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Old 11-14-2010, 03:30 PM   #1
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Need help with New HVAC Reducing Plenum Duct Design


Hi, I am installing a new HVAC system in my house. I will be installing a 4 Ton heat pump in Maryland. I believe that is 1600 CFM's. I will be using the reducing plenum duct design. Currently, I have the following:

Metal Return Duct size:
12x28 @ 686fpm w/ 2 Return grills FILTERED @ 24x24
The recommended grill size UNFILTERED is 20X20, does it hurt to go with the larger grill?


Metal Supply trunk:
8x32 (1600cfm's) @ 900fpm (not longer than 20 feet)
I will have 9 100cfm run offs (3.25 x 10 rectangular metal). This leaves me with 700 cfm's.

Reduced Supply trunk:
8x15.5 (700cfm's) @ 813fpm
I will have 7 100cfm run offs here.

Did I do this correct?? Are my duct sizes proper??


If I have 1600 CFM coming in, do I need 1600 CFM coming out?
Are there any issues in reference to static air pressure??
What is the minimum plenum size coming off of the unit?

I am thinking about the Lennox x17 (single stage) or xp16 (two stage). Any opinions?
Do bends in the run offs take away from the cfm's/fpm? if so what is it @ 45 and 90 degrees??

Any help or suggestions would be great.

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Last edited by Matt2010; 11-14-2010 at 04:17 PM.
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Old 11-14-2010, 05:48 PM   #2
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Need help with New HVAC Reducing Plenum Duct Design


You can't have 1600 CFM coming into the furnace/air handler, if you don't have 1600 CFM leaving it.

16X8 is small for 813 CFM. 20X8 is more appropriate.

The 32X8 is slightly small also. But you might get away with it.

The PD of a rectangular ell varies slightly with the way its made. Radiused, or not. A radiused ell will be around 25 foot. An non radiused will be around 45 foot. Depending on weather its a flat ell, or vertical ell.

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