12-11-2021 07:08 AM
Just installed the Nest Thermostat Gen 3 model last night followed all the directions took a pic of the prior wireing on old thermostat and labled all correctly on the new nest but no heat the fan is blowing cold air
12-11-2021 07:27 AM
Sounds like one of your R wires is not making good contact.
12-11-2021 07:29 AM
i removed the thermostat again a checked all wires same results smh
12-11-2021 08:13 AM
If you have them, send pictures of your old thermostat wiring before it was removed, and the wiring for your Nest Thermostat. Maybe there is a clue that will help?
12-12-2021 11:57 PM
May be:
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03-27-2022 04:53 PM - edited 03-27-2022 05:17 PM
Be aware of common failure mode when using a nest thermostat on a heat pump system which causes a no heat condition that the thermostat does not self diagnose or even identify is happening.
I have had 3 nest thermostats fail in the same way, and I have a strong suspicion the issue is way more common than the user community knows or nest is willing to admit.
When the heat pump reversing valve shuts off, the inductive kick from the collapsing magnetic field generates a voltage spike that causes the nest thermostat base to fail. It ends up holding 24VAC on the reversing valve(o wire) regardless of the settings, and stops the system from making heat. The reversing valve is normally energized when the system is in air conditioning mode or defrost mode.
The issue is that the nest doesn't use actual switching contacts to send out power, it just uses a transistor driven output vulnerable to damage from voltage spikes. I finally purchased metal oxide varisters to install between the common wire and the o wire on the terminal strip in my air handlers to prevent the issue from happening, but not everyone is an electrical engineer with time to troubleshoot $250+ thermostats.
03-27-2022 09:47 PM
And what rating MOV is suitable in this application?
03-28-2022 06:24 AM
03-28-2022 09:19 AM
Thanks for the discussion. Should I assume the operating characteristics of the thermostat's
03-28-2022 11:37 AM
Well, I'm not on a quest to **bleep** Nest for their product design first of all. I happen to have experienced the same failure mode three times since my early adoption. I also cannot make a 100% guarantee that the guidance I am providing here is flawless, but I do have considerable experience and knowledge to help create these thoughts. I am not an employee of nest or have any other agenda that would change my opinion here.
The "operating characteristics" of, what I am assuming is, the transistor driven/switched output of the "O" terminal are not primarily the focus of selecting MOV protection. I mean this is in the sense that it is a commonly understood concept that voltage spikes can cause failure of transistors, and that's about as far as the useful depth of knowledge needs to be on the subject in this instance.
The main concept is that the thermostat base provides a current path from the "+" of the 24vac control power transformer to the heat pump reversing valve. This path is most likely a through a switched transistor instead of a coil driven set of contacts(like an actual relay)
IF your setup is the same voltage, a MOV with a rating of 25vrms AC should be a good choice. Instead of installing it in parallel with the main 24vac output of the control power transformer, install it across the "O" terminal and common in the air handler so it can prevent any spikes generated at the reversing valve coil from traveling back to the thermostat base.
As far as the joule rating goes, I'm not 100% on what is adequate to prevent failure from in service exposure to surges. My MOVs are rated for 1 joule and I am reasonably confident that they will perform as intended without failure, but it is easy to find MOVs which exceed that value.
The magnitude of each spike produced when the valve is deenergized will be different due to the randomness of when exactly power is interrupted in relation to where the 60hz sine wave is in voltage. After a sufficient number of occurrences at a harmful voltage level, my "O" terminal on my thermostats would be stuck constantly outputting 24vac.
05-17-2022 10:29 PM
My thermostat does not give the heat option can anyone help
08-29-2022 06:45 PM
I have a similar issue. Is anyone aware of an issue with recent software update? A/C settings appear, but not heater. Details: models- display 3.6; backplate 5.4; software 6.2-27. Gas heat, functioned fine for years and recently (last couple months - not sure because it’s summer) lost the heat settings display. I have reset the thermostat to factory settings and deleted & re-added. Heater connection appears in the equipment menu & heater works in “test” section of menu. However, I cannot set the heater on the thermostat, only the A/C.