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Nest - furnace fan runs but AC doesn't work

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  • May 19th, 2018 12:06 pm
[OP]
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Feb 26, 2016
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Vaughan

Nest - furnace fan runs but AC doesn't work

I've had a nest for almost two years now and i'm really starting to regret this piece of junk..

first year of install, I had no problems with heat or cool. Now that winter is over, i've removed the ac unit cover and tried to turn the AC on. The nest thermostat says it's cooling down the house but then i realize that it's just blowing out air and not cold air. I let it run for about an hour and the temperature doesn't seem to go down but instead increases. I go outside to see if the AC unit is running and it's not. Is this a Nest problem or is my AC unit busted? i'm gonna install my old thermostat tomorrow to find out but wondering if anyone has experienced this before. Nest wiring seems fine too. everything seems to be connected properly: yellow wire to Y1 - Cool, white wire to W1 - Heat, green wire to G - Fan and red wire to RH - Power.
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Dec 12, 2009
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Based on what I have read online, it seems like the nest uses the AC wire for charging the battery. Apparently the way it works is that the impedance on the charging circuit is so high that generally the solenoid does not actuate. I am wondering if keeping a small current flow on this coil may damage it. Personally, I would definitely not go with a thermostat that has a rechargeable battery. It is a 4-5 year death clock. Add in the half baked work around for a 4 wire arrangement, it could easily be a disaster in waiting.
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Most likely problem with AC
low on refrigerant
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Feb 11, 2007
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Oakville
Did you remember to turn on the breaker at your panel AND outside next to the AC?
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Brampton
Keigotw wrote: Most likely problem with AC
low on refrigerant
Doesn't work that way. Typically the Compressor fan will still work, even if Low on refrigerant. The compressor just goes in to "Bypass" or doesn't run.
engineered wrote: Did you remember to turn on the breaker at your panel AND outside next to the AC?
This is where I would start. If your AC ran before and you didn't change anything.

I'd look elsewhere than the Nest.

Your Compressor unit has either a Start run cap or separate start and run caps. You haven't posted any information on your AC unit age or model etc....
There's a good chance your breaker is off at the main panel or at the compressor unit. The capacitor maybe dead. If you want to rule out the Nest just connect Y1 to RH and see if it runs. If it doesn't then you know the Nest has nothing to do with it.
It's also possible that the relay on your control board is dead.
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Oct 6, 2010
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Also, test/clean the contactor. When the thermostat calls for power, the contactor needs to engage otherwise, the fan will just spin. This is a little box that is connected to the main power and the capacitor.

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koffey wrote: Also, test/clean the contactor. When the thermostat calls for power, the contactor needs to engage otherwise, the fan will just spin. This is a little box that is connected to the main power and the capacitor.

Image
Right that's the part name I couldn't remember. It's usually either the cap or contractor that fails, especially now that caps don't have PCBs in it.
[OP]
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Feb 26, 2016
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Vaughan
engineered wrote: Did you remember to turn on the breaker at your panel AND outside next to the AC?
I have to double check my breaker panel in my basement but I would never have turned it off. i did check the one outside next to the AC and it appeared to be on.
[OP]
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Feb 26, 2016
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Vaughan
tebore wrote: Doesn't work that way. Typically the Compressor fan will still work, even if Low on refrigerant. The compressor just goes in to "Bypass" or doesn't run.


This is where I would start. If your AC ran before and you didn't change anything.

I'd look elsewhere than the Nest.

Your Compressor unit has either a Start run cap or separate start and run caps. You haven't posted any information on your AC unit age or model etc....
There's a good chance your breaker is off at the main panel or at the compressor unit. The capacitor maybe dead. If you want to rule out the Nest just connect Y1 to RH and see if it runs. If it doesn't then you know the Nest has nothing to do with it.
It's also possible that the relay on your control board is dead.
thanks! that's very helpful! I'm going to try that tonight. as far as the AC unit, my house is about 6-7 years old so the ac unit is about the same age. i can't remember the make or model and will have to check that as well.
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otaknap wrote: I have to double check my breaker panel in my basement but I would never have turned it off. i did check the one outside next to the AC and it appeared to be on.
OK, we'll check both again and flip them both off and on. One of them may have tripped on its own.
You should be shutting the breakers off to prevent accidental use in the winter.
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otaknap wrote: thanks! that's very helpful! I'm going to try that tonight. as far as the AC unit, my house is about 6-7 years old so the ac unit is about the same age. i can't remember the make or model and will have to check that as well.
I've seen contactors and caps go in the 5-6 year range. They really just don't make them like they used to.
It's very evident when you pull a contactor or Capacitor out of of a 30 year old unit

