______________________________________________________________________ Usually the furnace stays on for 5 min or so then cycles off for 20 min etc. Right now it has started cycling on for 60 sec off for 30 sec on for 60 etc. then stops for a while and repeats…..how can I fix? is this serious? Replies: anon “Re(1):Lennox Pulse 21 short cycles” , posted Wed 18 Feb 17:15 ______________________________________________________________________ I had the a similar problem with my Pulse 21. The problem with mine was that the exhaust nozzle that stuck outside the side of the house had fallen off. The furnace in my house has a side exhaust vent. I was told that the furnace works off of the back pressure of the exhaust tube and is very picky about it being in an acceptable range. The pvc piece that fell off actually had a venturi tube that sped up the exiting air and as a result, lowered the pressure. Without this 1/2 foot pvc tube outside the home, my furnace would cycle on and off like you descibe. It doesn’t help when it is -10 outside. xenos Webmaster “Re(1):Lennox Pulse 21 short cycles” , posted Fri 16 Jan 19:12 ______________________________________________________________________ I would check and make sure the filters are clean and the venting is clear. Then look at the control board and look and see what the diagnostic light are notifying you of. Also jump R and W to verify it’s not the thermostat. Xenos. The best way to escape a problem is to solve it. streeter “Re(2):Lennox Pulse 21 short cycles” , posted Sat 17 Jan 23:28 ______________________________________________________________________ Make sure you’re using the correct terminology. A short cycle means the burner comes on and then goes off. IF it is just the fan it is not a “short cycle”.