Burners go out

erniemit · 03-14-2009, 03:33 PM

The filter looks good and is almost new; problematic with or without filter installed. The limit switches are all fine, closed circuit through four temp switches in burner area and also 160/30 one in flu box. (Ohm meter checked zero; and if any are unplugged nothing starts, so I know they are fine.) Ingitor is fine too, gets red hot and burners do come on, but often go off as ignitor stops glowing. Cleaned the fire sensing rod with scotchbrite and replaced that. The fire sensor is on the opposite side, six burners away from the burner with the ignitor. So it appeared that the fire sensor was not telling that it was hot and the system shuts down. (did not know how to test a fire sensor) Cycling power several times will finally get it going. Replaced fire sensor (yikes...$50) and problem is still intermittant. Went outside and there seems to be a steady blowing flow of hot wet air coming out of the PVC pipe when the furnace is running. Once running, it will continue to run until thermostat temp is achieved and turns off. Then the odds of it coming back on are dicey, often wake up next morning in 60 degree home with thermostat set for 67. Cycle power at furnace, wait for burners to light and quit, cycle again, wait, three to six times later it catches and runs until it is warmed up the house again. Obviously, if a repair man came over, chances are that we would find it working fine at the time. What's next? Take apart the squirrel cage? Snake the PVC and look for an old bees nest? Don't want to tear it all up for nothing. There was some water puddled near the furnace after rain, vacuumed that up. Advice in this would be helpful.

yuri · 03-14-2009, 03:46 PM

Post the make, model and serial # and we may help.

erniemit · 03-14-2009, 03:56 PM

I can't figure out how to cancel a message i the post

yuri · 03-14-2009, 04:00 PM

Good. Look up one post, and post that info.

erniemit · 03-14-2009, 04:52 PM

GMPN120-5

The serial number is hard to make out, its either:

9607829045
8607829045
or maybe
9607828045
8607828045

Hope that helps.

yuri · 03-14-2009, 05:27 PM

I have worked on lots of them Goodmans. Try cleaning the face of the burner in front of the flame sensor with emory paper. If there is a viewing window in the bottom door look thru it for a flashing red LED which will give you an error code when that is happening. Find the code on the back of one of the doors. How old is the unit.

erniemit · 03-14-2009, 06:00 PM

It's got a factory inspection date of 1996. No peep window, so I held the door open and the door switch pressed to allow it to turn on and there was just the red LED on, which I thought was just a power indicator, and there was no blinking message. The flame on all burners looks good and consistant from burner to burner. I'm sure there is plenty of flame on the flame sensor, which I cleaned and then replaced. Do I now take the burner out to clean it or reach in and be careful I don't bump the new sensor? There is a screw on the back of the burner that aoolws a cover to rotate on the inflow hole...they were wide open...I lostened the screw and closed them a 1/16 turn (to mid vent position) and the flame is still blue, but that does not seem to do anything really. Besides the limit switches and flame sensor, what else can shut down the gas soon after the ignitor successfully lights the flame? (I thought that the nozzles may be pushing too much air into the flu box, and that is why I tweaked them down a bit.)
Now, I'm also curious how a flame sensor works, especially since it only has one wire going to it: Does it generate electricity from heat or does it conduct to ground through the carbon in the flame to the burner edge; e.g., why that burners edge needs burnishing? If it's a small current flow, could the flat spade connector at the wnd of the wire going to the flame sensor need some cleaning of sorts or replacement for better conductivity at the push-on connection?

yuri · 03-14-2009, 06:38 PM

DC flame rectification:http://www.fieldpiece.com/tech-artic...-rectifier.htm
I have worked on the newer ones from 2003 and on. If the face of the burner is corroded it weakens the flame signal back to the board, which in your case may be faulty from age.

kenmac · 03-14-2009, 07:20 PM

You might also try jumping out the vaccum switch.It may not be getting enough vaccum from the vent motor for the furnace to stay on or , it may have just gone bad.....also check t.stat

erniemit · 03-15-2009, 08:43 PM

Where's a good on-line store for getting a new board for this furnace?

kenmac · 03-15-2009, 09:09 PM

Try google furnace parts