If it's really the cap. Don't get fooled in to paying $100+ for a part. It's around $10-50, 50 would be for an original GE Gentec part.
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soupmaster666 wrote: You can easily rule out the thermostat by shorting "Y" to "RC". RC may have a link to "RH", or one terminal may be labelled RC/RH.



The Nest stat does charge itself through the furnace/AC by trickling power, but only if you don't have a "C" wire. The C wire exists specifically to power thermostats and accessories. This house and A/C is 6 years old so it would absolutely have one.
I suggested this could possibly be an issue because the OP did not identify the presence of a C wire.
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[OP]
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Feb 26, 2016
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Vaughan
engineered wrote: OK, we'll check both again and flip them both off and on. One of them may have tripped on its own.
You should be shutting the breakers off to prevent accidental use in the winter.
soupmaster666 wrote: You can easily rule out the thermostat by shorting "Y" to "RC". RC may have a link to "RH", or one terminal may be labelled RC/RH.



The Nest stat does charge itself through the furnace/AC by trickling power, but only if you don't have a "C" wire. The C wire exists specifically to power thermostats and accessories. This house and A/C is 6 years old so it would absolutely have one.
will888 wrote:
I suggested this could possibly be an issue because the OP did not identify the presence of a C wire.
tebore wrote: I've seen contactors and caps go in the 5-6 year range. They really just don't make them like they used to.
It's very evident when you pull a contactor or Capacitor out of of a 30 year old unit

If it's really the cap. Don't get fooled in to paying $100+ for a part. It's around $10-50, 50 would be for an original GE Gentec part.
So i've checked again the basement breaker panel and it's definitely on. I reset it just to be sure it's on. I've also checked the one outside (see attached picture) and it looks on to me as well. i also took a picture of my AC unit which is a Lennox. lastly, i've attached a picture of my nest wiring. As mentioned by some of you, I don't have a C wire. I tried switching the Y1 wire to the RH Wire but had no success. the nest won't even allow me to turn the ac on with this wiring layout.

any other ideas? what's interesting is that i saw the fan in my AC unit turn a little bit when i set the AC on but i'm not sure if it was just the wind... it made approximately half to one very slow rotation and then stopped. I don't want to go to the extent of trying to replace parts myself because i really don't know enough about opening the unit up. Might just have to call someone..
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otaknap wrote: . I tried switching the Y1 wire to the RH Wire but had no success. the nest won't even allow me to turn the ac on with this wiring layout.
I think the mean connecting/shorting the Rh/Rc and Y wire together to send a power on signal to the A/C unit, if that is not what you did. You don't use the Nest when you do this.
what's interesting is that i saw the fan in my AC unit turn a little bit when i set the AC on but i'm not sure if it was just the wind... it made approximately half to one very slow rotation and then stopped.
If it turned so slowly, it'd be humming. You might want to get a voltage sniffer and see if power is going to the condenser and fan, or if there is power to the units at all.
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[OP]
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Feb 26, 2016
872 posts
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Vaughan
thriftshopper wrote: I think the mean connecting/shorting the Rh/Rc and Y wire together to send a power on signal to the A/C unit, if that is not what you did. You don't use the Nest when you do this.

If it turned so slowly, it'd be humming. You might want to get a voltage sniffer and see if power is going to the condenser and fan, or if there is power to the units at all.
ah.. i didn't understand that. what do you mean by you don't use the nest when you do this? how would you turn the ac on without the thermostat? apologies for my lack of knowledge..

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