# [Equest-users] Benefit to modeling duct losses in cold climates?

Chris Baker CBaker at cci-alliance.com
Thu Mar 26 23:40:13 PDT 2015

```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.

_____________________________
From: Nicholas Caton <ncaton at catonenergy.com<mailto:ncaton at catonenergy.com>>
Sent: Wednesday, March 25, 2015 9:41 PM
Subject: RE: [Equest-users] Benefit to modeling duct losses in cold climates?
To: Chris Baker <cbaker at cci-alliance.com<mailto:cbaker at cci-alliance.com>>, Jones, Christopher <cjones at halsall.com<mailto:cjones at halsall.com>>
Cc: <equest-users at lists.onebuilding.org<mailto:equest-users at lists.onebuilding.org>>

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. 192<x-apple-data-detectors://8/1>nd<x-apple-data-detectors://8/1> St., #4-202<x-apple-data-detectors://8/1>
Shoreline, WA 98133<x-apple-data-detectors://9/0>
office:  785.410.3317<tel:785.410.3317>
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From: Equest-users [mailto:equest-users-bounces at lists.onebuilding.org<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<mailto: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<mailto: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<tel:416.644.4226> • Toll Free: 1.888.425.7255 x 527<tel:1.888.425.7255;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<mailto: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

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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

[cid:image001.png at 01D0674C.D95CFDF0]
From: Nick Caton [mailto:ncaton at smithboucher.com]
Sent: Thursday, September 08, 2011 3:49 PM
To: Bruce Easterbrook; Jeremy Poling
Cc: equest-users at lists.onebuilding.org<mailto: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:”
[cid:image002.png at 01D0674C.D95CFDF0]

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
[cid:489575314 at 22072009-0ABB]

NICK CATON, P.E.
SENIOR ENGINEER

Smith & Boucher Engineers
25501 west valley parkway, suite 200<x-apple-data-detectors://30/0>
olathe, ks 66061<x-apple-data-detectors://30/0>
direct 913.344.0036<tel:913.344.0036>
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From: equest-users-bounces at lists.onebuilding.org<mailto: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<mailto: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> [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<mailto: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:

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<mailto: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

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