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Old 12-11-2009, 02:37 PM   #1
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More RPM's = More Airflow?


A question from a HVAC dummy if you please.

We recently had installed an outdoor wood boiler to our forced air heating system at our house. The heat exchanger from the boiler was installed in the plenum and it appears to be working OK, but we are wondering how much the heat exchanger cuts down on airflow in the ducts to the registers. The blower motor for the air duct system is rated at 1725 RPM's. If this motor was replaced with a faster motor (ie. 3450 rpm) would we get noticeably stronger airflow (and therefore quicker heating capability) at the registers?

Just wondering.

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Old 12-11-2009, 03:27 PM   #2
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More RPM's = More Airflow?


Probably not. You would need to find an fan curve cart that shows cfm delivered at various rpms specific to your blower. Spinning a blower too fast will reduce cfm. And keep in mind that if you do increase air flow most likely you'll reduce your delta T.

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Old 12-11-2009, 03:31 PM   #3
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More RPM's = More Airflow?


Is it a direct drive motor?
Or a belt drive motor?

To change to a 3450 RPM motor from a 1725 RPM 1/3 horse power motor. The new 3450 RPM motor would need to be increased to 2.5 horse power.

Increasing blower speed by 100 to 200 rpm will increase air flow some.
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Old 12-11-2009, 03:40 PM   #4
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More RPM's = More Airflow?


OK, so I would need to get some information about the blower then. And the blower specs would let me know if increased speed would provide greater cfm? Also the blower specs would determine the size in HP of the motor? (which is belt drive btw)
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Old 12-11-2009, 04:03 PM   #5
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More RPM's = More Airflow?


Look at teh pulley on the motor. Is it adjustable?
If so, it can be turned in to increase blower speed. If its adjustable, and already turned in all the way. Then you need a bigger adjustable motor pulley.

The HP rating will be on the motor.
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Old 12-11-2009, 06:28 PM   #6
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More RPM's = More Airflow?


What is the make and model # of the furnace. You can check the temp rise thru the furnace and if it is too high then the fan needs to be speeded up. Or if the furnace starts cycling on its limit control.
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Old 12-11-2009, 07:18 PM   #7
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More RPM's = More Airflow?


http://en.wikipedia.org/wiki/Affinity_laws
Doubling the shaft speed is supposed to double your CFM according to law 1a, but law 1b has some influence and is determined by duct resistance.
You need the fan curve.
The ductwork that your furnace works into also has a pressure/CFM curve and where these lines intersect is the point at which the fan will operate.
http://www.governair.com/TechNotes/A...fan_curves.htm

Last edited by Yoyizit; 12-11-2009 at 08:03 PM.
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Old 12-11-2009, 09:57 PM   #8
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More RPM's = More Airflow?


Wow, fan curves, and in color. Who knew? I'll do some studying. Thanks for the info.

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