[Bldg-sim] Accounting for plenums/technical ceilings

[Jeff Haberl]

Fyi.

In DOE-2-based simulations modeling a plenum can have +- issues. For example if you model a space as a plenum then the loads in/out of the space do not show in LOADS only in SYSTEMS as a heat gain/loss to/from the plenum only when the air is flowing in the plenum. Also, there is no thermostat setting for the plenum and the load is not included in the sum of space loads.

On the other hand if you model it as an unconditioned zone then you can see the floating temperature, but it is not conditioned.

Finally, you can also model it as a space with a wider dead band so the system never comes on/off.

So, either way has its pros and cons.

This is easy to test with the SAMP1e.inp file that is distributed with DOE-2.

Jeff

Jeff S. Haberl, Ph.D., P.E., FASHRAE, FIBPSA
Department of Architecture
Texas A&M University
College Station, Texas 77845-3581
Office: 979-845-6507, Lab: 979-845-6065
Fax: 970-862-2457, jhaberl at tamu.edu<mailto:jhaberl at tamu.edu>, www.esl.tamu.edu<http://www.esl.tamu.edu>

On Oct 24, 2013, at 9:56 AM, "Shanta Tucker" <shanta.tucker at atelierten.com<mailto:shanta.tucker at atelierten.com>> wrote:

Hello Patrick,
The plenum areas of the building certainly need to be taken into account in the model and there are cooling and heating loads associatiated with plenums even if they are not conditioned. While they may not be used as a return plenum, there will still be a heat transfer between the plenum and all of the adjacent surfaces, especially the exterior wall and the ceiling. These heat transfers can also affect the occupied space load as there is an exchange through the ceiling. To answer your first question, plenums are typically modeled as separate spaces, yes. The height of the plenum should equal the floor to floor height minus the ceiling height. All of the math should add up so that the exterior wall heat transfers are correctly accounted for.

I recommend consulting the ASHRAE 90.1 users manual for more information on accounting for different definitions of spaces and partition types.

Cheers,

Shanta Tucker
Atelier Ten

T (212) 254-4500 x 210

From: bldg-sim-bounces at lists.onebuilding.org<mailto:bldg-sim-bounces at lists.onebuilding.org> [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Patrick Bivona
Sent: Thursday, October 24, 2013 9:26 AM
To: bldg-sim at lists.onebuilding.org<mailto:bldg-sim at lists.onebuilding.org>
Subject: [Bldg-sim] Accounting for plenums/technical ceilings

Hi all,

How do people generally account for plenums or technical ceilings? I'm not referring here to plenums used for air circulation, but closed ceiling space that is not considered as part of a volume to heat or cool for load calculations.

- Do you model them as separate spaces?
- Do you ignore them (but how then do you preserve the building's geometry)?
- Some other option?
- And for those using EnergyPlus, do you use the "ceiling height" field of the Zone object? The documentation says that this generates a warning if the ceiling height and calculated ceiling height differ too much, whatever too much is.

The building I'm working on has a 1.1 m (3.6 ft)  difference between the space ceiling height and the height of the floor above, so it's not something I can ignore.

Thanks for your feedback,
Patrick


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