I have a Ruud silhoutte II gas furnace. Since it has gotten cold in this last week or two we started using the heater. The problem is the house is still cold. The thermostat stays at 69 even though it is set for 74-75. I will turn it off & then back again to get it started. I hear the fan motor kick on, then the igniter, then I have flames for about 5 seconds then they shut off. They might come on again, but only for about 5 seconds. Then the fan motor shuts off & it is still 69. I have checked the air at the closet register & it is blowing air that is room temp, not heated. I was thinking the thermostat or maybe the thermocouple, but It doesn't have one. Any ideas? The furnace is 6 years old & the thermostat is 4 years old

chopshop, is the filter clean?

You might try cleaning the flame sensor since the cutting on and off is one symptom of this kind of problem. If it was the thermostat it would not cut off until satisfied at your 74 degree setting. Sorry but I see no direct way to post a picture here since they changed the format.

I agree that your problem has something to do with the FLAME SENSOR. It's design to cut off the flow of gas at 5 to 8 seconds. This is a safety feature. It my need to be replaced or cleaned or just my have come unplugged.