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Discussion Starter #21
1) remove the G wire on the circuit board ( board ) on the G terminal and put some tape on the end of it. Do not use the AC with it off. Wait a day or two and if the fan starts you may have a problem with the board.

2) If it runs with the G wire off then tap the board with the butt end of a big screwdriver in several places and if the fan stops then the fan relay in the board is faulty and shorting on. That is a older furnace ( 15-20 yrs old ) and the board may be dying from old age/have a problem.
Hi Yuri,

I followed your instruction and had the observation and test below:

* Before disconnecting the Green cable ( see the attached picture#1 )*
10/12, SYSTEM=OFF, FAN=AUTO:
I heard the very short(<1 second) buzzing sound(not sure if "buzzing" is the right word, but the sound was just like a motor tried to start but immediately stop, or like the sound when sump pump starts) from the attic, where the furnace is located, for about 5~7 times a day(mostly in the early morning or late evening as I noticed, but I still couldn't find the pattern), it sounded to me it tried to start the motor for 2~3 times within a half minute period, then stopped trying. This has been an ongoing issue for several weeks.
10/13 morning, SYSTEM=HEAT, FAN=AUTO:
My wife turned the HEAT on, after reaching the target temperature, the FAN kept running, while the air blew out no longer warm, so I assumed the furnace turned the fire off. I waited for >40 minutes, the FAN was still running. Then I forced to set SYSTEM=OFF. After about 5~6 minutes, the fan finally stopped.

Green_connected.jpg

** After disconnecting the Green cable ( see the attached picture#2 )*
10/13 noon, I went up to the attic, disconnected the Green cable from the board, and left SYSTEM=OFF, FAN=AUTO

Green_Disconnected.jpg

It has been more than 24 hours since I disconnected the Green cable(and kept SYSTEM=OFF), I haven't heard any sound from the attic yet, but I will wait for another two days.

=======================

When I was at the attic, I noticed another Green cable detached, I am not sure what's that for. I attached the picture below, can you advice?

Green3.jpg

Also, I want to mention again another test I did before(which I mentioned in my original post by following the Pepco staff's advice): I disconnected the front part of the thermostat(the main part of the thermostat, with circuit board, LED screen and control buttons…) from the wall part(where the color cables are connected), the problem persisted: either the fan ran for 30+ minutes then stopped, or I heard the short(<1 second) buzzing/starting sound from the attic.
So I don't think the problem is because of a bad thermostat control board, but I don't know about if something went bad with the connector part of the thermostat, or something wrong with the green cable(from the thermostat to the control board in the attic…

The symptom of the problem sounds to me like this: when SYSTEM=OFF or (SYSTEM=ON but FAN shouldn't be running), the fan tries to start(when I could hear those short buzzing sounds), when it fails to start, it may try again hours later. When it can successfully start, it runs for 30~40 minutes then stops)

Thanks!!
 

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The other green wire. Is just a spare wire, from the bundle that is ran to your outdoor unit.
 

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If the fan kept running for 40 mins after a normal heat cycle then I suspect the fan relay inside the board is sticking on. It may be shorting also.

I suspect you need a new board. Sometimes we have to replace parts based on symptoms and there is not always 100% diagnosable proof.
 

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Discussion Starter #25
If the fan kept running for 40 mins after a normal heat cycle then I suspect the fan relay inside the board is sticking on. It may be shorting also.

I suspect you need a new board. Sometimes we have to replace parts based on symptoms and there is not always 100% diagnosable proof.
It has been 2.5 days since I removed the green cable from the control board in the furnace at attic, I haven't heard any sound from the attic or the fan runs randomly, while the SYSTEM=OFF (I am afraid to set SYSTEM=HEAT, and I told my wife not to turn on HEAT, to avoid the heat turns on without a running blower/fan and then cause fire, I could be wrong though). In this case, do you think the problem could be on the "base" of the thermostat or the green cable?
Now I haven't seen the symptom(at least for 2.5 days), can I conclude the board is good? (I assume removing the G cable = I previously set SYSTEM=OFF with G cable connected, correct me if I was wrong)

Thanks!
 

