I have a Bryant Plus 80 model 395C that is 17 years old. I turned the furnace on this weekend and it starts up just fine. The draft blower starts, the igniter glows, the valve opens and it lights the burners. After it heats up the blower comes on as always. After about 3-5 minutes the burner goes out and the whole cycle starts again and goes through the same thing over and over. I have also cleaned the flame sensor, I do this every year at the beginning of the heating season.

Any ideas?

Thanks
Scott

For starters clean the flame sensor if so equipped. Locate the drawing on the inside of the blower door to see if you have a flame sensor. If so it will be located in the pilot/ignitor area.

NOTE the ignitor is very fragile so do not bang that part around or touch it with your hand. Clean the sensor with steel wool or sand paper. A clean sensor produces a stronger safety signal and that will hold the gas valve on instead of it turning off and on every 3 minutes.

If you have replaced the thermostat lately that could be a problem if programed incorrectly.

I have already cleaned the sensor with sandpaper, I do it every heating season, the furnace is in the laundry room. I replaced the ignitor in 2001. The thermostat is the round Honeywell with the mercury tube in it. The unit runs fine in air conditioning mode. I took out the pressure sensor and made it click by blowing and sucking on the vinyl tube. No luck so far. Anything else I should try?

Thanks
Scott

A defective high limit control? That would cut it off and when cooled bring it back on to start over. It could be out of calibration also which means you would still have to replace it to find out if that is the part.

I figured you already checked for a dirty airfilter. A dirty AC coil can also cause heat build up in the furnace.

The filter is clean and changed every month, the AC coil is almost impossible to check as it pretty much sealed into the plenum, but I'll see what I can do. Is the high limit the rectangular plate screwed into the air box with the 2 wires going into it between the burners and draft blower?

When you have a unit that is 17 years old usually the coil is dirty by that time.

Some Bryant units have a plate as you described.

I think I'll try a new limit switch first, getting that back cover off the plenum will be very involved. I'll let you what I find though.

Thanks for the help.

Ok

It was the high limit thermostat, thanks for all the help!