How to troubleshoot Nest offline?

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  #1  
Old 05-19-19, 02:22 PM
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How to troubleshoot Nest offline?

My Nest thermostat is offline. There doesn't seem to be power on the Nest thermostat regardless of how many times I press the thing. I have not tested each of the wires because I'm not sure what these wires represent.

Help is much appreciated!
 
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  #2  
Old 05-19-19, 02:31 PM
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Can you pull the thermostats off the base and post a pic of the connections ?
Do you have a meter..... check from R to C or R to Y or R to G and see if there is any 24vAC there.
 
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Old 05-19-19, 02:46 PM
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Heres's what I have.

R to W = nothing
R to G = nothing
R to Y = nothing

 
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Old 05-19-19, 03:43 PM
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Ok.... looks like a gas or electric furnace with A/C.
You have no C connection so the power has been lost at the furnace.
Is the service switch on ?
Is the blower door closed ?
Tripped furnace breaker ?
You need to find out why the furnace is dead.
 
  #5  
Old 05-19-19, 03:56 PM
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Yes, it's a gas furnace with A/C. The furnace is running though. The fan is running constantly now. If I shut the breaker or the switch at the furnace, the fan and furnace do turn off. However, it was on all this time though.
 
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Old 05-19-19, 04:35 PM
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You turn the service switch off at the furnace and the blower still runs ?
That doesn't sound like the switch is wired correctly.

You said the blower is running. Is the draft inducer blower on the front of the furnace also running ?
If it is..... there is open safety limit switch inside the furnsce.
 
  #7  
Old 05-19-19, 05:37 PM
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No, if I turned the switch at the furnace everything shuts down, the furnace as well as the blower, nothing runs.
 
  #8  
Old 05-19-19, 05:58 PM
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Yes, it seems to be the red wire has no power. I charged the Nest, put it back and this is what is shows. The thing, is where do I go from here to detect where the source of the power come from? The furnace is working fine.
 
  #9  
Old 05-19-19, 06:20 PM
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So, I found this YouTube video that show similar error to mine. However, is this panel located somewhere on the furnace on mine? Second, if this the main issue, why was it working for almost two years before the error shows up?
 
  #10  
Old 05-19-19, 06:29 PM
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Okay, so I open the bottom of the furnace and I saw this wiring. I think this is where the thermostat wires connect to. If so, how do I test for power?

 
  #11  
Old 05-19-19, 06:40 PM
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So, if the furnace is working but there is no power to my thermostat, could this mean that the Honeywell Furnace Fan Control Circuit Board ST9120G4012 needs replacement?
 
  #12  
Old 05-19-19, 07:37 PM
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First off..... the stat wire at the furnace is not the same one that arrives at the stat. There are different colors at each location which means there is a splice involved somewhere in the thermostat line.

No.... that board is fine. This is going to be a little tough to explain. The nest when used without a C connection.... steals charging power from the A/C part of the system. That means the charging loop is thru the Y. I'd imagine if you tried A/C you'd find it not working. Your A/C line is open. Possibly a break or low on refrigerant.

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Old 05-19-19, 08:25 PM
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First off..... the stat wire at the furnace is not the same one that arrives at the stat. There are different colors at each location which means there is a splice involved somewhere in the thermostat line.
I don't quite understand. At the thermostat, there are four wires: red, green, blue, and white. At the bottom of the furnace, it has four wires with same colors as well.

Regardless if it's stealing charging power from the A/C or not, I measured and it's zero. It should be 24v, correct? I can't turn on the A/C since the Nest thermostat is no longer working. So I can't verify if it's a break or low on refrigerant on A/C.
 
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Old 05-19-19, 08:36 PM
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My apology. I was looking at the nest supplied diagram. You are correct.... the wiring is the same.

You said when you charged the nest and put it back on the heat worked. That would mean you would have had to have 24v from the red(R) wire to the white(W) at the nest. The measurement would need to be made with the nest off the base. That should also mean you have 24vAC from red to green.

Touching the red wire to the white wire will start the heat.
Touching the red wire to the green wire will turn the blower on.
Touching the red wire to the blue wire will turn the A/C on although there could be a 5 minute delay timer in the outside unit.
 
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Old 05-20-19, 04:44 AM
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You said when you charged the nest and put it back on the heat worked.
No, the heat do not work. The Nest shows that there is no power. The furnace or blower turns on because I turn on the switch. Regardless if the switch at the furnace or breaker is turned on or not, there is no power to the Nest. The red wire has no power. The furnace blower automatically turns on when I turn on the breaker/switch regardless if I plugged in the Nest or not because at this point there is no power to the Nest.

Touching the red wire to the white wire will start the heat.
Touching the red wire to the green wire will turn the blower on.
Touching the red wire to the blue wire will turn the A/C on although there could be a 5 minute delay timer in the outside unit.
This is goo to know but at this point, the Red wire has no power.
 
  #16  
Old 05-20-19, 08:00 AM
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One possibility is that the thermostat wire may be shorted somewhere. Test by removing the red at the R terminal of the furnace to see if the blower no longer turns on when you power it up.
 
  #17  
Old 05-20-19, 08:07 AM
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Thanks, Astuff! I'll give that a try. If the blower stops when disconnect the R-wire from the board and switched the furnace back on then that means I still have power from the transformer, correct? And somewhere between this board and the t-stat is probably where the problem lies?
 
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Old 05-20-19, 09:50 AM
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Divide and conquer. Also should try disconnecting R at the Nest base in case it is what is shorted.

