# Venting fan runs intermittent in HVAC cooling mode...

#### Externet

Joined Nov 29, 2005
1,472
The motor fan that pushes burned flue gases out of a GMP 100-3 furnace is running a minute on, a minute off in cooling mode.
This may be also for #12 expertise.
The schematic shows the motor runs from a 120V enabling relay in a control PCB.

Seen that behavior before when gas was not igniting and 're-trying', 're-trying' in heating mode. But it is in cooling mode now, when such fan should be always off.

Any clues on what to check ?

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#### ebeowulf17

Joined Aug 12, 2014
3,274
The motor fan that pushes burned flue gases out of a GMP 100-3 furnace is running a minute on, a minute off in cooling mode.
This may be also for #12 expertise.
The schematic shows the motor runs from a 120V enabling relay in a control PCB.

Seen that behavior before when gas was not igniting and 're-trying', 're-trying' in heating mode. But it is in cooling mode now, when such fan should be always off.

Any clues on what to check ?
Can you share the schematic? Do you have details on the control PCB? Not much to go on for anyone who's not already familiar with that particular furnace.

Is the whole thing built around a microcontroller, or is it old school logic? Is it possible some other output is stuck on in a way that would make some subsystems act like it was trying to heat even while others were in cooling mode?

#### Externet

Joined Nov 29, 2005
1,472
Thanks.
The PV motor is the one that runs and stops when should be off as is in cooling mode. It is fed by 120VAC from a relay on a control board with unobtanium schematic.

The FS is the flame sensor that connects to the same control board. I ordered a new one just in case as the control board is reporting a "false flame detected" which could explain this PV flue venting motor coming on. But could also be a board failure. Its wiring is good.

The HSI is the hot surface igniter for the gas. (not turning on, should not turn on now. ) Good.

The W -white thermostat wire- (heat call) is now disconnected for diagnostics to ensure there is no call for heat.

The GV gas valve does not turn on as should be in cooling mode. Also, the gas supply is shut off. In series to it, a bunch of safety and overtemperature switches; not of concern now.

I would say the board is microcontrolled :
----> http://joescontrolboards.com/products/goodman-b1809913s-board

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#### ebeowulf17

Joined Aug 12, 2014
3,274
The FS is the flame sensor that connects to the same control board. I ordered a new one just in case as the control board is reporting a "false flame detected" which could explain this PV flue venting motor coming on. But could also be a board failure. Its wiring is good.
Certainly sounds like that "false flame detected" error is the culprit. Hope it's just the sensor, not the control board. Either way, sounds like you're on the right track.

It's been many years since I dealt with these, but I used to operate and maintain a coffee roaster with a variety of flame control and safety systems.

Which kind of flame sensor is it? Has anything else changed that could effect the flame sensor? If it's the rectifier style, have you checked for insulation issues, possible leakage paths, etc? If it's the UV optical type, is it possible there's a light leak somewhere allowing outside light to hit the sensor? I may be remembering this wrong, but I have vague memories that the UV sensors could be false triggered by sunlight, maybe?! I'm not sure of anything I just said - just brainstorming!

#### Externet

Joined Nov 29, 2005
1,472
Thanks.
Which kind of flame sensor is it?
It is the passive do-nothing insulated rod that conducts the grounded burner nozzles via the flame ions to the board detection circuitry... Nothing fancy at all.

Wondering if the last technician working in there probed the rod and its instrument placed a spike/into the sensing circuitry causing the board failure.

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#### KeepItSimpleStupid

Joined Mar 4, 2014
4,019

#### wayneh

Joined Sep 9, 2010
16,384
I have no idea what's going on but I'll offer one speculation. My system has a blower also, and it's partly under control of a pressure sensor. I think the logic is that, if the back pressure seen at the blower rises too high, the fan and the furnace shut off to prevent a possible CO event. Anyway, it's not unusual for condensate from the A/C to back up (due to a clogged takeaway tube) and interfere with the blower and pressure sensor.

It makes no sense to me what might cause your symptoms, but I thought I would throw it out there.

You should have tagged @#12 more aggressively so that you catch his attention.

#### thedoc8

Joined Nov 28, 2012
147
Even if the flame sensor is bad, should not do that in cooling mode. My vote is your controller board is bad.

#### Externet

Joined Nov 29, 2005
1,472
Smells like that.
But read somewhere that the flame sensor is always active. Perhaps part of safety in the design...

#### Externet

Joined Nov 29, 2005
1,472
Update...
Replaced the flame sensor today. The control board did not report 'false flame detected' any more in 20 minutes I was vigilant. Will check again tomorrow.
But the venting fan did the same. Starting and stopping repeatedly in cooling mode...

#### #12

Joined Nov 30, 2010
18,210
You should have tagged @#12 more aggressively so that you catch his attention.
I haven't been here this week, and more bad news: I've seen about 5 fuel burning furnaces in my life. They just aren't necessary in Florida, delivering BTUs in the range of 100,000 per hour when most homes here could get by on 15k to 20k from a cheap nichrome heater. I'm talking $50 for the heater option and$6 for a 5KW restring kit.

From where I'm standing, this looks like a new controller board should fix everything, then wayneh talks about the condensate drain having some influence on the chimney pressure??? I don't get it. Count me out. I'm incompetent on this one.

#### Externet

Joined Nov 29, 2005
1,472
Thanks, #12. I wish you never come across a gas burning furnace in Florida. That would be like meeting Satan.

#### Tonyr1084

Joined Sep 24, 2015
4,394
This past winter my furnace shut down because of a failed pressure switch. The switch didn't fail, it just got a little debris in it. I know you're talking about AC and not heat but I'm wondering if when the blower is on one of your air tubes is blocked and triggering the exhaust fan. This is only a guess, but worth looking into. It doesn't hurt to pull the tubes and clean out their orifice's.