Ducane furnace is acting up

[rangard59]

I have a Ducane 80% gas furnace. It will work OK for several days or a couple of weeks, then it will click on, the igniter will light, but the burners will not light, until I turn off the thermostat, let the blower stop,then turn on the thermostat and furnace will turn on and work ok. Or when the burners won't light , if I go to the furnace and take off the main cover and tap on the gas pipe going to the control valve the burners will light. This has been happing for several months now. A couple of days ago I took out the three burners and cleaned them up, they were not too dirty,and I put it back together and it worked for a couple of days, but it is still not starting up some times.I also took off the clear hose to the pressure switch and blew in that to clear out any dirt. The furnace is about 10 years old.
Anyone have any ideas what could be wrong with *
Thanks

[Freon]

You said, "...the igniter will light, but the burners will not light," Since the ignitor lights that tells me the draft pressure is adequate. Tapping on the gas line and getting the burners to light could indicate either a gas valve solenoid that's getting stuck or that there is no power to the valve due to a loose wire or control board problem.

First, CAREFULLY check all wiring to the gas valve. Do not try and fix any wiring unless the furnace is off. Make sure no wire is broken under the insulation.

Next, you need to have someone call for heat while you are at the furnace with your hand on the gas valve. You may need to call for heat several times until you get a burner flame failure. Your goal is to feel and hear the gas valve solenoid opening the gas valve. Your need to verify that when the burners do not light, you can not feel or hear the gas valve open. If you can feel and hear the gas valve opening with no subsequent burner flame then we go another direction.

Post the make and model of the gas valve and the furnace control board. Do not use dashes in your post, just spaces. Check your post after it's on this board for any * symbols that indicate something has been edited.

[rangard59]

Well I checked the wiring to the gas valve, from what I can see it looks OK. How can the wiring get broken under the insulation if it doesn't move, just wondering, does it get *
No one else is home right now to click on the furnace so I can watch it.
My furnace has worked fine for the last 2 1/2 days without skipping a beat. This might work OK for days without screwing up.
Gas valve is a Honeywell SV9501H2409
Furnace control board is ST912OC2010

[Freon]

I need to clarify one thing. You said the ignitor will light. It is important you describe this ignitor. Is it an electric spark that lights a smaller gas pilot light or is it a 3"x 0.5" cylinder object that glows red hot without any other flame present.

Sometimes the control wires may get kinked or abused in other ways during installation. It is not common but I have seen the single strand of a copper control wire broken yet the tough insulation is fine. In your case this possibility is unlikely.

[rangard59]

From what I can see, it has 2 blue wires and 1 black wire going to the ignitor,the igniter glows red then it lights, I don't see a smaller pilot light before it lights.
I did wake up this morning with the furnace running , blowing cold air, the house was 65 degrees but the thermostat was set at 67 degrees, I turned it off, waited a few minutes and turned it back on and it work all day like it should.

[Freon]

Your system is definitely behaving strangely. If the burners never light then the main house blower should never go on... the flame sensing circuit will stop the furnace and then do a retry.

But first let's stay with why the burners sometimes do not light. As previously described try and feel the gas valve solenoid working when there's a call for heat. It might be easier to turn off the power, label and then remove all the thermostat control wires from the furnace control board and then attach a wire to the W terminal with the other end showing one eighth inch of bare wire. Now when you touch the bare wire end to the R terminal at the control board it will simulate a call for heat. It will be easier to repeatedly call for heat this way while feeling and listening to the gas valve. Touch the wire and wait for the complete cycle of the burners lighting. Then remove the wire from R and wait until the draft inducer finishes the purge cycle. Then repeat.

Smart valves do have problems with their small printed circuit boards. But until we know for sure we'll do the test. Check all the plugs to be sure they're secure.

[rangard59]

Sorry I haven't got back to you, but I am layed up with a bad back right now, My furnace has worked OK for the last 4 days, once I am up and around I will get back on this.