In very very cold weather, our furnace is shutting down. Furnace is a lennox, about 17 years old The closest they've come to figuring it out appears to be that it's either overheating, or isn't starting and after a certain number of attempts it shuts down. We've had two different technicians check it over multiple times and change numerous parts, and do cleanings. No solution has been found yet. One person suggested that our duct work gets too narrow, too fast. That doesn't seem to explain why the furnace worked fine for the first 16 years though. Am I looking at a new furnace? If so, is it possible our ductwork is too small (it's a 17 year old house) so that it might burn out a new furnace? Is there perhaps another obvious fix the technicians are missing?

Did the burners light and then shut down right away?

Troubleshoot an ignition problem on a Lennox gas furnace ...

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https://www.youtube.com/watch?v=yXIxgzwMkPs
Nov 17, 2012 - Uploaded by grayfurnaceman
This furnace should light its pilot with a spark then the pilot proves its on ... Troubleshoot an ignition problem on a Lennox gasfurnace .... Pilot and no inducer but is the same model as that great video! ... You probably have dirty burners. ... How do you know spark module is not sending 24 volts to mv and pv ...

Lennox Furnace Model G11Q3-82-2 - RepairClinic.com

Sounds like a dirty flame sensor.
Also 9 degrees is a large swing to recover. Most specialist will advise no more than a 5 degree set back, otherwise recovery time eliminates any savings you may achieve.

The bearings are bad. You need to replace the motor. Motor cost $75 to $125. Labor is on avg. $150 to $400 with an decent A/C Company . You can try to oil it but its not recommend by Lennox. That furnace at the time they build it was top of the line. Good Luck.

Could be a defective thermostat in the plenum.
When a gas furnace shuts down the fan(blower) will run until the plenum cools to the safe range. Usually 1 and 1/2 minutes.
Intermittent operation is a defective thermostat or relay.

Hint number 1, get HVAC license. One mistake or misunderstanding and you could blow your house up.

I have a Lennox furnace that starts but won't ignite.
Turn up the thermostat, you can hear it start but the burners don't come on

Check your thermostat anticipator setting. If you dont know how to do this don't try on your own or you may damage your thermostat.

It is normal. Your house has to go down a couple more degrees because the heat pump is not working. You can solve this by rewirng you thermostat. Take off the wire that goes to Y and tape off. Then take the wires that are marked w and e or w1 and put the wires to yellow. Do not run ac with it wired like this. If you need more information please post me back

if your furnace is on 24 hours straight, before it heats up your home, then either your cold weather and indoor temps are extreme, or your unit is grossly undersized. if however your furnace cycles on and off when the temp is still way colder than you have set it you have either a bad stat that thinks its hotter than it is, or a bad furnace problem and even though the furnace is getting a signal from the stat to heat it is failing to do so. you'll have to watch and think this through to isolate the problem. if you set the furnace to max heat and it runs and heats the house reasonably quick then its probably going to be a bad stat. if you set it to max heat and it shuts off way before it gets to setpoint then the problem is in the furnace. be sure filters are clean, blower wheel is clean , returns are no obstructed, etc. if you by chance have an old mercury bulb stat they had a thing in them called a heat anticipator which basically worked like this. a dial with a little pointer determined how soon it would be till the furnaced reached the desired temp and anticipated how soon to shut off the burners in anticipation of the cool down period so you dont overshoot the desired temp. if these are bad or moved to different setting they can short cycle the furnace on of on off etc. or shut down the burner almost as soon as it lights. setting is usually at .4 to .6 on most, but really a digital stat is only way to go. by the way check to be sure you don't have a hot air supply grill blowing directly on the stat or it will shut off as soon as stat gets hot and rest of house will be cold. usual location is 5 feet off floor directly over the main return air grill or under it as the case may be. good luck