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Old 11-28-2010, 01:59 PM   #1
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Lennox furnace shutting down before desired temp reached


Have a Lennox G23Q3/4-125-4 furnace. Started not igniting last year, so upgraded to the Honeywell S8670K circuit board. Ran well last winter and AC all summer. Now with ignite, burners come on, blower comes on, but runs only long enough to warm up a couple of degrees. Using a White-Rogers digital setback thermostat which appears to work correctly (calculated with other thermostat). After setting down to 64 at night, is taking forever to heat up to 73 in am. Overheating? Not sure where to check for auto shut off switch. Blower not working right? Motor does not seem to get overly hot. Anything else?

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Old 11-28-2010, 03:19 PM   #2
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Lennox furnace shutting down before desired temp reached


The limit is between the burners and the inducer assembly, brown rectangle with 2 wires. When the furnace shuts off test for voltage between the terminals that those wires connect to. If it's overheating you'll have 24 volts show on the meter.

Check the evap coil and make sure it's clean along with the filter.

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Old 11-29-2010, 01:57 PM   #3
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Lennox furnace shutting down before desired temp reached


Evaporative coils are fine, not restricting air flow. Filter is changed. Limit switch (small round black above burner) reads 26 to 28 volts in and 0 out when tripped. When switch trips, everything shuts off, including main blower even though the blower thermostat switch is in ON position not AUTO. The blower does come back on after a time.
The only brown rectangle box is by the circuit board and is a sail switch (air pressure) has no voltage.
Furnace still is heating up a couple of degrees at a time, then shutting down. Once desired temp is reached it runs ok. Cycles on / off when needed with thermostat set to auto.
Limit switch is working, what would be causing the overheating? Is there something on the blower control board I should be checking?
Thanks for any suggestions.
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Old 11-29-2010, 06:17 PM   #4
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Lennox furnace shutting down before desired temp reached


Smell the circuit board and you can tell if the blower relay is burning. Safe to assume you checked for high voltage at the heat/H terminal when the blower fail to run? Post a picture of the limit if you can.
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Old 11-29-2010, 06:52 PM   #5
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Lennox furnace shutting down before desired temp reached


There is no smell at the circuit board or any where else. I am not sure what you mean about the heat/H terminal. Will try to take pics now and post soon. Thanks
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Old 11-29-2010, 07:05 PM   #6
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Lennox furnace shutting down before desired temp reached


The g23 circuit board(part number 48K98) has the blowers heating speed on a terminal marked H. The AC fan speed is on A. With a limit tripped the blower should be running so need to find out if it's the circuit board not sending 120V to the motor or the motor not working when voltage is present.
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Old 11-29-2010, 07:08 PM   #7
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Lennox furnace shutting down before desired temp reached


Won't let me attach photos or insert images. not sure why. The limit has the number 8OK8901 on the top.
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Old 11-29-2010, 07:13 PM   #8
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Lennox furnace shutting down before desired temp reached


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The switch sits just above the pipe in the other pic

Last edited by dringrn; 11-29-2010 at 07:18 PM.
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Old 11-29-2010, 07:24 PM   #9
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Lennox furnace shutting down before desired temp reached


Ok. Trying to get it to trip so I can check voltage at the circuit board.
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Old 11-29-2010, 07:54 PM   #10
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Lennox furnace shutting down before desired temp reached


Switched out the circuit board to a Honeywell S8670K last year. Can be seen here: http://cyb-tec.com/S8600_2010.pdf

Circuit board has no power going out from any terminal when limit switch shuts it down.
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Old 11-29-2010, 08:37 PM   #11
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Lennox furnace shutting down before desired temp reached


that part number is for a primary limit according to lennox parts. i cannot see where its at in the pics. if its actually the primary limit, then there is an air flow issue, are all your register open?
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Old 11-29-2010, 08:49 PM   #12
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Lennox furnace shutting down before desired temp reached


yes, registers are open air intake vents are not covered
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Old 11-29-2010, 09:04 PM   #13
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Lennox furnace shutting down before desired temp reached


You can enlarge the pics by holding control and hitting the + sign. The limit shows as a light pinkish dot below the black fan motor and above the burners. Is there another limit I should look for? Thanks for all the help.
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Old 11-29-2010, 09:16 PM   #14
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Lennox furnace shutting down before desired temp reached


the blower may not be operating at the proper speed, does it make noise when it starts up?
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Old 11-29-2010, 09:23 PM   #15
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Lennox furnace shutting down before desired temp reached


A S8600 is an ignition module. The circuit board is in the blower compartment.

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