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  1. #1

    Lennox G23 furnace cycles multiple times before reaching set thermostat temp

    Background Information:
    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.


    Current Issue:
    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?
    Undersized motor?
    Problem with part of the motor housing not installed?
    Heat Exchanger?
    Something else?

    Any comments from the experts would be really appreciated.

  2. #2
    Join Date
    Jul 2006
    Location
    Virginia
    Posts
    4,799
    sounds like it needs a correct motor installed properly,also shoild have a 20 year warranty on heat exchanger. think they need to send a competent tech to come fix your furnace motor
    We really need change now

  3. #3
    Join Date
    Dec 2002
    Location
    Houston,Tx.
    Posts
    16,110
    Quote Originally Posted by jimdavisjr View Post
    Also, he left a small piece of the motor housing uninstalled in the bottom of the blower area.
    Can you take a picture of this? sounds to me like the cut-off plate, that guides the air into the furnace correctly was left off the blower housing, if this is the case, this is were your problem is.
    __________________________________________________ _______________________
    “Experience is a hard teacher because she gives the test first, the lesson afterwards". - Vernon Law

    "Never let success go to your head, and never let failure go to your heart". - Unknown

  4. #4
    Attached are two images of the blower compartment and the uninstalled part of the blower housing.Name:  Blower image 1.jpg
Views: 512
Size:  56.7 KBName:  Blower image 2.jpg
Views: 559
Size:  66.7 KB

  5. #5
    Join Date
    May 2012
    Location
    Upper Michigan
    Posts
    3,589
    That is the cover for the control box, it's quite mess in there.

  6. #6
    Join Date
    May 2012
    Location
    Upper Michigan
    Posts
    3,589
    Just noticed the cap laying on the bottom lol pretty low quality.

  7. #7
    What blower motor specifications should I expect to have to support the Lennox G23 125K BTU furnace? If the workmanship was not correct to replace the motor I may also have an issue with having the correct motor as well.

  8. #8
    Join Date
    Feb 2010
    Location
    Calgary, Alberta
    Posts
    1,130
    Post the full model number.

  9. #9
    Lennox G23 Q3/4 - 125 - 4

  10. #10
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,433
    Low airflow across the heat exchanger is an issue that can cause the symptoms you have. Do you know the temperature coming from the registers right before the furnace kicks off on limit?

  11. #11
    No. I don't have any guages to be able to identify the temperature at the registers.

  12. #12
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,433
    Quote Originally Posted by jimdavisjr View Post
    No. I don't have any guages to be able to identify the temperature at the registers.
    No fancy gauges needed, an ordinary thermometer will do.

  13. #13
    Join Date
    Dec 2002
    Location
    Houston,Tx.
    Posts
    16,110
    What in the wide, wide world of sports did he do, set off a bomb in there? is that motor turning the right direction? the cap laying on the bottom, read the mf on it, and we can maybe tell if the hp on the motor is correct, it's either, 5mf..7.5...10...or possibly 12.5....or 15.
    __________________________________________________ _______________________
    “Experience is a hard teacher because she gives the test first, the lesson afterwards". - Vernon Law

    "Never let success go to your head, and never let failure go to your heart". - Unknown

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