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It has been 2.5 days since I removed the green cable from the control board in the furnace at attic, I haven't heard any sound from the attic or the fan runs randomly, while the SYSTEM=OFF (I am afraid to set SYSTEM=HEAT, and I told my wife not to turn on HEAT, to avoid the heat turns on without a running blower/fan and then cause fire, I could be wrong though). In this case, do you think the problem could be on the "base" of the thermostat or the green cable?
Now I haven't seen the symptom(at least for 2.5 days), can I conclude the board is good? (I assume removing the G cable = I previously set SYSTEM=OFF with G cable connected, correct me if I was wrong)

Thanks!
If your R wire is somehow shorting out with your G wire then you could have intermittent fan operation. It could be caused by a screw or nail piercing the cable, squished by drywall at time of installation causing trauma inside the cable jacket, or any other occurrence. If the problem goes away when the wire is removed at the unit, but still present when only the thermostat is removed from the wires, then I would say the issue is probably within the cable and that should be what you check for fault.

You can check for continuity between all wires when they are disconnected and isolated at both ends. If you have an extra wire you may be able to utilize that spare wire in place of the G wire if you find the G wire is in fact shorted. It may be easier to simply pull a new wire if you don’t have the ability to check continuity. Sometimes a megger is a better solution for checking the wires in this situation. Be sure the wires are disconnected first either way you go.
 

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The green wire doesn't need to be connected to the furnace board to use heat. The furnace board brings on the fan by time during a heat call.

Sounds like you just need to pull a new thermostat wire bundle.
 

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The furnace does not use the green wire for heat, only AC. You can safely run heat w/o that wire as the board uses a timer to start the heat cycle fan. Plus you have a high air temp limit control to stop it if the fan is not running.

I would leave G off and test the heat. If the fan does not stop after a heat cycle then the board is shot as the fan relay is sticking or shorting.

If it works OK then likely you have short in your thermostat wiring which is very very rare but can happen.
 

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Hi, I haven’t read the whole post but , with the green wire connected at the furnace and removed from the subbase is the fan still Off?
 

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Discussion Starter #30
The green wire doesn't need to be connected to the furnace board to use heat. The furnace board brings on the fan by time during a heat call.

Sounds like you just need to pull a new thermostat wire bundle.
Ah, good to know, thanks!!
 

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Discussion Starter #31
If your R wire is somehow shorting out with your G wire then you could have intermittent fan operation. It could be caused by a screw or nail piercing the cable, squished by drywall at time of installation causing trauma inside the cable jacket, or any other occurrence. If the problem goes away when the wire is removed at the unit, but still present when only the thermostat is removed from the wires, then I would say the issue is probably within the cable and that should be what you check for fault.

You can check for continuity between all wires when they are disconnected and isolated at both ends. If you have an extra wire you may be able to utilize that spare wire in place of the G wire if you find the G wire is in fact shorted. It may be easier to simply pull a new wire if you don’t have the ability to check continuity. Sometimes a megger is a better solution for checking the wires in this situation. Be sure the wires are disconnected first either way you go.
Yeah, I do have three extra lines as Yuri pointed out before, I will try that. Thanks!
 

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Discussion Starter #32
The furnace does not use the green wire for heat, only AC. You can safely run heat w/o that wire as the board uses a timer to start the heat cycle fan. Plus you have a high air temp limit control to stop it if the fan is not running.

I would leave G off and test the heat. If the fan does not stop after a heat cycle then the board is shot as the fan relay is sticking or shorting.

If it works OK then likely you have short in your thermostat wiring which is very very rare but can happen.
Thanks for the test plan! (y)
I will leave the Green wire disconnected from the control board in the furnace, and set SYSTEM=HEAT. Good to know it's not dangerous to do so.
I will provide update soon.

