Need Help With New HVAC Reducing Plenum Duct Design - HVAC - DIY Chatroom Home Improvement Forum

Go Back   DIY Chatroom Home Improvement Forum > Home Improvement > HVAC


Thread Tools Display Modes
Old 11-14-2010, 03:30 PM   #1
Join Date: Nov 2010
Posts: 1
Rewards Points: 10

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.


Last edited by Matt2010; 11-14-2010 at 04:17 PM.
Matt2010 is offline   Reply With Quote
Old 11-14-2010, 05:48 PM   #2
An old Tradesmen
Join Date: Oct 2008
Location: Somewhere
Posts: 29,994
Rewards Points: 11,460

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.


beenthere is online now   Reply With Quote

new ductwork , new hvac install

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Similar Threads
Thread Thread Starter Forum Replies Last Post
Planning on New duct work..any feedback... kilosos2 HVAC 28 01-26-2011 10:21 AM
ECO / green options for insulating HVAC duct work ACtawk HVAC 3 10-03-2010 07:02 PM
General HVAC Design Issue Robert Green HVAC 4 05-29-2010 12:02 PM
plenum splitter design & linkage rebar HVAC 8 01-19-2010 08:18 AM
Plenum Design winkydink HVAC 1 11-22-2006 11:28 PM

Top of Page | View New Posts