FPM question


Old 05-05-06, 05:08 AM
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Join Date: Apr 2006
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FPM question

I downloaded that HVAC Calc program I have seen suggested here a few times. Pretty sweet program & an invaluable tool for the DIYer. Anyway, I plugged in all my house info & am working on designing the duct system. I am goiong to be supplying all my rooms with Flex, which the program recommends 700 FPM, they will come off of extended plenums. The plenums will be rigid which the program recommends 900 FPM for rigid. I think I want all my flows to match correct? No matter if they are the rigid extended plenum or the flex supply lines right? I would think I would want the rigid to be 700, same as the individual branch lines.

Also, the program is recommending 5" & 7" flex on a couple branch lines to get 700 FPM. I have not seen this size available, but have not really looked yet. 6" will change my FPM to ~650 & ~750 respectively. Is this even enough difference to worry about? Should I just go with 6" here rather than buy the 5" & 7" for just 2 runs? They are upstairs bedrooms if that helps.

Thanks in advance!
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Old 05-05-06, 02:05 PM
Join Date: Mar 2006
Location: Wet side of Washington state.
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I am not familiar with the program you used but the answer to your question of having the same flow is no.

The flow is measured in CFM (Cubic Feet per Minute) and the required flow into any room (and consequently from any branch duct) is dependent upon the temperature of the air flowing and the size of the room it is heating or cooling.

The FPM (Feet Per Minute) figure is how fast the air is flowing, not how much air is flowing. The air flow (CFM) is dependent upon both the speed (FPM) and the size of the duct. You can increase the flow by either increasing the speed or by increasing the size (cross-sectional area) of the duct.

Higher air speeds in ducts mean higher noise levels. Larger ducts (lower air speeds for the same CFM flow) are generally quieter than smaller ducts but larger ducts also are more prone to dust and dirt collecting (because of the lower air speeds allowing dust and dirt to drop out of the air stream), are less energy efficient (due to larger area presented to the surroundings) and are more costly in material and installation.

Yoou need to design with all of the above taken into consideration.

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