BTW, it has been three days since I disconnected the G wire and set SYSTEM=OFF, I haven't noticed any sounds from the attic and the fan no longer started randomly.
 

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Discussion Starter #33
Hi, I haven’t read the whole post but , with the green wire connected at the furnace and removed from the subbase is the fan still Off?
Previously(before I disconnected the green wire per Yuri's suggestion) the fan could run randomly even I set the thermostat SYSTEM=OFF.
What did you mean "subbase"? I once tested with the main part of the thermostat removed from its base(the base had a few color wires connected), the fan could still run randomly. So I agree with the Pepco staff that in this case, the problem is not because of their thermostat(well, it could still be the wire connection they did, or a bad wall/base part…).
 

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The sub base is where the wires connect to Stat on the wall, remove the wire that is on the “G” terminal, and be sure to connect the green wire on the control board, report what happens, the sub base could be bad.
Did this all happen after the Stat was changed?
 

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New update for my latest test:

I disconnected the Green cable from the "G" terminal on the furnace control board(50A50-241 White Rodgers), while I didn't change any connection from the thermostat end, but set SYSTEM=HEAT on 10/16, since then the system has been working well as expected for more than a day, that is, the fan ran when the furnace started heating, then stopped within one minute after the room temperature reached the target number, repeated for a few times through the day to warm up the house, that's a good sign; also I have no longer heard any weird random/intermittent, short sound from the attic(the sound might indicate the fan tried to start but failed within one second, even I previously set SYSTEM=OFF, FAN=AUTO).

I will leave the Green cable disconnect and observe for another few days, considering I learned that I don't need the G cable for heating for the winter so no risk to set SYSTEM=HEAT without the Green cable connected. If everything looks back normal after a few days, then I think both the furnace control board and the thermostat control board are NOT bad(correct me if I was wrong), the problem could be on the Green cable, or its connection to the thermostat(something wrong with the thermostat wall base, where all the color wires are connected), or its connection to the furnace control board. I have 3 spare cables to replace the Green cable.

I will post another update early next week.

Many thanks!
 

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Discussion Starter #36
The sub base is where the wires connect to Stat on the wall, remove the wire that is on the “G” terminal, and be sure to connect the green wire on the control board, report what happens, the sub base could be bad.
Did this all happen after the Stat was changed?
Originally when I noticed the problem back in early September, I suspected a bad thermostat and called the utility company. The staff came onsite and replaced the main control part of thermostat but left the base(the wall part with wires connected) untouched/unchanged. He asked me to remove the main control part of the thermostat to test while he was leaving the house, and I followed, then found the problem persisted(fan ran randomly) with the thermostat off from its base, then I thought I had a good thermostat(at least the main control part, not sure about the base or its wire connections)…
 

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Originally when I noticed the problem back in early September, I suspected a bad thermostat and called the utility company. The staff came onsite and replaced the main control part of thermostat but left the base(the wall part with wires connected) untouched/unchanged. He asked me to remove the main control part of the thermostat to test while he was leaving the house, and I followed, then found the problem persisted(fan ran randomly) with the thermostat off from its base, then I thought I had a good thermostat(at least the main control part, not sure about the base or its wire connections)…
I really think it’s the cable that is the problem based on what you have been describing and reporting. Typically a tstat base is just a terminal block. Not much to go wrong there, you can get bad connections but hard to get a short on them, not to say it can’t happen, just uncommon in comparison. The control board typically won’t cause the fan to randomly turn on due to a bad G terminal connection. The most common issue is wire damage. Sounds like you have a short in your cable. Pull a new wire if you can, if not then check the other wires for shorts using a meter (after disconnecting the wires at both ends). Use a wire that checks out good as the new G wire and it would be good to swap out the R wire with another known good as well since that would have damage to it also.

I would be curious to see the thermostat front, back, and base. If you could post pictures of that it might help provide additional info. I don’t recognize it from your description but many thermostats are relabeled and sold as a different name.
 

