[Equest-users] Roof vs Exterior Wall Load
Nick Caton
ncaton at smithboucher.com
Thu Sep 8 12:48:53 PDT 2011
Hey everyone,
Sorry to chime in late... but I have some input to wedge into the
conversation as it's wrapping up:
First: As has already been said, LS-F is inclusive of all 'conditioned'
spaces. I didn't realize the DOE2 entries explicitly (more or less) say
that, and had figured it out for myself via logic/gray hair growth... =)
I'll take this as a reminder that the DOE2 help files are perpetually
more useful than they may seem!
Second: I'm also no developer, but Bruce's conclusions didn't sit well
with me so I did a quick study to confirm... I don't think you can
characterize plenum loads as being *entirely* separated from the
conditioned spaces... Do such loads (really hot/cold plenums) get
directed straight back to the central air handler, or do they also
partially interact with the zones as well?
I know ceilings generated by eQuest wizards by default are not
adiabatic, but do use a construction with defined heat transmittance
properties.
I slapped together a 1-zone, 1-floor dummy model and tried changing the
default ceiling construction (U~0.5) to adiabatic in a parametric run.
Return air path was set to "plenum zones." The plenum zone itself was
changed from "unconditioned" to "plenum:"
Lo and behold: The two runs do indeed have differing
cooling/heating/ventilation consumptions (attached)!
* Investigation of the LS-F reports shows the whole building
deals with identical load components altogether - which is expected.
* Comparing of the LS-E reports reveals the plenum with an
adiabatic ceiling has less heating and less cooling to deal with over
the year. Both runs' conditioned space however deal with identical
loads for each month.
Conclusions!
1. Bruce is NOT to be questioned: Plenums modeled as a return air
path do not transmit their loads to the corresponding spaces below.
They carry their loads directly to the parent system.
2. Return air plenums DO however receive heating/cooling loads
transmitted from the conditioned spaces below via the ceiling surfaces.
It's basically a 1-way street.
3. I'd agree real-world behavior is somewhere in-between:
Return-air plenum ceilings should act as a 2-way street thermally, and a
single plenum can be hot on one side but cold on another at any given
instance. In many cases however, I suspect the assumption of all loads
staying in the plenum is closer to reality than assuming they all dump
into the corresponding space below, from an supply airflow-sizing
perspective.
~Nick
NICK CATON, P.E.
SENIOR ENGINEER
Smith & Boucher Engineers
25501 west valley parkway, suite 200
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 Bruce
Easterbrook
Sent: Thursday, September 08, 2011 12:18 PM
To: Jeremy Poling
Cc: equest-users at lists.onebuilding.org; Jonathan Smith
Subject: Re: [Equest-users] Roof vs Exterior Wall Load
Thanks Jeremy,
I went back and reviewed my spaces and removed the plenums from my
tally. The LS-E and the LS-F now agree. The information in the reports
summary also had zeros for the roof components but a note indicated that
only 6 of the reports were shown and these were from the ground floor of
a 3 story building, so spaces with roof components weren't shown. So
the plenums are not included in the reports. The bigger question now is
is the plenum load properly accounted for in the eQuest simulation. I
believe you are correct on the way eQuest accounts for the heat
gains/losses. This lumps all the various zone plenums onto the AHU
system as one value. In the cobwebs I think this is why I don't use
this report. I prefer to keep all the loads in the zone and so I add in
the plenums. This way later when I'm changing my zoning I don't forget
what can many times be a significant load on the AHU. It also makes
checking easier. In reality with firewalls, ducted returns, this is not
the way an actual system deals with the plenum loads. The heat or cold
can actually penetrate the room space and possibly throw off the ability
of the supply air flow to deal with this extra load. So definitely a
caution to think about when fine tuning a building system in the model
and when actually designing the HVAC system. The load is accounted for
but maybe not in the way you expected and can easily be forgotten about
as we just proved! lol.
