I have a Rheem 80 plus gas furnace with an automatic igniter in my home that has a problem lighting the gas. The glow plug or igniter has no problem glowing bright orange to light the gas, but the device that controls the gas doesn't seem to let enough gas out to be ignited. Once the igniter is hot, the device allows a shot of gas and there will be a flash of blue but the burners don't light, the ignter turns off, the blower fan turns on and no heat. There is also a seprate fan that turns on before the igniter glows and sometimes will stay on for hours if the burners are not lit. This problem was intermintent, but now it has been 2 days since the burns have actually been lit. What is the cause of the burners not lighting?Thanks!