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no heat!!

Rieracorp
Community Member

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 

11 REPLIES 11

kiltguy2112
Silver Product Expert
Silver Product Expert

Sounds like one of your R wires is not making good contact.

i removed the thermostat again a checked all wires same results smh 

sporfrog
Bronze
Bronze

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?

anengineer
Community Member

May be: 

3. Change the O/B wire settings

If your system starts to blow cool air when your Nest thermostat is in Heat mode (or if it blows warm air when its in Cool mode), change the settings for orientation of your heat pump.
The instructions depend on which thermostat you have. 
Select the thermostat you have:

Nest Thermostat
 

anengineer_0-1639382183018.png

 

Nest Thermostat E
 

anengineer_1-1639382182939.png

 

Nest Learning Thermostat

 

anengineer_2-1639382182965.png

 

How to tell which thermostat you have

Nest Thermostat 

  1. Open the Home app anengineer_3-1639382182930.png

     

    .
  2. Tap your thermostat.
  3. At the top right, tap Settings anengineer_4-1639382182934.png

     

    .
  4. Select Thermostat.
  5. Scroll until you find Heat pump. It should show which wire configuration is currently selected.
    • If the O option is highlighted, select B.
    • If the B option is highlighted, select O.
  6. To make sure the issue is resolved, test your heating and cooling again.

Nest Learning Thermostat and Thermostat E

  1.  On your thermostat, go to Settings anengineer_5-1639382182908.png

     

    .
  2. Select Equipment.
  3. Select Continue anengineer_6-1639382182873.png

     

     Continue.
  4. Select Heat Pump.
  5. The Nest Thermostat defaults to an O setting for heat pumps. 
    • If the O option is highlighted, select B.
    • If the B option is highlighted, select O.
  6. To make sure the issue is resolved, test your heating and cooling again.

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.

And what rating MOV is suitable in this application?

25 volts ac rms seems to be a popular rating in the "little fuse" ZA product line.  This matches the control power transformer output on my particular HVAC system, but I think it's fair to say 24vac is universally common. 
 
As far as the joule rating goes, more is better.  I have not laboratory tested the reversing valve coil, and was not sucessful in finding an inductance rating. I can't imagine that the overall energy level could be very high in respect to joules.  I am confident the it is the voltage magnitude produced and not the overall energy level doing the damage in this case.  I do have first hand experience testing the spikes generated from a small electrical relay opening that was causing quite the issue at my workplace, and was astounded at the voltages produced from such small components.
 
It appears that the most correct application of MOV "surge" protection is a bit of an art form for the following reasons:  If the rating of a MOV is 25vrms, it does not act as a short circuit to excessive voltage at 26vrms - there is kind of an undesirably wide gray region until full shorting or clamping occurs.  The best selection requires keeping the voltage rating high enough to not have the MOV act as a load on the circuit by operating in the "gray" area, while also keeping it as close as possible(low enough) to the operating range so that clamping/shorting of excessive voltage will begin at the lowest value possible and offer the best protection.   
 
Keeping the concept brutally simple - you wouldn't want to put a 120vrms MOV on a 24vrms circuit for protection because it would be clamping the voltages until they were substantially higher than 120v based on the inherent operating characteristics of MOVs.
 
Too low of a voltage rating and it's just a circuit load.
 
It has to be the correct voltage with a high enough joule rating to safely "eat" any surges.
 
Based on the product data sheet, I just realized the MOVs I installed and purchased are rated at only 20vrms.  I need to go and make sure they aren't loading the circuit or failed from operating at an incorrect range. Palm on head.
 

Thanks for the discussion.  Should I assume the operating characteristics of the thermostat's 

switching transistor(s)/circuit would/should be determined? Tearing down the thermostat to determine
part numbers may/probably would void the thermostat's warranty.

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.

Bayee33
Community Member

My thermostat does not give the heat option can anyone help

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.