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Re: installed Nest but won't blow A/C Cold air

Nestuser9
Community Member

Been using my my Nest Learning Thermostat for 3 years with no issues. Once it updated to 6.2-27, it turns the fan off too soon but the compressor keeps running as it's not completed with it's cycle.  Tried another Learning Thermostat from my work that was on the same version and did the same thing in cooling mode.  Wonder if there is a bug with this version.  Even swapped the control board thinking that it was it since the voltage zeros out to the blower.  It doesn't happen if the yellow and red are hot wired together or when I connect my old basic thermostat.  Wish I could roll back the update to troubleshoot.

11 REPLIES 11

CoolingWizard
Silver Product Expert
Silver Product Expert

I am running the same software and not experiencing that problem you write about.  My HVAC system is a Goodman package system model GPG1360090M41D. This unit is a Gas / Electric system.  My thermostat meets the set temperature, removes R-to-Y and the blower runs on for a few minutes. This additional blower fan time is part of the software on the Goodman control board. Because I like to keep air in the house moving, I use a fan schedule on the Nest Learning Thermostat.  

You state that if you touch Y to R, then let it run for say 10 minutes, then remove Y from R, the blower stops at the same time?  If this is the case, the HVAC control board controls the blower Fan.  Try this test, remove the wire in the G terminal on the nest backplate.  Then run your cooling test again.  See if the blower runs till the compressor shuts down.  

NestPro and HVAC company owner

Thanks for your reply.  Nest just finally posted that software version on their website, so I was still wondering yesterday if it wasn't fully released.  My home Nest received the update in April before a hot day when the AC was then used.  The other Nest at my work updated in May and I was able to swap with the same result but was more intermittent.  I have a Lennox and it only short cycles the fan on cool.  The gas heat runs okay still.  That helped me determine that the blower isn't overheating.  The blower is also cool to the touch when it short cycles.  It can turn off anywhere from 1 minute to 20 when the short cycle happens.  The thermostat still shows it is cooling and the compressor still runs.

I could use some help. Same issue. Blows but gets warmer not cooler. It ran for an hour before I realized it wasn’t working. Just installed this weekend and everything seemed to fire for testing. But I didn’t realize it wasn’t cooling. This is a new build moved into in April. I think it is Lennox.

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Hi Scubajanetn,

 

I'm sorry to hear that you're also having the same issue of not activating your AC when calling for cool. Let's see what's going on.

 

To isolate: 

 

  • What's the make and model of your HVAC system?
  • Did you verify if Fan and Heating still work?
  • Do you see a power report or message code on your thermostat?

 

I did find a helpful article with some troubleshooting steps.

 

Please let me know if that helps.

 

Thanks,

Mark

Hi Scubajanetn,

 

I wanted to follow up and see if you are still in need of any help. Please let me know if you are still having any trouble, as I would be happy to take a closer look and assist you further.

 

Thanks,

Mark

Hello Scubajanetn,

 

Chiming in to check if you have additional questions or concerns as we will be locking this thread in 24 hours.
 

Best regards,

Mark

CoolingWizard
Silver Product Expert
Silver Product Expert

What type of Lennox system do you have? Choices are 1) Packaged Unit, all in one unit  2) Split System with part outside and part inside. The fan terms to understand are Condenser Fan Motor; the fan above the compressor you can see it. This Fan Motor is critical for cooling to work.  The other is the Blower Motor. This is the motor that circulates the air inside the house. The blower motor is used in cooling and heating.  The Fan Motor is only used during cooling.  So, which motor is “short cycling?”

NestPro and HVAC company owner

It has the outdoor compressor and indoor standard furnace.  It is gas.  The blower is what I am referring to that is stopping and not circulating air.  The model is a C33-30A-2F.  I replaced the control board because I originally though I cleared the thermostat by having a voltmeter inline on the green wire.  When the blower would stop, the voltage was present.  So then I hard wired the red and yellow and used the switch on the furnace to turn on and off.  The blower always worked until I shut off the unit.  I found my old original thermostat and reconnected, and don't have any issues.  Before going out and purchasing a new Nest, I then borrowed the one at work that is the same model and it just updated the software.  I was able to narrow it down to just happening with both Nest Thermostats.  I had the Nest for 3 year with no issues.  It was using power sharing on 4 wires.  I was going to replaced the cable with a 5 wire so that the C lead can power the thermostat but can't pull the cable as it is stapled inside the wall.  The basement just got finished and in order to fish another cable, I would have to cut a hole in the ceiling and drill a new hole to fish a cable.  I guess I could connect the cable at the control board and the Nest thermostat to clear the C wire power.  Nest support claims it is because of the C wire but it worked for 3 years with power sharing.  Again, the issue only happens on cooling and not the heat.  I just don't trust leaving the Nest connected when I'm not home because the unit will freeze up with the blower turning off and the cycle not completed still running the compressor.

CoolingWizard
Silver Product Expert
Silver Product Expert

So the current cable has 5 conductor wires.  So, 1) R, 2)C, 3)G, 4)Y, 5)W.  The control boards in these systems either work or they are failed completely.  External to the control board is the 40VA transformer.  

The 120VAC or 240VAC line voltage is connected to the primary side of the transformer and the secondary powers the control board and sends 24VAC to the thermostat.  First, turn off the power to the air handler.  Use a volt meter to confirm there is no power between Line 1 and ground, and Line 2 and ground.  

locate the cover on the air handler where to power and thermostat wires enter.  Remove this cover and you should see the blower, the control board and the transformer.  Look at the thermostat wire, is one of the conductors not connected? If so, this will become your common wire. 

You can find this in the air handler where the thermostat cable is terminated.  Follow the red wire towards the control board and locate where is connects with the secondary side of the transformer.  The common will be the second pole of the secondary side.  Simply connect the unused wire the secondary side somewhere and you now have common at the thermostat.  The common is used throughout the air handler.  I hope this helps. 

 Before you turn the power back on, remove the C-Wire Modules, and connect the common wire color conductor in the air handler to the C terminal in the nest backplate. Then, go back and turn the power back on. 

NestPro and HVAC company owner

Unfortunately, the current cable has 4 wires.  It was using power sharing with no issues for 3 year to charge the thermostat.  I was going to replace it with a 5 wire but not able to run without cutting out the basement finished ceiling.  I provided the wrong model number earlier.  The furnace model is G40UH-36A-070-02.

CoolingWizard
Silver Product Expert
Silver Product Expert

We should move this conversation to its own place.  I just realized we are in Clee1228’s posting. 

NestPro and HVAC company owner