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My Carrier Performance 96 series furnace (similar to the 59TP6 currently shown on Carrier's website, but I don't have the exact model number in front of me) has been showing codes 13 ("Limit Circuit Lockout") and 33 ("Limit Circuit Fault") intermittently for the last two years, but several repairmen have come and been unable to find anything wrong.

According to the manual, the codes indicate that the flame roll-out switch has been tripped, but it seems like that can't be the case since the flame roll-out switches require manual reset and I have never done so. The troubleshooting section of the manual refers to other "limit" switches, but there are no such switches installed on this unit and nowhere else in the manual talks about them.

Yesterday the unit spent most of the day alternating between these two codes, and I decided to spend some time watching it. I sat there in front of it for half an hour (holding a carbon monoxide detector and) watching the flames. The flames look exactly as they should, in both low and high stages, and I see no roll-out.

After seeing nothing for 30 minutes I assumed that maybe one of the roll-out switches is faulty and I should replace them, so I put the cover back on and went to order new limit switches. Minutes later, the unit shut off again with code 13.

The second time I sat down in front of it, it sounded like the exhaust inducer motor was struggling a little, and that leads me to my questions:

This unit uses a concentric vent for intake of combustion air and for exhaust, and the pipes for these are around 30ft long. At the furnace end, they have multiple complex bends because of space constraints, then once they reach the ceiling above the unit they head straight across the room to the outside wall.

  1. Is it possible that a clog in either (or both) of the pipes could cause this condition?
  2. How can I check for such a blockage? If I find one, how can I clean it out?
  3. I have identified a much straighter vertical path for the vent pipes. Is it worth cutting them and redoing them?
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To me it sounds like a motor lubrication issue, your fan motor that sounds like it is struggling is probably the issue. Check for lube ports on the fan, if there is a port on the motor case both front and back try a few drops of motor oil in the ports, if there are no ports the bearings may be zz or double sealed and normally need to be replaced, in a pinch one of the shields could be pulled and fresh lube added in the form of a light grease as oil will just drip away if not designed for oil and grease is used. The last type is an oil light bearing. These are not usually used on quality fans but possible the bearing is a sleeve that wicks oil from a wadding adding oil until the wadding is saturated will usually give new life to the bearing (these are usually found on low load fans like bathroom and low cost window ac units). There is probably a cane switch or a pressure switch that monitors the flow and may be triggering the fault code, oiling may help the fan to produce full speed/flow and solve your problem.

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  • When I say struggling, I don't mean the motor is grinding or squeaking. I mean it sounds like a vacuum cleaner does if you block the hose - like it's trying to push air but there's nowhere for the air to flow. Would you still say the same answer? – Moshe Katz Mar 2 at 21:24
  • With a vacuum cleaner stopping the airflow increases the fan speed as there is no work of moving air at a reduced pressure. With changes in fan “noise” and no other changes I would look at bearings / lubrication. With multiple speed induction low speed usually is ok if it starts but after switching to a medium or high the bearing heats up and slowing the squirt cage down as the higher speed winding has less torque than the high speed winding. – Ed Beal Mar 2 at 21:34

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