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Old 09-23-2012, 06:27 PM   #1
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furnace cuts off, then on, then off...


Amana furnace model GHI90A30.

- Gets call for heat, igniter heats, main burner lights, furnace heats up, blower motor comes on, does not overheat, runs for a couple of minutes then the main burner turns off.

- Blower turns off after furnace cools enough, igniter heats, main burner lights, furnace heats up, blower motor comes on, runs for a couple of minutes, does not overheat, runs for a couple of minutes then the main burner turns off. (repeat until call for heat ends).

Has an S8910U Universal Hot Surface Ignition Module. Troubleshooting section suggests a poor or erratic ground wire from the main burner. I have replaced the round wire and I have replaced the S8910U.

Any ideas?

Thanks,
-dog


Last edited by landog; 09-23-2012 at 06:29 PM.
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Old 09-23-2012, 06:50 PM   #2
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furnace cuts off, then on, then off...


check the air side filter rounds on the squirrel cage your not moving the air thru the furnace and it is going off on HI LIMIT that's why the burner goes off and the cycle starts again as it is doning.you have a hi limit sensor mounted sensing the heat exchanger area...might be worn out(it is just a bi metal) if all the other things are clean disc looks like this http://www.bestbuyheatingandaircondi..._Code=LIMD1900 or this http://www.bestbuyheatingandaircondi...ode=FLT64B2228


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Old 09-23-2012, 06:57 PM   #3
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furnace cuts off, then on, then off...


as Biggles said, it sounds like it is cycling on the high temp limit. Check for poor airflow due to plugged filter or dirty blower. If you have a/c check that the evaporator coil is clean on the furnace side.
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Old 09-23-2012, 07:29 PM   #4
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furnace cuts off, then on, then off...


as stated sounds like cycling on and off high limit....cleaning is in order ..or just a defective high limit...which would need replaced..
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Old 09-23-2012, 09:25 PM   #5
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furnace cuts off, then on, then off...


I thought the S8910U had a led diagnostic indicator?
What is it flashing when the burner goes off while there is still a call for heat?
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Old 09-23-2012, 09:31 PM   #6
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furnace cuts off, then on, then off...


The call for heat seems to remain, but the burner cuts off. The LED on the S8910u stays pulsing the whole time.

Thanks for your replies. Is the high limit switch you refer to in addition to this: http://www.diychatroom.com/attachmen...h-help-006.jpg

While the blower runs, the temp on the limit switch (the one similar to my link) stays constant. It does not come close to reaching its upper limit.

Would there be another limit switch... ?

Thanks for your help!
-dog
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Old 09-23-2012, 10:46 PM   #7
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furnace cuts off, then on, then off...


as stated your limit switch is just a bi metal and could be defective
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Old 09-24-2012, 05:47 AM   #8
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furnace cuts off, then on, then off...


Is the limit switch this one:

http://www.diychatroom.com/attachmen...h-help-006.jpg

..or am I looking for one like the ones that biggles posted?
If so, where should I be looking.

Thanks,
-dog
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Old 09-24-2012, 06:10 AM   #9
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furnace cuts off, then on, then off...


if you don't have that black dial one then you have to have a disc/and or sensor but there is only 1 hi limit..if you have a meter check the 24V to the main gas valve see if it drops out or is constant.the fact that the heating signals continue as the main burner goes off is tell us it has to be a hi limit...should be mounted above the burner section or on the side with an access plate with 2 wires to it...you have a control module on that furnace it goes to that check the terminals for a HI LIMIT point
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Old 09-24-2012, 06:29 AM   #10
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furnace cuts off, then on, then off...


check the batteries in the thermostat
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Old 09-24-2012, 07:16 AM   #11
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furnace cuts off, then on, then off...


YES the pic you provided is your "fan limit control" just watch it operate when you start your furnace ..it will turn as the heat exchanger warms up" so, it is likely shutting off ..on "HI" limit then not allowing re start until it cools down..this action may be due to dirty filters etc. as previously stated OR it may just be defective...which is likely if everything is clean and temps are within normal operating limits ALSO..check setting of HI limit...may be set too low/lower than operating limit ..both are indicated on the dial settings and can be changed by moving the little tabs that stick out and sliding them to desired settings...you can increase the HI limit to see if that helps but, the bi metal sensor is likely defective
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Old 09-24-2012, 10:20 AM   #12
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furnace cuts off, then on, then off...


This picture is of a lennox style fan/limit control. To these old eyes, it looks like the limit side is locked in at 150F and will not be adjustable. It also contains it's own preheater which adds an extra wrinkle to it's temp response. Let us know if that far right limit nib rotates clockwise past the 6 oclock position when the furnace is running.

Last edited by how; 09-24-2012 at 10:22 AM.
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Old 09-24-2012, 11:49 AM   #13
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furnace cuts off, then on, then off...


I have a dial type limit switch. The picture was the closest I could find searching google images. The temps are adjustable.

The temp rises and when it hits 150, the blower kicks on. With the blower on, the temp stays pretty constant. It never approaches the upper limit where it would shut down the burner.
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Old 09-24-2012, 12:01 PM   #14
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furnace cuts off, then on, then off...


so, it is shutting off the circulating fan, then the hi limit is reached thereby shutting off the burner therefore the control is not allowing the fan to continue running at normal operating temps but, is still shutting the burner off as it should ..seems to me the limit fan operating control IS defective and should be replaced.
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Old 09-24-2012, 12:38 PM   #15
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furnace cuts off, then on, then off...


Quote:
Originally Posted by techpappy View Post
so, it is shutting off the circulating fan, then the hi limit is reached thereby shutting off the burner therefore the control is not allowing the fan to continue running at normal operating temps but, is still shutting the burner off as it should ..seems to me the limit fan operating control IS defective and should be replaced.
It does not shut off the fan (the blower motor) until it has cooled. The burner shuts off after about two minutes, even though the teperature does not reach the upper limit - the temperature stays just above the middle trigger that turns on the fan. After the burner cuts off, the fan runs until it reaches the lowest temperature trigger.

It looks to me as if the limit switch is operating properly - but since the burner shuts off prematurely, I suppose that I will replace it. I really don't know what else to try.

Thank you all very much for your help!
-dog

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