[Equest-users] question about inefficiency of duct

[Paul Diglio]

Nick:

For a supply duct passing through a plenum return ceiling during the cooling 
season, the temperature difference between the supply air and return air can be 
as much as 25 degrees.

During the heating season, this difference could be as much as 30 degrees during 
morning warm up.

Paul Diglio





________________________________
From: Nick Caton <ncaton at smithboucher.com>
To: Paul Diglio <paul.diglio at sbcglobal.net>; "McArdle, Mike" 
<Mike.McAardle at state.vt.us>; xiaoyang shi <xs2144 at gmail.com>; 
equest-users at lists.onebuilding.org
Sent: Tue, March 29, 2011 8:03:32 PM
Subject: RE: [Equest-users] question about inefficiency of duct

 
I think Mike’s general point is fair and isn’t to be taken in absolutes.  
 
When a ducted return air stream is passing through a conditioned space or 
plenums/chases in between other conditioned spaces, it’s probably a safe bet to 
claim any duct gains/losses are *negligible* (not nonexistent), particularly if 
said return ducts are insulated, due to the relatively small delta-T.
 
On the other hand, winter losses and summertime gains at extreme temperatures 
may be considerable if any ductwork is routed exterior to the building, even if 
insulated.  

 
Xiaoyang, if you wish to calculate it as a percentage at design conditions:   ( 
U x A x deltaT )DUCT / (total building heating or cooling load) x 100
 
If you wanted to model such exterior duct heat losses/gains in eQuest… I haven’t 
considered that before – but depending on your system setup, it might be as 
simple as  a small exterior wall surface with appropriate U-value and area equal 
to the duct surface area… in the event the solar loads incident on the duct 
might be significant, make a point to correctly locate the surface in 3D (over 
the roof, along a wall, etc.) to avoid any incorrect self-shading from other 
surfaces.
 
If anyone has delved into this and knows a better/simpler way please do share 
=).
 
~Nick
 
 
 
NICK CATON, E.I.T.
PROJECT ENGINEER
Smith & Boucher Engineers
25501 west valley parkway
olathe ks 66061
direct 913 344.0036
fax 913 345.0617
www.smithboucher.com
 
From:equest-users-bounces at lists.onebuilding.org 
[mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Paul Diglio
Sent: Tuesday, March 29, 2011 4:59 PM
To: McArdle, Mike; xiaoyang shi; equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] question about inefficiency of duct
 
Michael:

"if the air in duct is the same air as the air being supplied to the space, 
there is no net effect"?  What do you mean by net effect?

The duct heat loss/gain depends on the difference of the temperature of the air 
in the duct and the temperature of the air surrounding the outside of the duct.  
It is highly unlikely that these temperature will be the same.

Paul Diglio
 

________________________________
 
From:"McArdle, Mike" <Mike.McAardle at state.vt.us>
To: xiaoyang shi <xs2144 at gmail.com>; "equest-users at lists.onebuilding.org" 
<equest-users at lists.onebuilding.org>
Sent: Tue, March 29, 2011 5:39:40 PM
Subject: Re: [Equest-users] question about inefficiency of duct
Dear xiaoyang shi
 
It Depends on the temperature differentials between air in duct and space and 
insulation thickness 

 
But more importantly , if the air  in duct is the same air as the air being 
supplied to the space there is no net effect 

 
 
Thank you,  Sincerely,
 
Michael J. McArdle , P.E.
Buildings Engineer II, Project Manager 
State of Vermont, 
Buildings and General Services (BGS)
2 Governor Aiken Avenue
Montpelier, Vt. 05633
 
Office: 802-828-5650;  cell: 802-272-4828
 
From:equest-users-bounces at lists.onebuilding.org 
[mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of xiaoyang shi
Sent: Tuesday, March 29, 2011 5:37 PM
To: equest-users at lists.onebuilding.org
Subject: [Equest-users] question about inefficiency of duct
 
Hi folks,

In the winter time, some heat loss in HVAC is from duct.

What is the percentage of heat loss through the duct to the total building heat 
load, due to the inefficiencies of duct delivery system?

Thank you guys.
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