I have a side-mounted Rheem heating system in my attic. It burns natural (city) gas The air conditioner is separate and is outside. The thermostat is an old style dial with a mercury bubble, but I can't see any model numbers on it.

Heater model number is RCAH-Z025S

I got in the attic to watch the cycles.

The blower comes on. 1 minute later the burner comes on for 10 seconds and stops. The 1 minute off, 10 seconds on cycle repeats until the 8 minute mark when it apparently gives up for a while. The air out of the vents, obviously is very cold.

Written on the top of the unit (in sharpie) it says control board was replaced in March of '05, and motor was replaced in February of '06. Near the unit was an empty box for a remote flame sensor that I'm assuming someone put on the system.

I don't know anything else about the system's history. I bought the house 2 years ago. The house is about 13 years old.

Any information you could give would be appreciated. Thank you.

10 seconds is long for a flame sensor problem but you might try cleaning it with steel wool or fine sand paper. Make sure you turn power off to unit before cleaning and back on to test the unit when done.

Ok. I did that. It seems to have worked. It's been intermittent for a few days, but it has worked fine for the last 4 hours since I cleaned it off. I had little hope of it working when I saw that the flame sensor had no corrosion, soot, or heat discoloring on it. It basically looked like brand new metal, but I cleaned it with sandpaper anyway. Is this something that I may have to address again soon, or should I think about replacing the sensor?

Clean every year to be trouble free. Some day you might need to replace. Glad you got it going.

Great work! Thanks a lot for the help. You saved me time and money. My wife and dog thank you as well.

No problem