Is this furnace connected to a humidifier?

No, it's not connected to a humidifier.

Another possible problem could be the G wire is shorting to 24 V somewhere. If you have a voltmeter take a measurement between the G terminal and ground. With the system set to Off the reading should be zero. Do this when the fan is not running and when the fan is running by itself.

I couldn't find a electrical schematic online for your unit so if the furnace has one glued to the cover take some really good pictures and post them.

You say it's in the attic? How hot does it get up in your attic? The reason I ask is related to the high temp limit switch located to the right of the main gas valve. These switches are set to turn the fan on and off at between 100 -130 degrees and the attic temperature temp may be triggering the switch because it's out of calibration (broken) or crud on it. I need to see a schematic to determine how it's wired in to the control board and also while I was searching for a schematic I notice the replacement OEM control board (031-01266-000) is upgraded and doesn't exactly fit right without drilling some new holes. If it's the control board there is nothing you can repaired unless the same components can be found to solder in.

@mike_home

Did you mean to measure the G terminal on the board inside the furnace(that is, not the G terminal on the base the thermostat in the room)?

I will have a problem to check the G terminal on the board while the fan is running by itself, because the fan runs randomly, easily to miss. Also, in order to access the board, I need to open the door of the furnace, as soon as I remove the door, the switch on the door is released and there is no more power supply to the whole system...

Thanks!

@outpaientzero

It's fall now so the attic is not hot. Where's the high temp limit switch? Can you please show me in the picture I attached?

You are right, I tried to google "50A50-241 White Rodgers", I couldn't find any new part, but used part on eBay.

You mentioned that when the fan is not running by itself, you are able to turn on and off the fan from the thermostat. That would imply you still have an interrupted connection back to the control board. Therefore making a measurement at the thermostat would give an indication of what is going on. It is easier to measure at the thermostat, but at the control board would be better if you could bypass the safety switch.

You should take a voltage reading when the fan is not running by itself and verify it is zero volts. It could turn out the G connection is at some voltage between 0 and 24 V. That voltage could be intermittently energizing the fan relay on the control board. That could explain why the fan runs with no set pattern.

@mike_home

When I measure the voltage of the G connection, I think I should touch the red probe of the meter to G, but where should I put the black probe of the meter? (sorry I am new to this).

What may have cause the G connection some voltage between 0~24V? short? If so, should I switch the green cable with one of the spare cables available?

Thanks!

HU-373650237, the fan won't run if you disconnect the three wires but you need to push in the button on the frame to turn on the furnace to get the 24Vac transformer to work. The meter should be set to Vac and not Vdc.

You should make a measurement of the G wire to ground. Kevin9408 makes a good suggestion of disconnecting the wire at both ends so neither the thermostat or the control board will influence the measurement. If the thermostat has a C wire, then that is a ground connection. Otherwise you have to this at the control board.

This troubleshooting is getting a bit involved so if you are not comfortable doing all this then it may be best to hire a competent tech.

If you do have a spare wire in the bundle, then switching the current G wire to another would be a good test. If after switching wires the fan still runs intermittently, then the problem is probably not a short.

New update for my latest test:

I disconnected the Green cable from the "G" terminal on the furnace control board(50A50-241 White Rodgers, see the attached photo), 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. I heard 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 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 will then follow the suggestions above to measure the G wire with a meter, and I have 3 spare cables to replace the Green cable.

Many thanks!

@outpaientzero

Thank you so much for your suggestions in great detail, really appreciate!

I did a test with Green cable disconnected from the control board and I posted the test detail above.

I can't tell the speed of the fan. When the fan previously randomly turned on(even SYSTEM=OFF), I noticed it ran like the heat was on. But frankly speaking, I didn't realize the fan ran faster for A/C. The winter is coming, I don't think the A/C will turn on before next May.

When you said "pulling the "Y" and "W" wire" when I measure the voltage of "G" wire, did you mean I should disconnect ALL 3 wires: G, Y, and W from the furnace control board, before I do: red probe -> G(green) wire(NOT the G terminal on the board), and black probe -> ground(metal of the furnace)?

Great analysis about the leakage(short) and the coil!

Does the condenser only work for A/C? If so, I doubt it's relevant, but I agree no harm to do that test.

Thanks for the reminder about "The meter should be set to Vac and not Vdc". If I disconnect G, Y & W wires from the board, and I believe my thermostat doesn't run on battery but get the power from the control board in the attic. If G wire has voltage, can I assume the thermostat gets power from R or C of the control board?

Thanks again!

@mike_home

Thanks for your continued support! I did a test with Green cable disconnected from the control board and I posted the test detail above, good sign!

I thought Kevin9408 suggested me to disconnect ALL 3 wires: G, Y & W from the furnace control board, while keep the wire connections to the thermostat untouched. But you mentioned to disconnect the G(Green) wire at both ends(furnace and thermostat?). I am confused here, please clarify. Thanks! Also, I found two wires(blue & white) connecting to the C terminal on the furnace control board, is that the "ground" for the measurement?

I love to do this troubleshooting and I love to learn from you guys, but hopefully I don't have to deal with this every year :)

Yes, I do have another 3 spare wires(orange, brown and black unused) for test, so if the G wire is the root cause, I will swap the wire.

I suggested disconnecting the G wire from both ends to isolate it from the control board and thermostat. If you read any voltage then that would indicate a partial short to 24 V. You could then connect it to the control board and repeat the test. Your problem is only the fan coming on intermittently and not the furnace or AC. So I don't see the need to also disconnect the Y (cool) or W (heat wiring). If you do measure any voltage on the G wire you could disconnect the other wires are remeasure to see if it causes any change. Intermittent problems are usually traced back to a strange situation. It is good to trace every possible cause.

Disconnecting the G wire from the control board is a good idea if the furnace control board turns the fan on when the furnace fires. Disconnecting the G wire eliminates the possibility of a wire short as the cause. That was good idea. I wish I had thought of that!

The condenser runs for AC only. Most furnaces have a least one fan speed setting for heat, and another for cooling. This allows the fan speed to be set for various furnace and condenser sizes.

The R connection is the 24 V supply. The C connection, if your thermostat has one, is the ground. Most thermostats have a battery back up so they won't lose their settings during a power outage. Thermostat acts as a switch between the R connection and W, Y, and G connections. During a call for heating or cooling, the thermostat energizes the W, Y, or G connections to 24 V.