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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Chris,
I don't understand your concern. You say that eQUEST models duct losses in terms of how
(they) affect unconditioned zones, but isn't that what Plenums are? As far as I know, when
eQUEST/DOE-2.2 models duct losses, they degrade the HVAC output to the conditioned spaces,
and add those losses into the Plenum spaces.
Am I missing something here?
Joe
Joe Huang
White Box Technologies, Inc.
346 Rheem Blvd., Suite 205A
Moraga CA 94556
yjhuang at whiteboxtechnologies.com
http://weather.whiteboxtechnologies.com for simulation-ready weather data
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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
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
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]
*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
] *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
Yes that's basically the idea nick thanks for the info.
This is just a theory right now but If equest contains said heat from duct losses within an unconditioned
yet highly insulated plenum With a constant volume of air and corresponding loss, the air in the plenum
would eventually reach a state of temperature equilibrium with Relation to the conditioned space below.
I'm not sure yet how equest models this but in the real world the heat would build up in the plenum.
Factors would include volume of the plenum for each shell, the insulating properties of
The roof and walls, the outside air temp and the temp set point of the corresponding
Conditioned Zone below. The temp maintained in the plenum would thus vary
By zone and/or shell.
That temp level being maintained in the plenum by the constant volume losses
Would vary by zone and system obviously but if the conditioned space in any way interacts thermally
Even with just the lid, there could be increased energy efficiency of the conditioned space in the
Form of less energy needed for a baseboard, for example.
It just depends on how your software models the interaction.
If the increase in efficiency makes up for the extra cost of making up for the lost heat/air by the system
There would be overall cost savings in a given model year.
In climate zone 8, where every btu is important and where systems can sometimes heat for over half the
year, this cost savings would be expected to be amplified, if you want to call it that.
I'm planning an experiment on my current model to see if the lost heat in any way affects the energy
performance. This is all just an idea I had the other day and I'm in no way an expert.
Common sense tells you it would use more energy to make up for lost supply air and heat.
But there would be other factors at play. If the conditioned space doesn't require as much
Heat from, say, a baseboard, because of the temperature within the plenum the overall cost
could potentially be lower Than not modeling any duct losses.
Sent from iPad
Just to clarify, I'm talking about duct losses in terms of a fully ducted system.
In my case packaged single zone.
In other words the plenum above lid would have no air flowing through it - return air or otherwise.
Just the lost air and heat from the duct losses.
It will be interesting to see the results. I'll be sure to share with the users.
Sent from Outlook
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