[Equest-users] External wall area for simulation

Daniel Knapp danielk at arborus.ca
Tue Mar 1 10:24:37 PST 2011


Since the area of the outside surface of the insulation scales with the thickness of the insulation (surface area = 2*Pi*L*radius), I don't see how it is possible to increase the heat loss by adding insulation to a pipe.  The formula for radial heat flow from the inside to the outside of a cylinder is given by Q = [2*Pi*k*L (T_i - T_o)] / [Ln(r_o/r_i)], where k is the thermal conductivity, L is the length of the cylinder, T_i is the inside temperature, T_o is the outside temperature, r_o is the outside radius, r_o is the interior radius.  As the thickness of the cylinder increases (that is, as r_o increases) the radial heat flow will decrease as 1/Log(r_o).

Cheers,
Dan

—
Daniel Knapp, PhD, LEED® AP O+M
danielk at arborus.ca

Arborus Consulting
Energy Strategies for the Built Environment
www.arborus.ca
76 Chamberlain Avenue 
Ottawa, ON, K1S 1V9 
Phone: (613) 234-7178 ext. 113
Fax: (613) 234-0740



On 2011-03-01, at 10:47 AM, Michael Hardy wrote:

> As Nick previously mentioned, you may need to use different areas for different aspects of the project (lighting, PV, energy, ventilation, etc.) For the energy modeling portion, you should probably talk to your project design engineer as the building construction you describe is a bit unusual.
>  
> Going back to the fundamental heat flow equations that simulations use, the heat flow calculation is based on some area  and includes outside and inside air films, wall assembly construction, material properties, heat transfer path, etc.  For your specific building, the thermal properties of the wall material may not offset the additional wall areas due to the thickness of the wall.  In a normal size building, the wall area of the corners is usually small in comparison to the total wall area so the corner details become negligible.  Since you’ve said the building area changes by 4% when you include the wall thickness, you are correct to raise the question.  I think the question needs to go back to the design engineer who is familiar with your specific building construction.
>  
> In a very simple example, when specifying insulation for piping, it is actually possible to increase the heat-loss by adding too much insulation because of the increase in surface area relative to the insulation thickness.  I know your building is not a pipe (and probably not round) but this simple example just goes to show how important it is to understand the limitations of the equations being used and not assume that what is negligible for 8-12” thick wall construction, is also negligible for 24” thick wall construction.
>  
> Regards,
>  
> Michael Hardy, PE, F.NSPE 
> Sr. Mechanical Engineer
> Black Oak Engineering, Inc.
> Cell 503.314.5244
> From: equest-users-bounces at lists.onebuilding.org [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Praveen Jain
> Sent: Tuesday, March 01, 2011 4:44 AM
> To: Nick Caton
> Cc: equest-users at lists.onebuilding.org
> Subject: Re: [Equest-users] External wall area for simulation
>  
> Thanks Nick for good explanation.
> 
> Actually in my project we are using almost 2ft wide external walls to get the effect of thermal mass. if I consider external wall area in eQUEST model my area deviation is almost 4% and mismatch in conditioned area is more than this. So project energy consumption would be more and also not able to match TR for the project.
> Actually we are also planning to go for 2.5% renewable energy credit, if I consider external wall than PV cost would be more.
> 
> Can I exclude external wall area for eQUEST modeling ?
> 
> --
> With Thanks and Regards
> Praveen
> 
> On Mon, Feb 28, 2011 at 10:02 PM, Nick Caton <ncaton at smithboucher.com> wrote:
> Hi Praveen,
>  
> I’ve copied below a discussion from some time back outlining my general practice for “where to draw the line” when it comes to envelope/wall boundaries.  This may be generally useful and seems to answer at least part of your inquiry.
>  
> When I’m doing electrical and/or HVAC design alongside the model, my area takeoffs for each inevitably will match up, because I hate doing the same work twice. 
>  
> I use the outermost surfaces when defining my building footprint and midpoints for all internal partitions for all calcs.  Space-by-space LPD calcs in a strict reading do not require the space areas to be measured to the outermost surface of an exterior wall (they do say to use the midpoint of interior partitions, as of 90.1-2007).  I’d feel comfortable saying the extra square-footage “handicap” I’m imposing on myself as a lighting designer is an insignificant fraction in 99.9% of cases when determining baseline LPD…   
