Furnace not igniting if fan is not set on auto

[user242]

I have a Heating element pilot- it gets red hot and then ignites the burners. it has not been working for the past day but I realized by trial and error that if the main (blower) fan is set to auto and runs before the heat is called, the ignition works. The main fan stops when heat is called, there is a small fan that creates some kind of suction above the burners and after ~30 seconds, I see the ignition turning red, the burners start burning the gas and the main fan turns back on after a couple of minutes. When the main fan is not set on auto, the heat is called, the small fan starts and then stops after a minute (without the ignition turning on). After a few minutes the cycle starts again with no ignition and so on and so on.

Any idea what to look for?

Thanks

 

[Fitter30]

Look at the burner flame the blower off and on if the flame looks like it pushed down or changes the heat exchanger could be compromised. If burner looks good check voltages 120 vac to the igniter.