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Nest Thermostat - Heat and Air Turning on at same time...

MCSlim888
Community Member

So we got our brand new nest thermostat back in March and installed it according to how it told us to. Was fine until early May when all of the sudden the heat would turn on with the air and over power it. It only happened about once every 2 weeks and we would have to turn the system off and take it off the wall to get it to stop. We have done several factory resets and it is now doing it a couple of times a week now. I tried chatting with someone from Google and showed them the wiring and it was correct and their only fix was to switch the wires for heating and air which is not a fix. Can anyone help???

15 REPLIES 15

Patrick_Caezza
Platinum Product Expert
Platinum Product Expert

How is it wired?

Does it have the C wire installed?

It doesn't look like it.... is that the issue? I only see a blue wire that isn't being used. I tried attaching a picture but it didn't work

CoolingWizard
Silver Product Expert
Silver Product Expert

First of all, what type of comfort cooling/heating system do you have? That is to say, do you have a heat pump, or do you have an AC with gas heat?

NestPro and HVAC company owner

It is a trane system. The heat is gas

CoolingWizard
Silver Product Expert
Silver Product Expert

So make sure I have this correct.  When you turn on the AC, the Y wire is energized and the W wire is getting energized at the same time.  The Trane IFCB would typically not allow this to happen. If a heat call is initiated during a cool call, the system generate an error code and performs a hard lockout.  

One additional bit of information if you can.  Do have a split system or a packaged Trane HVAC system? 

NestPro and HVAC company owner

So it doesn't turn the heat on all the time. The heat turns on randomly while the ac is running and overpowers it. It happens about once or twice a week right now. 

I'm not sure about the last question what you mean, I know we have our ac unit outside and our furnace is in the attic. 

Could not having a ground wire do this? Bc we were gone and our house got an the way to 87 degrees before a relative noticed at the house. The only thing that works right now is turning the system off and taking it off the wall for 5 min. Then putting it back on and starting the air. That's all that works and we just want to get this fixed

CoolingWizard
Silver Product Expert
Silver Product Expert

The lack of the C wire, would not necessarily cause this.  Since the furnace is in the Attic, your AC evaporator is there as well.  So let’s check the wiring on your thermostat. You should have 24V AC arriving at the thermostat on the Rc wire. The AC is called on the Y wire. How this happens is the thermostat connects the Rc to Y, the voltage travels to the furnace control board, and then onto the outdoor unit.  At the outdoor unit the Y wire and a C are attached to an electrical contactor, it is energized and that starts the compressor and condenser fan.  

The refrigerant travels to the indoor evaporator where the cooling of the air takes place. When the call of cooling arrived on the Y wire at the furnace, it cause the furnace blower to turn on.

The heating call is similar only the thermostat R is connected to the W and the call goes to the furnace control board.  There the heating sequence begins. The induction fan is started to remove any and flammable gas residue in the heat exchanger.  Once the negative pressure is detected, the gas valve is opened, the igniter initializes, and the flame is ignited. After the temperature rises enough, the furnace blower is started.  

So, if Cooling has been initiated,  and then once in awhile heating is initiated, then the furnace control board would override the the cooling call, turn off the compressor, and start the heating initialization.  

So, the question becomes, why is the thermostat calling for heating when the cooling is active? We need to double check the wiring of the thermostat and the air handler in the attic.

1. Need to verify the wires between the thermostat and the air handler. That is, which color wire in the furnace control board are connected to which terminal, Y, G, R, W, C etc. 

2. Need to verify if any schedule programing in the thermostat is active. 
3. Is the thermostat set is Heat mode, Cool mode or the Heat/Cool mode. 
To verify the Heat Call when a Cool Call is active. Need to be at the air handler to use a VOM to look at the calls coming in so see if indeed a Heating call comes when the Cooling call is active. 

 

NestPro and HVAC company owner

Thanks for the info.

1. I am trying to attach a picture of the wiring but it won't let me. How can I show you a picture?

2. Yes there is scheduling for the AC only everyday about 5 times a day.

3. It is set on Cool, we live in AZ, we never turn our heat on lol

CoolingWizard
Silver Product Expert
Silver Product Expert

I am not sure why people cannot upload a picture. What we need is to see the original thermostat wall plate with the wire attached before you switched to the Nest.  If we cannot see that, the other option is to see the Integrated Furnace Control Board and which wire are connected to which terminal screws.  

8264A89F-3ACA-428D-A6A7-023B351581EA.jpeg
 Most HVAC companies use a thermostat wire that has conductors that are Red, Yellow, White, Green, Blue, Black.  We need to note which color wires are on which terminal screws.  

NestPro and HVAC company owner

Jeff
Community Specialist
Community Specialist

Hi MCSlim888,

It looks like the conversation dropped off a bit here, so I wanted to check in to see if you were ever able to get this worked out or if you're still looking for some answers. If you're still needing help, please let me know and I would be happy to work with you on this.

Thanks,
Jeff

MCSlim888
Community Member

Hey Jeff,

Thanks for reaching out. I didn't get it figured out bc to be honest I am not going to switch wires around where heating is cooling and vice versa, that isn't a fix. I can't add pictures in here which is dumb so I can't show what it looks like and I don't have any of the original hardware. 

So my fix was I disconnected the heating wire in the thermostat 2 months ago and it doesn't happen anymore because it cant trigger it now lol. So yeah. Just sucks because I would rather have it connected and be able to have this fixed but no one was suggesting anything doable. 

CoolingWizard
Silver Product Expert
Silver Product Expert

MCSlim888,

If you can describe which wire colors are connected to which letter screw on the air handler, We  might get this figured out.  Also, the nest can be set up in a mode to Heat/and Cool. Make sure you not doing this and just use a single mode,,  in your case, only Cool mode. 

NestPro and HVAC company owner

Ryan_G
Community Specialist
Community Specialist

Hey MCSlim888,

 

I'd like to jump in here and check if you've seen CoolingWizard's response. Let us know if you need more help by replying to this thread.

 

Thank you for your help, CoolingWizard.

 

Regards,

Ryan 


 

Ryan_G
Community Specialist
Community Specialist

Hey folks,

I just wanted to follow up and see if you're still in need of help. Please let us know as we would be happy to answer any questions you may have. 

Best regards,
Ryan
 

MelbaDT
Community Specialist
Community Specialist

Hi, 

 

We haven't heard from you in a while so we'll be locking this thread now. If you have any new issues, updates or just a discussion topic, feel free to start a new thread in the Community.

 

Best, 

Melba