[Equest-users] question about inefficiency of duct

Wed Mar 30 16:50:03 PDT 2011

Nick:

My experience in the real world has been that the temperature in the plenum is 
about 4-6 degrees warmer than the space temperature.  Don't forget the heat 
introduced by the lighting ballasts and the lamps themselves.  I have performed 
some stratification studies and the temperature of a space at ceiling level can 
often be 4 degrees warmer that at the standard 5' thermostat elevation.  I was 
recently commissioning a project and noticed that the T-8 lamps are very warm to 
the touch when they have been on for some time.   T-12's run cooler.  The rule 
of thumb that I have used is that 30% of the lighting wattage is radiated into 
the plenum.  It depends on the type of lighting fixture.

On a ducted return system, the plenum is much hotter than the space temperature, 
as much as 90 degrees on a hot day with a space temperature of 72 degrees.  As I 
work with automation systems quite a bit, it is not unusual for the discharge 
air to rise 6 degrees between the discharge of the fan unit and the discharge of 
a VAV box.  And this is with a duct insulated with 2" of fiber.  This is not a 
negligible heat gain, especially when working with large air handlers.

Paul Diglio

________________________________
From: Nick Caton <ncaton at smithboucher.com>
To: John Bixler <JBixler at Sebesta.com>; Paul Diglio <paul.diglio at sbcglobal.net>; 
equest-users at lists.onebuilding.org
Sent: Wed, March 30, 2011 7:04:32 PM
Subject: RE: [Equest-users] question about inefficiency of duct

Thanks John, I think that sums it up nicely – we’re very much on the same page 
now =).

My latent concerns regarding return duct inputs are resolved as well… I seem to 
have glazed over the right half of the basic system tab!

~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:John Bixler [mailto:JBixler at Sebesta.com] 
Sent: Wednesday, March 30, 2011 5:30 PM
To: Nick Caton; Paul Diglio; equest-users at lists.onebuilding.org
Subject: RE: [Equest-users] question about inefficiency of duct

Nick,

The original post as well as follow ups from Paul and myself discuss loses that 
occur in supply and return ducts, not limiting the discussion to return ducts 
nor limiting it to plenum loses, for that matter.

Equest has inputs for duct loses, in several different formats/locations.  In my 
experience, it has been difficult to accurately model these loses and obtain 
logical results.

I believe Mike’s point was “if the air in the duct is the same temp as the air 
the duct is passing through, the effect is negligible”.  I certainly agree with 
that, and the classic example of that scenario is a return duct (carrying room 
temperature air) passing through conditioned space or plenums (assuming the temp 
of the plenum is near the temp of the space).

From a practical standpoint, unless you are working on an existing building with 
outdated construction standards, or trying to model a  specific energy 
conservation measure that analyzes the impact of duct insulation (or potentially 
changing supply air temps), there are few compelling reasons to account for 
these loses (from an energy use standpoint).  

On the other hand, if you are trying to run load calcs for equipment sizing, you 
may want to account for them.  Again, equest has inputs under “duct loses” to do 
so.  It sort of sounds like this is what the original poster is asking for.  
(original poster:  try to calculate the ducts UA value and use that).

From:Nick Caton [mailto:ncaton at smithboucher.com] 
Sent: Wednesday, March 30, 2011 5:09 PM
To: John Bixler; Paul Diglio; equest-users at lists.onebuilding.org
Subject: RE: [Equest-users] question about inefficiency of duct

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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