[Equest-users] Benefit to modeling duct losses in cold climates?
Nicholas Caton
ncaton at catonenergy.com
Wed Mar 25 22:41:49 PDT 2015
This query now reminds me of a discussion some years back I participated in
here on the lists. I recall getting myself turned around reading up on
plenums and heat transfer interactions. The attached thread (also copied
below – I’ve had trouble with attaching emails lately), may help contribute
to this discussion.
I’m not certain of the exact interactions (some prodding with a simple
dummy model could work out the kinks), but I think the concern Chris B. is
trying to raise/quell is whether duct heat losses “leave the thermodynamic
box,” so to speak during the simulation. Intuitively, an amount of heat
added one hour to a roof/skin-load bearing plenum in the winter should
reduce the relative difference in temperatures to a conditioned space
below, thus reducing the transfer of heat in that direction for that hour…
right?
All things being equal, I would expect modeling return air duct thermal
transfer losses through plenums (as opposed to not modeling those losses)
and/or duct air losses (leakage) to result in overall higher
fan/space-heating consumptions in a climate like Alaska, but perhaps
negligibly so in the grand scheme of things, particularly if your project
involves plenums without substantial skin load exposure (i.e. inter-floor
plenums).
Hope that helps clear the air, somewhat…?
~Nick
*NICK CATON, P.E.*
*Owner*
*Caton Energy Consulting*
1150 N. 192nd St., #4-202
Shoreline, WA 98133
office: 785.410.3317
www.catonenergy.com
*From:* Equest-users [mailto:equest-users-bounces at lists.onebuilding.org] *On
Behalf Of *Chris Baker
*Sent:* Wednesday, March 25, 2015 2:20 PM
*To:* Jones, Christopher
*Cc:* equest-users at lists.onebuilding.org
*Subject:* Re: [Equest-users] Benefit to modeling duct losses in cold
climates?
Thanks Chris.
I’m only implying that a small amount of duct loss (compliant with ASHRAE
90.1) in that the loss would heat the “DUCT-ZONE” somewhat.
That heat would still be contained within the building which could
potentially better insulate it during months of extreme cold.
In other words the heat going into “DUCT-ZONE” due to the duct loss would
still be heating the building.
It would just be going into an unconditioned space.
I’m just not sure if the heat contained within an unconditioned zone is
factored into an annual simulation.
Duct losses lower the supply air rate somewhat which have to be made up (in
the form of more energy usage by fans and heating systems).
If there is any potential cost benefit caused by this you would have to
cancel that out in the form of the added heat performance in winter caused
by the duct/heat losses.
In other words equest would have to model an advantage to having warmer
unconditioned spaces in the cold winter months to make it worthwhile.
I think I’m going to experiment with this once I finish my current model.
I doubt it will matter much but if there is cost savings – why not model it
if it is still ASHRAE compliant.
-Chris
*From:* Jones, Christopher [mailto:cjones at halsall.com <cjones at halsall.com>]
*Sent:* Wednesday, March 25, 2015 11:48 AM
*To:* Chris Baker; equest-users at lists.onebuilding.org
*Cc:* Joe Huang
*Subject:* RE: Benefit to modeling duct losses in cold climates?
Chris,
I suggest you go back and read the DOE22Vol2-Dictionary manual. If you are
talking about the DUCT-AIR-LOSS, then yes, the duct zone must be a plenum
or unconditioned zone.
If you are talking about the heat/cool energy lost (DUCT-DT or DUCT-UA)
then the duct zone can be a plenum if one exists or any zone on the system,
you specify the DUCT-ZONE. Only 1 zone is specified.
*Christopher Jones**,* P.Eng.
Tel: 416.644.4226 • Toll Free: 1.888.425.7255 x 527
*From:* Equest-users [mailto:equest-users-bounces at lists.onebuilding.org
<equest-users-bounces at lists.onebuilding.org>] *On Behalf Of *Chris Baker
*Sent:* Tuesday, March 24, 2015 7:33 PM
*To:* equest-users at lists.onebuilding.org
*Subject:* [Equest-users] Benefit to modeling duct losses in cold climates?
So I haven’t yet modeled my current project with duct losses. This is a
13,000 sf building in Alaska (Climate zone 8).
Equest only models duct losses in terms of how it affects unconditioned
zones (it does not model duct losses to conditioned areas).
But over the course of an Alaskan winter (October thru March) any duct
losses would act to somewhat heat the plenum space.
So would there be a benefit in thermal performance over that period?
Has anyone experimented with this?
This would obviously add an additional load requirement to the air-side
systems and the pumps/air systems would obviously use slightly more power
(especially for PSZ systems) to make up for any duct losses.
