a C-wire or not. Further guidance would be appreciated. Do you think it’s possible there’s a problem with this Nest thermostat?