I have a Ruud downflow system about 18-20 years old. When I switched from a/c to heat I noticed the pilot was not lit. So I lit the pilot, no problem, the furnance kicked on warm air. A few hours later I noticed cold air coming through the vents. I checked the pilot it was out. I lit again and same thing. The pilot lights easily and stays lit and the furnance comes on and works great. Then a little while later the pilot goes out. Was working fine after last season. Can anyone advise me on this please.
Thank you
Rick

try changing the thermocouple. It is the skinny copper looking tube that runs from your gas valve to the pilot. You would need to shut the gas off to your furnace, unscrew the t couple from the valve and unscrew the silver pilot line from the gas valve. Then remove the pilot assembly from the burner, usually has 2 flat blade screws holding it onto a bracket.

Buy the 36" universal thermocouple from a hardware store. It come with any adapters you may need to install it. Just look how the old one is installed into the pilot assembly.

While you have the pilot out. blow thru the end of the tube that was screwed into the gas valve to clear out any debris you may have in there..