furnace fan motor sizes

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Old 02-26-05, 06:54 AM
bevl
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furnace fan motor sizes

Is it acceptable and safe to replace a 1/4 hp furnace fan motor with a larger hp motor ie: 1/3 or 1/2 hp, this is on a wood furnace.
Thanks in advance for any help.
BTW this forum is very well maintained and very informative. Great job!
Bevl
 
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Old 02-26-05, 07:11 AM
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Blower motor sizes cannot be interchanged. They are sized to things like the blower wheel the return and supply opening.
 
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Old 02-26-05, 07:28 AM
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If you have a direct drive blower where the motor lies within the blower wheel then you cannot install a physically larger motor but can install a greater hp rated one.
You have to make sure that the mount can take the extra weight.

A larger motor may not give you more airflow though.
All replacement motors operate within the same rpm range.
What a larger motor allows is the motor to not overload when it is at its highest setting in a multi speed motor.

What problem are you trying to overcome with a larger motor?
 
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Old 02-26-05, 07:40 AM
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Might check with the furnace company and see if your furnace can take the bigger size blower and motor first.

ED
 
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Old 02-26-05, 07:40 AM
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So greg I have always been told that going up in h/p will also increase the wattage of the motor and if there is not enough air going through the unit it will over heat the motor. If this is not true please let me know so I can tell the engineer from A.O. Smith and Emerson that they don't know what they are talking about.
 
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Old 02-26-05, 08:46 AM
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Well scottg..................I think you are safe with what you were told but there is a bit more to it.

The bottom line on what motor will work on what blower wheel is determined by the blower mfr's performance tables.
You can't just willy nilly shove a bigger motor in and get more air which is what most people think.
The size of the motor is determined by the required airflow in conjunction with the static pressure of the ductwork.
You therefore need a bigger motor (as long as the fan performance curves allow it) to maintain the required cfm if the static of an installation is higher than "normal".
The engineers are correct in theory, but in practice I have never have heard of or seen myself where an increase from 1/4 to 1/3 hp has ever caused a motor to overheat, 1/4 to 1/2 hp would be pushing it I think..
Again, this is very general advice and if the poster provides more info we can help with more detail.
Also if the model of the actual blower is provided the motor range it can handle can be confirmed.


The poster can easily confirm if a bigger motor will help.
He needs to make sure the speed setting is set to the maximum. A clamp on ammeter will then be needed to determine if the motor is drawing at or less than its rated amperage. (The motor access door must be closed and filter in place for this test.)
If the motor is set to its highest speed setting and the amperage is at or below rated a larger motor will not help.
In this case the whole assembly will need to be replaced provided as Ed said, the furnace can take it.
 
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Old 02-26-05, 09:17 AM
bevl
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we had a problem with the old motor overheating and we were curious if a larger hp might correct the problem, but you guys answered our question. . The fan is not direct drive, there is a pulley and belt, and 1/4 hp is what was installed from factory. We have replaced it with the same hp and there seems to be no problem with overheating so far.
Thanks for all the speedy replies.
 
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Old 02-26-05, 09:29 AM
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You say belt drive then the motor pulley should be variable diameter. With GregH here . If you do run the speed up you should amp the motor out to make sure your ok on load.

ED
 
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Old 03-01-05, 11:38 AM
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To add to what Greg is saying, the 1/2, 1/3, 1/4 etc, ratings on motors are there as a guide to properly replace a motor with a given amount of power to overcome resistance to desired airflow. If you replace a 1/3hp motor with a 1/2hp motor, the two motors both run at the same rpm. The only difference is that the 1/2hp motor is more able to overcome the resistance of the ductwork and therefore may deliver more air. But that is not always the case as many 1/3hp motors are overrated for their use and in the cases of both the 1/3hp and the 1/2hp much of the extra power is not used.

As for the motors overheating, that is not realistic for an oversized motor in this type of application and size. Heat is generated by an overworked motor, not an underworked motor.

As for changing from a 1/4hp motor to a 1/3hp in a woodburning appliance, I would be leary but if you check to make sure the tempurature rise across the unit is proper and the flue tempurature is nice and hot I don't see it being a problem. With wood burning appliances you need tons of heat escaping the chimney to make sure there is near complete fuel combustion and so that there are not high levels of creasote developing in the chimney...
 
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