Furnace constantly shutting off and on.


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Old 12-07-10, 08:29 AM
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Furnace constantly shutting off and on.

Lennox Model G23Q2/3-75-5, White Rodgers programable thermostat. Programmed 65 degrees day, 60 degrees night. In early morning hours when temperature drops below 60 furnace will come on, gas will ignite, then blower comes on. Runs for a minute then shuts off for about 5 minutes then starts cycle again. Even though temperature maybe registering 57 furnace will shut off. Will continue to do this for several hours. Did this for 6 hours this a.m. Tried to override thermostat but to no avail. It appears after heat from downstairs furnace reaches upstairs and upstairs warms furnace will then continue to run until it reachs 65 degrees and then shuts off. Need suggestions on how to fix. Faulty circuit board? Thermostat? ????
 
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Old 12-07-10, 11:19 AM
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Does the fan circulating air around the house continue to run while the burner turns on and off, or are the burner and fan both shutting off?
 
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Old 12-07-10, 11:56 AM
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Fan

The fan runs just like it normally does when getting to the temperature level I have the thermostat set to. It justs only runs for roughly a minute then every thing shuts off. You can hear the furnace kick on, then shortly the gas burners turn on, then the fan blower turns on.
 
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Old 12-07-10, 12:33 PM
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Do you have a multimeter so you can measure AC voltages?


I'd start by verifying that you have low voltage power during these episodes. Measure the voltage from chassis ground to the R contact, of from C to R. Should be 24 VAC when the power is on to the furnace.

Then do the same for the W terminal. When you have 24 VAC here the thermostat is calling for heat.

Could be the thermostat isn't keeping the furnace switched on reliably. Check for that condition first.
 
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Old 12-07-10, 02:51 PM
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I have a very similar issue on mine G23Q2/3-75-1. Thermostat I have is MagicStat. I set temperature to be 60 degrees all around. Current temperature is 52 degrees, furnace comes on, fan starts blowing air, gas will ignite, and warm air starts to blow. Runs for about 6-7 minutes then shuts off for about 2 minutes and starts cycle again. Even though the temperature is below the set temperature, furnace and fan both shut off. Once the temperature reaches the set temperature of 60 degrees, the gas valves shuts off (no warm air) BUT the fan keeps circulating.

Also changing the FAN from Auto to On at the thermostat does not do anything; furnace will still cycle through the procedure.

Also if I move the Thermostat into the "Off" position the fan keeps circulating, the only way to shut off the fan is to cut off the power at the furnace. But note that the fan mostly keeps circulating if the set temperature is reached, if its below it, it will just cycle as described above.

One thing to note the furnace air filter has not been changed probably about 9-12 months and after I took it out to replace it was dirty way beyond my imagination.

Any Help would be appreciated. Thank You
 
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Old 12-09-10, 09:14 AM
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Still having problems. Cleaned furnace and checked voltage. Checked voltage from G to R on thermostat when running and measured 26-27 vac, W to R nothing. When turned furnace off both G to R and W to R measured 26-27 vac. Themostat on 65, registering 65 but furnace was running with no gas burning. Recently shut off. Don't understand when upstairs warms up it appears to run OK and doesn't cycle every few minutes as when the upstairs is colder. Last night thermostat auto reset temperature to 60 at 11 p.m. Temperature was reading 65 at time. Around 2:00 a.m. when temperature dropped to 60 furnace started it's recycling issue every minute or so. Any more suggestions?
 
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Old 12-09-10, 09:17 AM
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Failed to mention that when monitoring this a.m. when furnace kicked on, then gas ignited for a brief time both the red and green lights on the control board flashed simultaneously. A few seconds after the blower/fan kicked on the gas burners shut down and flashing sequence changed. The red flashed quickly as it did before but the green light flashed slower.
 
 

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