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I really think it’s the cable that is the problem based on what you have been describing and reporting. Typically a tstat base is just a terminal block. Not much to go wrong there, you can get bad connections but hard to get a short on them, not to say it can’t happen, just uncommon in comparison. The control board typically won’t cause the fan to randomly turn on due to a bad G terminal connection. The most common issue is wire damage. Sounds like you have a short in your cable. Pull a new wire if you can, if not then check the other wires for shorts using a meter (after disconnecting the wires at both ends). Use a wire that checks out good as the new G wire and it would be good to swap out the R wire with another known good as well since that would have damage to it also.

I would be curious to see the thermostat front, back, and base. If you could post pictures of that it might help provide additional info. I don’t recognize it from your description but many thermostats are relabeled and sold as a different name.

Please see the pictures attached below for the thermostat:

631278

631279

631280



I was surprised that I couldn't see the "spare" wires(black, orange, and brown) from the thermostat side, while I could see them from the furnace control board side:

631281


631282


( for now, I keep the green cable disconnected from "G" on the control board in furnace. )

If I really don't have the spare wires, then I may have to figure out the Green cable.
If I do "disconnecting the wires at both ends" as you said, how can I measure the voltage? Sorry I am confused here, and you please explain more detail. Also, why the "R" cable? I thought only the green cable could be bad…

Thanks!
 

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Please see the pictures attached below for the thermostat:

View attachment 631278
View attachment 631279
View attachment 631280


I was surprised that I couldn't see the "spare" wires(black, orange, and brown) from the thermostat side, while I could see them from the furnace control board side:

View attachment 631281

View attachment 631282

( for now, I keep the green cable disconnected from "G" on the control board in furnace. )

If I really don't have the spare wires, then I may have to figure out the Green cable.
If I do "disconnecting the wires at both ends" as you said, how can I measure the voltage? Sorry I am confused here, and you please explain more detail. Also, why the "R" cable? I thought only the green cable could be bad…

Thanks!
The R wire supplies power to all the other wires at the thermostat connection point (with exception only for the C wire). The thermostats job is to switch this power on and off when it’s required. In your case the switches are situated like this...

R to G
R to W
R to Y

C is your common wire and is used only by the thermostat at that connection point

If G is receiving a command to turn the fan on then it must be getting that power from another wire. Since R is the only wire that is constantly energized when the tstat is pulled off the wall then it must be the one that is supplying the power to the G wire. Since it’s not consistent then it is probably a poor contact short making it more difficult to find and confirm (this is why I recommend replacing the wire if it’s easy to pull a new one). So with all that said, if G wire is damaged and receiving voltage then another wire also needs to be damaged to supply the voltage, R wire fits that bill in this situation.

... regarding testing ...

You are disconnecting the wires so there is no voltage on them. Using a voltmeter set to Ohms (resistance) you can check for continuity between all wires against themselves and to ground. If the meter reads OL then that is good, if you detect any sort of resistance then that is bad. I think you are going to have a hard time confirming the damage this way since it seems hit or miss as to when it’s making contact. I’m hopeful that the short is poor because you will pick that up, but if it’s intermittent based on temperature or some other variable causing the wires to physically separate and connect then you may only detect it if you happen to test it under the right circumstances (hence the reason for replacing the wire if possible).


The other wires are tucked in behind the thermostat base inside the wall. You will find them when you pull out the cable further. They hide them there because there is not much room for storage inside the thermostat base.


You could just leave the R wire even though there is suspected damage to it but you run the risk of a spare wire grounding out and causing issues when the short energized that spare wire. It wouldn’t be a bad idea to tape off all the spare wires so the ends are not exposed if you decide to keep this cable. You could strip the ends and cap them all together or individual, it wouldn’t matter either way as long as they are secure and isolated.

Hopefully that is clear, let me know if it doesn’t make sense,
 

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If you pull the base off the wall(remove the screws) you will see the spare wires.
 
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