[Equest-users] question about inefficiency of duct

[Nick Caton]

Paul/John,

 

Thanks very much for the clarifications!  I’m learning a thing or two, but I suspect my choice of words might be getting in the way of what I was trying to convey…  

 

Please consider and affirm or correct this statement:  

 

If there is rarely a case where return duct plenum losses/gains are negligible, then it follows all energy models should explicitly account for these losses/gains to those spaces.

 

A specific follow-through would be:  If an eQuest model does not model conductive heat gains/losses in the return air path, does that make the eQuest model invalid?  

 

Trying to pull this discussion back into daily practice with eQuest… I am still fuzzy on exactly how and to what extent eQuest/DOE2 does model such internal supply and return duct losses/gains (outside of light load fractions)…  Can anyone in the know clarify this query?

 

~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 John Bixler
Sent: Wednesday, March 30, 2011 9:15 AM
To: Paul Diglio; equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] question about inefficiency of duct

 

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

 

 



 

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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