A problem recently developed with my furnace. I increase the thermostat and I hear the furnace "click" on. I see the orange ignition light turn on and the flame begins to flow within 10 seconds or so and then the orange light fades away and the hot air flows - which I presume is normal. However, while the orange light is on, and before the flame begins to flow, cool air comes through the vent. What has started to happen is the light would turn on, the flame would begin to flow and as the orange light would begin to fade, the flame would stop and thus no hot air was being produced.

It is a Rheem Criterion II gas furnace.
Model RCBA-2457GG14
Serial M2897
Forced air
Unit is approx 12 years old

Please excuse me for the lack of technical terms.

Thanks for any help.