I think the only potential benefit would be thermal performance but I’m not
sure if equest takes into account thermal energy in a plenum (unconditioned
space)
when it models the thermal performance of a conditioned space beneath it
during the course of 1 model year.
If it does, I think the only potential benefits during winter months would
be in extremely cold climates like Alaska.
Otherwise you are just adding more heating and power loads on the pumps,
fans, etc… Which over the course of 1 year would have an impact.
HOWEVER…
If you model proposed duct losses the same as baseline might there be a
greater overall cost savings in proposed model while keeping the baseline
ashrae compliant?
ASHRAE (at least the 2013 version) has equations for determining duct
losses in the baseline model.
The proposed building would obviously retain the thermal energy more
efficiently than the baseline and it might be an overall benefit in cost
savings in terms of heating costs
Regardless of how much additional energy is used to make up for the duct
losses on the system.
Both models would act to contain the heat expelled by the duct losses but
the proposed model would do a better job, basically, than baseline even
though the duct losses are the same.
Chris Baker
CCI CAD Drafter
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Beginning of copied discussion thread “Re: [Equest-users] Roof vs Exterior
Wall Load”
Nuances of modelling. I agree with you Nick on your conclusions. I
didn't mean that I actually apply all the plenum load to the space but keep
this information grouped so it is front and centre and moved to appropriate
spaces as I fine tune my design, deal with the type of return(s) and
lighting loads. I also track OA like this as well. I also had ducted
returns and multistory building pop into my head later to further
complicate my thoughts on this matter. Then also too, dealing with old
software versions and a certain amount of acquired distrust, I may not be
as "old as dirt" as in another fine modellers resume, but I have been
around the block a few times.
With one air handler per floor on a multistory building with a plenum
return everything is pretty simple for the thermodynamics. The occupants
might not be to pleased but on average the system works. Today's reality
is not even close to this and many methods are used to keep every
occupant's spot the perfect temperature with the proper amount of fresh
air. My main point was not to forget the plenum next to the roof, the roof
load imposed on it and how you are going to deal with it when you move away
from a plain system. It is a significant load which could possibly be
forgotten about in a complex system. As you and Jonathan have nicely shown
eQuest deals with it well but in a cloaked fashion.
On the always learning, I just discovered 3.64 now accounts for
insulated return ducts, to second the reading of the help files on occasion.
This discussion has just scratched all the variables and implications
of just a return flow to an AHU. eQuest being openly available for
everyone to use and being an excellent modelling program, it can make
things seem too easy. Put in some data, press the button and you are a
building designer or modeller. I was consulting on a new car dealership
and my customer took me to one they had finished a year or 2 earlier.
Everyone knows the layout, a 2 story glass box attached to a building
facing south. He wanted a quick heat loss done by hand on piece of paper
and no modelling, "just like they always did it". It was a beautiful sunny
fall day, we were in short sleeves, and he was chatting about how great the
building was and how simple the engineering should be (cheap) to re-size
the new system. The first thing we saw inside was the salesman sitting
there in his *winter* coat in the sunshine. These are complex systems and
so is the modelling software. I just looked at my detailed sim file I was
referring to on this issue, 1,509 pages of data. It is not quite as simple
as it seems and it pays to run a spreadsheet alongside the model to keep
track and organize everything. They are great for troubleshooting too.
Bruce
On 08/09/2011 04:47 PM, Jonathan Smith wrote:
eQuesters:
I ran an hourly report comparing zone temperatures for a plenum and
adjacent zone. Temperature delta for the plenum was on average 4 degrees
less in winter than the occupied zone. I could not identify a System or
Zone hourly report variable to track heat flux of interior surfaces.
Looking at the DOE 2.1 Engineering Manual there is an explanation of the
calculation procedure whereby the LOADS program temperatures are adjusted
in the SYSTEMS program to account for a delta between adjacent zones. This
adjusted load impacts the system sizing when the user selects
“ADJUST-LOADS”.
IV.25-26, Calculation Outline, A.
IV.174-185, Interface between LOADS and SYSTEMS (Subroutine TEMDEV)
Jonathan
*From:* Nick Caton [mailto:ncaton at smithboucher.com <ncaton at smithboucher.com>]
*Sent:* Thursday, September 08, 2011 3:49 PM
*To:* Bruce Easterbrook; Jeremy Poling
*Cc:* equest-users at lists.onebuilding.org; Jonathan Smith
*Subject:* RE: [Equest-users] Roof vs Exterior Wall Load
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
[image: cid:489575314 at 22072009-0ABB]
*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
<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
<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 <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*®*AP*
[image: signature-logo_60%]
40 Beaver Street
Albany, New York 12207
p: 518.463.8068 x268
f: 518.463.8069
www.csarchpc.com
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