[Bldg-sim] Suspended ceilings vs. open plenums

[James V. Dirkes II P.E.]

Dear Paul,
 
In theory, the return plenum is warmer than the occupied space, so the wall
and, especially, roof loads do not affect the space in the same way.  This
will give a warmer coil entering condition and resulting smaller coil,  fan,
duct, motor, wiring, electrical service, etc.
 
The ACTUAL difference will be small and may not be worth the additional
analysis and load calculations.  You could argue that a VAV system will
realize the benefits, since it will "see" the space load only and reduce
volume to suit.
 
Another consideration is whether the life cycle energy benefits outweigh the
differential cost to install and maintain the suspended ceiling....  I
suspect they do, but am uncertain re: ROI.
 

The Building Performance Team
James V. Dirkes II, P.E., LEED AP
1631 Acacia Drive NW
Grand Rapids, MI 49504
616 450 8653

 

 

  _____  

From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of P. Hay
Sent: Thursday, January 08, 2009 10:19 AM
To: BLDG-SIM
Subject: [Bldg-sim] Suspended ceilings vs. open plenums



Hi all,

 

I recently came across a Life Cycle Analysis of Suspended Ceilings vs. Open
Plenums, initiated by CISCA [Ceiling and Interior System Construction
Association ( www.cisca.org )] which concluded that suspended ceilings are
more energy efficient because they have:

 

a)      higher reflectances,

b)      better heat-removal from luminaires, and

c)       use lower static pressure and fan power.

 

I can understand (a) and (c) but (b) leaves me suspicious that the
return-air plenum was not considered in this study.  As I understand it,
return-air plenums can be defined as separate zones from the usable spaces
(in this case: an office or a food store) but the suspended ceilings are
adiabatic.  So , other than the savings for lighting and fan, I really don't
understand how there could be significant savings for the HVAC system if the
overall height of the rooms (plenum & usable space) is identical, and
cooling load of plenums are considered.  

 

Is there something I'm missing?

 

Paul Hay

Managing Partner

PAUL HAY Consultants

 

Capital Project Planning and Management

 

15a Cassia Park Road

Kingston 10

Jamaica, W.I.

 

tel: 1 (876) 756-0631

cel: 1 (876) 324-4274

fax: 1 (876) 756-0631

 

web: www.phcjamaica.com

e-mail: paul.hay at phcjamaica.com

 

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