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Tom, HVAC Technician
Category: HVAC
Satisfied Customers: 770
Experience:  16+yrs HVAC/R service. Residential Commerical Industrial
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Hi. We have a Lennox G23 series furnace about 11 years old.

Customer Question

Hi. We have a Lennox G23 series furnace about 11 years old. The unit will work for awhile before the burners will go out and the unit will not turn off. Therefore, the tempature will keep decreasing and the house becomes cold. We must shut off the therostat and then start it again. Sometimes the burners will not come on but the fan does and it blows out cold air. We just changed the therostat because thats what a hvac tech told us to do. We also had the unit cleaned and they could find no problem. Please help. This still happens even with the new therostat.
Submitted: 7 years ago.
Category: HVAC
Expert:  Tom replied 7 years ago.

Hi, Tom here,


Does the main blower start when you restart the furnace??



Customer: replied 7 years ago.
Hi Tom. Is the main blower the second fan that lets out the heat? If so yes. However, sometimes the burners will not come on before the heat comes out.
Expert:  Tom replied 7 years ago.

Ok, just to clarify,,


If you remove the front panel on your furnace, you should see a small blower motor about 3 1/2 inches in diameter, connected to the exhaust pipe on your furnace. It is the first thing to run on a call for heat and should run the entire time the burner is on.


The main blower is at the bottom of the furnace and is quite large and supplies air to the vents in your house. It should start after the burner lights and run for a minute or two after the burner shuts off.


  1. Does the main blower continue to run even after the burner shuts down?
  2. If you look at your thermostat is the fan set to auto or fan on?
  3. Does the house get up to setpoint?

Just need a little more info...I think we'll get it figured out.





Customer: replied 7 years ago.


The main blower continues to run after the burner shuts down. The fan is set to auto. The house sometimes gets up to set point. Thanks.

Expert:  Tom replied 7 years ago.

Ok, here is what it sounds like is going on.


IF the heat exchanger in the furnace gets too hot, a hi limit sensor shuts off the main burner AND energizes the blower relay on the control board to keep your fan running until the heat exchanger has returned to a safe temperature.


The hi limit control looks something like this:


It is located in the front section of the furnace behind the gas valve.

The problem is usually a lack of air flow through the furnace.


This can be caused by a dirty furnace filter, too many vents shut off in the house, or a dirty A coil. (The A coil is the A/C coil that sits on top of the furnace that provides cold air in cooling mode, assuming you have central air). Over the years dirt and pet hair (if you have one) can accumulate on the bottom side of the coil and reduce air flow to the point where the high limit acts up.


Check the airflow out of your vents to see if you have a strong airflow or seems weak, you have to get your hand close to vent to feel airflow.


The other possibilities that cause this could be a slight crack or hole in the heat exchanger causing flue gas to go into the air stream around the limit sensor. This is a serious problem and should be checked out by a tech. One way to tell if you have a crack is to look at the burner flames on your furnace. You should have stable blue flames shooting into the heat exchanger. If one or more of them suddenly change, burn back or become disturbed when the main blower kicks in you probably have a problem in the heat exchanger and need to have it checked immediately.


Last, you may simply have a defective hi limit sensor. For testing purposes ONLY, you can remove the two wires, (usually red) and temporarily hook them together to see if it turns the fan off. Although it is only 24 volts going to the wires, make sure power is off if you try jumping them and make sure your jumper doesn't touch any metal in the furnace or you'll likely cause more problems.


If you are not comfortable trying any of these suggestions, by all means get a qualified furnace guy back out there. A hi limit fault is serious as it indicates an overheat condition and needs to be rectified.


Check the simple stuff first. (filter, airflow).


I hope it turns out to be minor. Feel free to reply with follow up questions. Thanks.



Tom and other HVAC Specialists are ready to help you
Customer: replied 7 years ago.



Thanks for your suggestions. We don't have any questions now. Thanks for your help!

Expert:  Tom replied 7 years ago.

OK, thank you for the accept. Let me know if you have more questions. You can reply to this thread without clicking on accept.


Thanks, Tom

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