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AC starts funny - sometimes

nbpt100

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I was just reading some AC threads and thought I may get some insight here before I call my HVAC guy. Just some back ground for good context:

I have a furnace and whole house AC on the same air handler.

The furnace and all has been working fine. The AC unit was old and I replaced it just a few months ago to be ready for the cooling season. I got a new Google nest "smart" thermostat last October.

We just went through a few days in the high 80s and 90's. This is the first time I really tested the unit since it was installed. I noticed a handful of times that when the AC kicks on the blower will start and then stop after 3 to 8 seconds and then start up again after 1 to 3 seconds. I am not sure if the AC unit outside does this as well. I would have to be outside when it happens to notice if the AC unit does this or not. I first notice this when I was in the basement near the air handler unit when the AC kicked on. You have to be there to really hear it. If you are upstairs you really have to be paying attention for it. Otherwise you will likely not notice it happening. It does not happen every time. Right now, Its very hard to say how frequently it happens.

I never noticed this in the past. Never last year with the older AC unit or this Winter with the heat on. I think if this always did this, I would have noticed it a few times in the past.

I was planning to get some more info before I call my HVAC guy and see what he says. The Air handler is the same old one. He only changed out the condenser and evaporator with the new unit install.

It is a new install so I assume he will check it out as part of his warranty. But if he finds something wrong with the blower I assume he will say it is something unrelated to his work.

Any thoughts on what else I should be looking for and info to collect before I call him? Thanks in advance.
 
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fitter30

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In cooling with fan in auto the fan gets a signal from the stat to the g wire. Turn fan in the on position see if it run constantly it should. What is the model of the Nest thermostat?
 

fitter30

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Didn't see anything in the manual other than the g wire is connected to star instead of g. If the fan runs on "fan on". Contact nest support might have a hardware/ software problem.
 
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nbpt100

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Didn't see anything in the manual other than the g wire is connected to star instead of g. If the fan runs on "fan on". Contact nest support might have a hardware/ software problem.
Maybe. I have not been able to connect it to my network connection now for months. It did when originally set up but for some reason it stopped working. I tried everything I could to get it to connect and it just will not recognize my network or any in the area. I did not really care because I have not used any of those fancy features.
 

BrandonV

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This thing powered with a common wire? Last Nest I had used to work fine without and then started acting funny as it got older without constant power from the transformer.
 
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chinboys

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Other than the indoor blower cutting in and out during the call for cooling did the blower finally come on and is the new condenser unit making cool air for you?
The Nest thermostat does require a common, ground potential reference. The Rh wire or signal is the 24V AC being used by the thermostat to redirect to the heat or cooling side's circuitry (safety interlocks, gas valve, winter season blower delay start, steady state run, compressor fan start, compressor run, cooling season blower start\run (need 400 ft3 per ton of cooling hence blower ought to run faster), etc). The thermostat Rh (power for the heat control) will pass over to be used by the cooling control during the call for cooling when you have only one 24 V AC transformer usually located in your furnace's circuit area.
You ought to ensure your transformer still provides> 24 V AC or replace it with a higher-rated one.
 
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nbpt100

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Did you turn off the furnace by any chance?
I have the thermostat set to Cooling. So the Furnace can not come on. There is no chance I will need heat until Early Oct. at the soonest.

I assume there is only one transformer. I will have to look. There was no problem this winter with the heating.
 

fitter30

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I have the thermostat set to Cooling. So the Furnace can not come on. There is no chance I will need heat until Early Oct. at the soonest.

I assume there is only one transformer. I will have to look. There was no problem this winter with the heating.
If there is just one R wire it's single transformer. Can pull the blower door, tape closed the safety switch, cycle power and measure G to C voltage then R to C compare should be within a .2
 
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nbpt100

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Would this symptom suggest the blower motor is starting to fail or if there is a start capacitor, that is starting to fail?
 
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