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Old 02-04-2010, 01:06 PM   #1
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help size blower motor


I need to replace the blower motor in my 25+ year old 80000 btu natural gas furnace. The motor has *no* information on it whatsoever except for "SFR18" ... probably a mfr code? The furnace plate does say "maximum electrical load 7.9 amps consumption 400 watts". The motor is 120 volts so the 7.9 amps makes me think it is 1/2hp. Does that sound reasonable for a furnace of this size? Thanks

Peyton

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Old 02-04-2010, 03:04 PM   #2
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help size blower motor


Could be a 1/3 or 1/2 HP motor.

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Old 02-04-2010, 04:37 PM   #3
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help size blower motor


Quote:
Originally Posted by beenthere View Post
Could be a 1/3 or 1/2 HP motor.
Thanks beenthere.

This brings up another question. In surfing the web, I have seen the statement made that using a blower motor larger than required (e.g. a 1/2 hp motor to replace a 1/3hp motor) is bad for the motor somehow. Seems counter-intuitive to me but if true, could someone explain. Thanks

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Old 02-04-2010, 04:53 PM   #4
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help size blower motor


It can be. if the 1/3HO was only doing the work of a 1/4 or less HP motor.

A PSC motor needs to work under a load. If it doesn't it will over heat and burn up.
Has to do with motor slip under load. No load, no slip. Begins to approach Asynchronous RPM, which would be 3500 RPM.
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Old 02-04-2010, 07:33 PM   #5
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help size blower motor


Quote:
Originally Posted by beenthere View Post
It can be. if the 1/3HO was only doing the work of a 1/4 or less HP motor.

A PSC motor needs to work under a load. If it doesn't it will over heat and burn up.
Has to do with motor slip under load. No load, no slip. Begins to approach Asynchronous RPM, which would be 3500 RPM.
Thanks again beenthere. I won't pretend to understand motor slip and such but it does show that intuition based on ignorance isn't always right. But what happens if the motor is undersized?

I still don't know how to size the blower motor. But going from the furnace plate info: 400 watts is slightly more than 1/2hp and the 7.9 amps is also in the range of 1/2hp motor fla suggesting 1/2hp at max.

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Old 02-04-2010, 09:10 PM   #6
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help size blower motor


Buy a universal motor payton. A 7.5 mfd capacitor will make it a 1/3rd hp and a 10 mfd cap will make it a 1/2 using ther same wires.
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Old 02-04-2010, 09:44 PM   #7
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help size blower motor


A 1/4 HP motor on a blower wheel that was intended to have a 1/2HP. Would also overload.

Since a motor isn't 100% efficient. 400 watts is not always a 1/2HP motor. Could of been an older 1/3.
Or 1/2.
Many times. If you put a 1/2 on where a 1/3 was. You can get away with using the slower blower speeds.
If the heat and cool were using high and med high for cooling and heating.
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Old 02-06-2010, 06:13 PM   #8
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help size blower motor


Thanks beenthere and Marty. Maybe the multi-horsepower motor is the way to go. Grainger sells an AOSmith 1/6hp-1/2hp motor for about $90. I could install it as a 1/2 and if I get thermal shutdowns, change it to 1/3hp. Sound reasonable?

Any idea of AOSmith quality?

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Old 02-06-2010, 07:42 PM   #9
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help size blower motor


That's what I have on the service van.
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Old 02-11-2010, 07:33 PM   #10
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help size blower motor


I Installed the AOSmith multi-hp motor as 1/3 hp. It works great ... thanks for the suggestion.

Totally ignoring the schematic on the service door, I installed the heater blower speed as medium high (1090rpm) rather than medium low (1030rpm). This resulted in cycling, caused by the limit switch, as the fan pushed the warm air out faster than the furnace could heat it.

Is there anything I can do to reduce the cycling other than reverting to the medium low speed? The limit switch is currently set to work from 140 (on) to 100(off). Would raising the on-switch to 150 cause any problems?

Thanks very much for all the assistance that you have given me.

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Old 02-11-2010, 07:38 PM   #11
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help size blower motor


Better off lowering the speed.

Raising the on temp setting just makes the heat exchanger hotter for a short period of time. But it will still cool off enough so that the blower shuts off anyway.


PS: Sometimes those motors are stronger then the old motors. And you have to set them on a lower speed then the original motor.

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