[Equest-users] question about inefficiency of duct

[John Bixler]

In addition to significant delta Ts, please also consider the additional airflow required to cool/heat a space when the supply air delta T (between the supply air and the conditioned space) is lowered due to duct losses.

Example:  Say the cooling load requires 100 cfm to cool a space assuming a supply air temp of 55 and a perfectly insulated duct (that is, a duct with no change in temp as it leaves the AHU and gets to the space).  The room is 70 degrees, so which makes the sensible cooling in the space about 1630 btu.

Now take this same room, and assume the duct gains 5 degrees between the AHU and the space (note, I am not talking about heat gain from fan energy here).  Now the air being supplied to the space is 60 degrees, and to maintain the same sensible cooling capacity, the airflow must be 150 cfm, a 50% increase (!).

This heat is not lost to the space being cooled (most likely).  The idea of *neglible* loss at the room level does not apply.

At the system level, the idea of *neglible* energy loss may or may not apply.  CLEARLY there is additional fan energy used.  The heat that is lost from the duct to somewhere, may be recovered to a large degree (in the case of a 100% return air system, where the primary area of energy loss COULD be additional heat gained in the plenum due to increased delta t between the plenum and the OA temp) or it may be lost entirely (in the case of a 100% OA system, which is obviously already a big energy user and this simply adds to the problem).  In the case of significant relief of the air the heat is lost to, there is also a large impact on cooling and heating energy as you are moving more air, and trying to maintain the same setpoints at the system level.

The notion that there is no net effect is really only true if the heat is being lost/gained directly at the location of the temperature sensor/tstat.

(Most buildings these days have appropriate insulation on the ductwork which minimizes this effect, but it wasn’t that long ago when this wasn’t the case)

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 7:32 PM
To: John Bixler
Subject: Re: [Equest-users] question about inefficiency of duct

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


[cid:489575314 at 22072009-0ABB]

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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