That's the theory as the blower should not normally be running without the thermostat calling for it. I assume the blower did not run 24x7 before. There is a "constant"/continuous option if blower wired to "CONT" terminal - see page 4 https://customer.honeywell.com/resou...0s/69-0781.pdf
 
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Old 05-20-19, 10:05 AM
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Ok..... I've been trying to find the technical diagnostics for that module.

I asked this previously.....
You said the blower is running. Is the draft inducer blower on the front of the furnace also running ?
If it is..... there is open safety limit switch inside the furnace.

Are both blowers running ?
 
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Old 05-20-19, 11:32 AM
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You said the blower is running. Is the draft inducer blower on the front of the furnace also running ?
I don't know which blower is the inducer and which is not. I will look into it tonight. What I have noticed is that I do feel the air coming out of the vent on every room though.
 
  #21  
Old 05-20-19, 11:36 AM
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I assume the blower did not run 24x7 before.
Before the Nest thermostat died, I did schedule the blower to run for 15 minutes each hour. However, the Nest is dead (no battery) and every time turn on the furnace switch, the blower runs.
 
  #22  
Old 05-20-19, 12:05 PM
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You still didn't answer my question. There are two blowers. One moves the air thru the house and is usually down in the bottom section. The other is the draft inducer for the exhaust gases as on the front of the furnace. Are they both running ?
 
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Old 05-20-19, 02:34 PM
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PJmax, both blowers are running.
 
  #24  
Old 05-20-19, 03:14 PM
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Also should try disconnecting R at the Nest base in case it is what is shorted.
I disconnect the red wire on the t-stat and when I switch the furnace back on, both browsers are running.
 
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Old 05-20-19, 04:03 PM
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Ok.... perfect. You've narrowed down the search.

Both blowers run when a limit switch is open to purge the system and to let you know there is a problem. Not all controllers do this and I couldn't confirm if yours did. So either you have an open limit switch or are missing the 24v from the transformer.

You're going to need to check for 24vAC from the transformer. If that 24v is present you will need to check the safety loop. Going by the generic wiring diagram there should be two limit switches. Those should be closed/shorted to be normal which means if you check from pin 1 to pin 6 on the multi connector.... there should be 0v.

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  #26  
Old 05-20-19, 05:17 PM
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Okay, so here's what I got so far.
I connected my multi-meter to the yellow and blue wires from the transformer and I do get 28v. However, when I connect the alligator clips to the white and red screws, I got zero volt. Next step is to check on the limits as suggested but I'm not sure which of these wires/switches are the one. Are there the ones labeled 1-4?

Th very bottom wires are the yellow and blue wire that come from the transformer. Here's a closer look at the two wires from the transformer.

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Last edited by PJmax; 05-21-19 at 03:52 PM. Reason: removed pic/resized/labeled pic
  #27  
Old 05-20-19, 06:49 PM
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You want pins 1 and 6 in that plug right under (or to the left in your picture) of the transformer connections.
It looks like pin 1 is red. Can't see pin 6.
 
  #28  
Old 05-21-19, 10:25 AM
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Looks like on page three it shows the diagram of the board I have.

https://customer.honeywell.com/resou...0s/69-0781.pdf
 
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Old 05-21-19, 04:46 PM
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PJmax, I put my probe to pin 1 and 6 and it shows 27v on the multi-meter. So, what do this leads me to next?
 
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Old 05-21-19, 04:49 PM
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As per the diagram I posted..... follow the those two wires to the limit switches.
One is going to be open.
 
  #31  
Old 05-21-19, 05:52 PM
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Okay, I followed those two wires and one goes to this.

And the other red wire goes to this location.

These two locations has one wire connecting to each other, making it a loop. Now that I found these two locations, what do I do with them?
 
  #32  
Old 05-21-19, 06:02 PM
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Here's a wider shot of the two locations.
 
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Old 05-21-19, 06:51 PM
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Check across those switches with your meter. Since they are supposed to be closed/shorted..... if you read 24v across them they are open. The one on the right side may have a push button between the terminals. Try pushing that if it checks open.
 
  #34  
Old 05-21-19, 08:01 PM
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Check across those switches with your meter.
I got 2.7v when measured.
The one on the right side may have a push button between the terminals. Try pushing that if it checks open.
I saw this little brown knob between the two pins and tried pushing but it's not going anywhere. I'm not sure if I should try pushing harder.
 
  #35  
Old 05-21-19, 08:04 PM
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Did you check the other switch ?
 
  #36  
Old 05-21-19, 08:12 PM
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The other switch measured .38v.
 
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Old 05-21-19, 08:24 PM
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You are measuring a loop. If you take a wire off of one switch..... you can't measure the other.

Maybe this will be easier...... take one wire off of each switch.
Put your meter to the ohms setting. Check both switches. They should both show as dead shorted which would be the same thing as shorting your two meter leads together.
 
  #38  
Old 05-22-19, 04:30 PM
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  1. Take the wire that goes from the Roll-Out-Switch to the Limit Control off and measure that in Ohms and it shows OL
  2. Take the wire that comes from pin 6 to the Roll-Out-Switch and the wire that comes from pin 1 to the Limit Control and measure two two switches. It shows OL as well.
Here's a diagram of what I did.
 
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Old 05-22-19, 05:20 PM
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This should be a quick and easy process.

So you are saying both of those switches are open ?
OL means open circuit. That would an extreme rarity.
When you touch the two meter leads..... you should measure near 0...... is that correct ?
 
  #40  
Old 05-22-19, 06:39 PM
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When I touched the two probes together, I do get zero. Regardless which switch I tested with the multi-meter it shows OL that is why I thought it was very weird. The only thing I didn't do is turn on the furnace while measuring. Should I be turning on the furnace to measure ohms?
 
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