Bruce
On 08/09/2011 11:49 AM, Jeremy Poling wrote:
For what it's worth, the example LS-F report in the help file
documentation also shows only 0's in the roof component. From within
eQuest, go to the Help menu, select DOE-2 Help. Expand the "Volume 4:
Libraries & Reports" and then the "Reports" topics. LS-E and LS-F are
under the loads reports. The text says the monthly space components are
summed across spaces. The definition of the LS-F report excludes plenum
spaces:
This report gives a breakdown of loads on a monthly basis for the entire
building, according to the source of the load. The loads in
unconditioned spaces (ZONE-TYPE = UNCONDITIONED or PLENUM) are not
included; all entries are in millions of Btu/month.
This just explains why they don't show up in the LS-F...I believe that
the mechanism used by eQuest to account for heat gain in the plenums is
to apply that heat gain to the return air before the system, thereby
increasing temperature, humidity, etc. in the RA stream due to the loads
in the plenum spaces connected to a given system. Someone from Hirsch
can jump in if I got that wrong.
I'm curious if anyone else has dug deeper on this than I have, though?
Jeremy R. Poling, PE, LEED AP+BDC
From: equest-users-bounces at lists.onebuilding.org [
mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Bruce
Easterbrook
Sent: Thursday, September 08, 2011 10:09 AM
To: Jonathan Smith
Cc: equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] Roof vs Exterior Wall Load
To tell the truth I haven't a clue. You would think the sums would
agree. I did open one of my sim's and summed the roof components for
all my zones from the LS-E report and found the sum didn't match the
figure in the LS-F report, it was much higher. I rarely use these
reports and hadn't noticed this before. eQuest does treat roofs as
horizontal walls but on my sim I know I didn't do any modifications
which might have mixed things up. I took a look at the "Detailed
Simulation Reports Summary" and there was nothing which stood out as an
explanation. So the quickest thing to do is pass this back to the group
to see if we can find an answer.
Bruce
On 08/09/2011 09:33 AM, Jonathan Smith wrote:
Hi Bruce:
Thanks you for your reply.
The LS-F report I'm reviewing is a building wide report which should be
inclusive of all spaces. As stated previously, MBTUs through ROOF
components is listed as zero. However, the space level report (LS-E) for
the one of the plenums with a roof component shows heat loss of the roof
component. This is true for some of the other plenum spaces. Any ideas
why the space level report (LS-E) ROOF components are not reported on
the building wide report (LS-F) as roof components?
Jonathan
From: Bruce Easterbrook [mailto:bruce5 at bellnet.ca]
Sent: Wednesday, September 07, 2011 9:08 AM
To: Jonathan Smith
Cc: equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] Roof vs Exterior Wall Load
In the room space the roof is actually the ceiling. Because the room
and plenum spaces are close or the same temperature there is minimal or
no heat loss between them. You need to go to the plenum space for the
heat loss through the exterior roof. If you want the heat loss for the
full floor to floor space you have to add the room and plenum losses.
The plenum will have some wall surfaces as well.
Bruce Easterbrook P.Eng.
Abode Engineering
On 06/09/2011 01:46 PM, Jonathan Smith wrote:
RE: Detailed Report, LS-F Building Monthly Load Components
Why would roof heat losses be zero in a model defined with plenum spaces
that have roofs components? In report LS-F, there are no losses reported
in 'Roof' and 'Wall' heat losses seem exceptionally high, as if 'Roof'
losses are reported in 'Walls'.
All spaces were created using the eQuest wizard mode. Inspecting the
"exterior surface properties", the word "Roof" appears below the surface
name. All roof constructions are made with layers input, having a
U-Value 0f 0.043. When creating exterior surfaces in detailed edit mode,
no option exists for defining an exterior surface as a roof or wall.
So, how are the loads associated with roofs reporting in LS-F? What
other reasons might roof loads not report, or at least not report in the
correct load component?
Jonathan R. Smith AIA LEED(r)AP
40 Beaver Street
Albany, New York 12207
p: 518.463.8068 x268
f: 518.463.8069
www.csarchpc.com <http://www.csarchpc.com/>
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