I have a Lennox G23 Elite Series 125K BTU furnace ... 1997. It appears to be in excellent physical condition ... clean and without any rust, etc. This unit is installed in a reverse ranch home with about 2400 square feet on the main level and about 2000 finished square feet in the walkout lower level. We recently added about 800 square feet on the lower level. The contractor tapped into the supply side of the HVAC and added three vents into the new space. He also tapped into the return air side and added three vents of approximately the same size.
About two months ago I had an HVAC tech replace the blower motor when it would not fire up when the thermostat called for heat. The system ran through all of its routine and would start the burners, but would not kick on the blower fan. It would shut down in a few minutes when the primary limit switch reached its high temp limit. The tech replaced the blower and the run capacitor and the system appeared to run correctly after that. When he was removing the old blower he cut all of the wires to quickly pull it out before noticing that there was a quick connect plug. He then appeared to have some trouble in understanding how to rewire the motor but he finally got it done. The system seemed to run OK. Also, he left a small piece of the motor housing uninstalled in the bottom of the blower area.
The HVAC company asked if they could send out someone to quote a new system to me. If I bought the new system they would give me full credit for the blower replacement cost. The sales guy came out and spent about 10 minutes looking at the furnace. He used a gage with probes to check the air in the flue and the return air lines. He told me that the excess air percent was very high indicating that the heat exchanger was cracked and that I needed a new furnace. Their new system quote was very high compared to other companies. I had another HVAC tech come to look at the system and he told me that the heat exchanger looked to be in good shape. It seemed to me that the original HVAC company that replaced the blower may have been jerking me around.
Recently the system began acting up again. Thermostat would lose power intermittently, furnace would not respond to calls for heat intermittently, etc. It still had the old Johnson Controls blue box ignition module so I replaced it with a Honeywell S8670K module and the matching Honeywell flame sensor. It appears to be running through all routines normally except that the system will shut down after about 10 minutes of run time and before the thermostat set temp is met. The primary limit switch is kicking open due to high heat ... blower continues to blow and when the temp drops enough the system restarts. This cycling continues until the call for heat ends. When I jump the connectors to the limit switch the system does not shut down. I'm pretty sure that this is the issue.
I have been watching the system more closely after replacing the ignition module or I would probably not have noticed this. Since everything appears to be working correctly and the set thermostat temp is met I would not usually pay attention to this. I opened up part of the plenum and looked at the evaporator coils ... they are clean. There appears to be no obstructions. All supply vents are open. Return vents are clear. If I turn the blower to "on" from the thermostat it runs at high speed and the cycling does not occur ... but the motor runs continuously.
Primary limit switch defective?
Problem with part of the motor housing not installed?
Any comments from the experts would be really appreciated.