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Old 03-14-2010, 12:10 PM   #1
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AMER STD open high limit


I have an American Standard natural gas furnce (ADX1B080A9421AA) manufactured 3/2006.
First problem draft inducer motor stopped. I removed motor and checked for obstructions, spun the fan and reinstalled. Motor started slowly, ran and unit worked for two days.
Second problem unit shut down on temperature high limit. Draft inducer still working but thought not at optimal to remove excess heat. Bought OEM motor with the exception there was no limit switch on the inducer motor itself as original motor had. Installed new motor and still unit goes out on high limit.
Things that I have checked, cleaned or replaced.
Replaced filter - had no excess dirt.
Checked intake and exhaust for obstructions - none found.
Checked A coil for dirt - very clean
Removed temp limit - visually fine
Unit fan runs normally.
Seems all airflow issues are OK.
Seems temp high limit is working because the unit is shutting down.
What am I missing
Thanks for the opportunity to ask for help.

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Old 03-14-2010, 01:08 PM   #2
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AMER STD open high limit


Your fan motor may not be running fast enough. Replace the capacitor for it.

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Old 03-14-2010, 05:26 PM   #3
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AMER STD open high limit


how about the rounds on the squirrel cage of the supply fan and does the air either at the return or your supply ducts seem to be normal(keep in mind the heat runs low fan in HEAT) hi in AC there are 2 limits there one for fan ON and that hi-limit for burner OFF..filter,fan coil dirty..etc as you checked.that missing proof for ID motor should be there??? of running air into the burner chambers to tell the burner to lite..lets hear back on that air noise on the returns and or supplys...did you match the RPMs when you picked up that new ID
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Old 03-15-2010, 02:58 PM   #4
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AMER STD open high limit


Thanks for replies.
Capacitor rated 10 MFD +/-6%, tested at 9.49 MFD.
Squirrel cage is fine and clean.
Removed limit switch from old draft motor and installed on new motor. I thought it was molded into motor housing, found two screws to remove from old housing.
When heat is called for draft motor starts, igniter glows, flame starts, unit fan comes on and all systems are fine for approx. 4-5 minutes.
Flame goes out.
Diagnostic light blinks 4 ( Limit switch open) , unit blower and draft fan both continue to run, after 3-4 minutes unit restarts as normal.
Is capacitor to close to low end of tolerance?
Could the high temp limit be defective?
Is there a test for the high temp limit?
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Old 03-15-2010, 04:22 PM   #5
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AMER STD open high limit


you would have to read the temp of the switch where it is mounted into the Heat-X during the cycle of heat coming up...it should be stamped with its trip temp.could be fatigued tripping right above when the supply fan comes on theu its disc
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Old 03-15-2010, 06:19 PM   #6
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AMER STD open high limit


The unit could be tripping the limit control because it is overfired or from a lack of airflow. Only way to know is to do a temp rise test. Check the temp downstream from the plenum about a foot and out of sight of the heat ex after running4- 5 minutes. Subtract the return temp. Check the rating specs. Should have a range there ie: 40-80F.
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Old 03-16-2010, 08:22 PM   #7
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AMER STD open high limit


check the secondary heat exchanger and blower wheel for dirt, and put fan on high.

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