[Equest-users] question about inefficiency of duct

[Nick Caton]

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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