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Old 07-22-2012, 06:47 AM   #1
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Furnace blower fan speed


I recently replaced the blower motor on my Ducane 125,000 BTUh furnace. It called for a 3/4 HP, 1075 RPM 11.7 amp. I am not getting the desired RPM's out of the motor as my airflow is poor. The fan can barely push the cooled air up to the second floor. The black high speed fan lead is on the "cool" terminal on the Honeywell control board model ST9120C-2010. I had the motor shop test the fan ahead of time in the squirrel cage fan and housing and it was spinning at the desired rpm and amperage draw. I am convinced the controller is the culprit. Is there a way to program the controller to speed up the fan? There is a "speed up" terminal on the board just above the thermostat terminals.

Thank you for any help you can give!


Last edited by Bfam4; 07-22-2012 at 06:50 AM.
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Old 07-22-2012, 06:51 AM   #2
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Furnace blower fan speed


...Filter?

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Old 07-22-2012, 06:54 AM   #3
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Furnace blower fan speed


Brand new.
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Old 07-22-2012, 07:16 AM   #4
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Furnace blower fan speed


Is the evap coil clean? Need to look at the bottom side of it to be sure.
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Old 07-22-2012, 07:31 AM   #5
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Yes it is clean. I know the issue is with motor/fan speed and not inhibited air flow.

Last edited by Bfam4; 07-22-2012 at 07:32 AM. Reason: Typo
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Old 07-22-2012, 07:53 AM   #6
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Furnace blower fan speed


All the control does is supply power to the correct motor lead so it has 120V and runs or it doesn't. A weak capacitor can make the motor run slow. Try testing that. The motor is spinning the correct direction right?

Last edited by Marty S.; 07-22-2012 at 07:55 AM.
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Old 07-22-2012, 08:12 AM   #7
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Furnace blower fan speed


forget the amperages on that bench test the coil/filter and duct rise gives you the actual amps in place with those restriction.motors inside the squirrel cage so it has to be right there with the fins and direction installed as removed
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Old 07-22-2012, 08:32 AM   #8
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Furnace blower fan speed


Are you sure its spinning the right way.
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Old 07-22-2012, 08:48 AM   #9
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Furnace blower fan speed


If the blower is spinning the correct direction, and the evaporator coil is clean, I recommend calling a pro to check the external static pressure of your duct work.

PSC motors will not deliver the desired airflow with a high ESP.
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Old 07-22-2012, 09:26 AM   #10
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Yes- the motor is spinning the correct way- tried that already. This is actually the second motor I tried. I started with a 3/4 hp motor and had the same problem. The motor in question is now a 1 hp.
The OEM motor was a GE model. The new one is an AO Smith.
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Old 07-22-2012, 09:47 AM   #11
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Is it possible that GE manufactured a motor for Ducane that outperforms its nameplate stats?
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Old 07-22-2012, 10:09 AM   #12
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new capacitor with the new motor?
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Old 07-22-2012, 11:38 AM   #13
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Yes - it came with the motor -could that be the culprit if starting voltage is not high enough?
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Old 07-22-2012, 03:01 PM   #14
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Furnace blower fan speed


A higher horsepower motor does not help you any, and in fact can hurt your airflow. Get the right motor for the furnace and install that. Test the sytem with a load against the motor (i.e. installed, with the system running). If you're still not getting good airflow you'll have to start looking at other things.

The "speed up" terminals just speed up delays on the board. They have nothing to do with fan speed.
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Old 07-22-2012, 09:42 PM   #15
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Furnace blower fan speed


Quote:
Originally Posted by Bfam4 View Post
Yes- the motor is spinning the correct way- tried that already. This is actually the second motor I tried. I started with a 3/4 hp motor and had the same problem. The motor in question is now a 1 hp.
The OEM motor was a GE model. The new one is an AO Smith.
every 1 hp 120v motor i have ever seen has a 5/8 shaft. the 3/4 motor would have had a 1/2 shaft.

are you shure your coil did not freeze when the original blower failed. If it did the ice will block your air flow.

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