>  
> Inevitably, areas summed for all spaces in a building between architects, HVAC, and lighting designers will not match – that’s a fact of life and in my book that’s okay, so long as nobody is way off.  Model reviewers will inevitably/reliably gripe when the numbers don’t match exactly, and it’s usually  an easy thing to either fix or explain after the fact.  If you to try to make everyone use the same numbers from the DD/CD design phases, you’ve chosen a losing battle.  For my part in the role of the project’s energy modeler, I’m satisfied to allow my fellow designers do their own calcs, and just ensure nobody is way off along the way… quibbles over small differences in the final tallies, if they come up, are easy to reconcile.
>  
> ~Nick
>  
> Error! Filename not specified.
>  
> NICK CATON, E.I.T.
> PROJECT ENGINEER
> Smith & Boucher Engineers
> 25501 west valley parkway
> 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 Praveen Jain
> Sent: Monday, February 28, 2011 3:37 AM
> To: equest-users at lists.onebuilding.org
> Subject: [Equest-users] External wall area for simulation
>  
> Dear All
>  
> While modeling in eQUEST should we include external wall area or not?
> While creating single line diagram I exclude external wall and draw sld on inner of external wall to match conditioned area.
> But for lighting power density calculation ASHRAE 90.1 user manual suggest to including external wall area.
> We are getting mismatch in area calculation for all HVAC, Lighting and Architectural design sides.
>  
> Can anyone suggest what correct way of modeling in eQUEST ?
> 
> -- 
> With Thanks and Regards,
> Praveen K. Jain
> Roorkee
>  
>  
> From: equest-users-bounces at lists.onebuilding.org [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Nick Caton
> Sent: Wednesday, August 04, 2010 1:20 PM
> To: John Aulbach; Nijenmanting, F.C.; equest-users at lists.onebuilding.org
> Subject: Re: [Equest-users] area of (internal) partitions
>  
> Filique,
>  
> This sounds a lot like a question I brought up when I was getting started with eQuest and energy modeling in general:  What is “standard practice” regarding where the inter-zonal and footprint boundaries are defined, relative to actual wall thicknesses?  The quick answer is that there aren’t any hard/fast answers to this.  It was a good discussion and is probably archived in [bldg-sim] if anyone cares to dig around…
>  
> In short: 
> 1.       eQuest areas correspond to the polygons used to define the floor areas/building footprint.  These polygons are defined by the shapes defined or traced from a CAD file in the wizards.  I’m pretty sure (someone correct me if I’m wrong) the floor area taken up by internal partitions (which have thickness) on the floor slab is NOT subtracted from the total area, so if you double the thickness of your internal partitions you will not see a change reflected in space areas.  This is normally not a big deal in the context of thermal modeled behavior, but if you have many unusually thick partitions (1ft or more deep), you might want to either account for them by modeling them as unconditioned zones between spaces. 
> 
> 2.       Regarding “best practice,” here’s a set of general guidelines that would apply to various loads & modeling software packages (beyond eQuest), prioritizing the interest of modeling with thermal accuracy:
> 
> a.       Define interior partitions using the midpoint between the two surfaces.  This is generally not terribly critical – I will take small liberties on this to reduce the number of vertices and simply actual internal zones.
> 
> b.      Define the building footprint areas using the outermost surface.    This is more important as you want to accurately model the actual amount of surface area subject to exterior loads.
> 
> c.       Define top of each zone (in the z-axis) using the top surface elevation the respective floor or roof construction.
> 
> 3.       On a related note, this ties into the general advice to not use energy modeling programs with the intent to create perfect 3D representations of your buildings.  If you need a pretty picture, they make 3D modeling software for that purpose.  When it comes to building geometries, simplicity is a virtue, and ASHRAE will even back you up on that one (re: 90.1 User’s Manual).  Avoiding overly-accurate building geometries lets you spend more time modeling the more critical building elements of a building’s energy behavior (loads/systems/schedules/etc).
> 
>  
> Best wishes,
>  
> ~Nick
>  
> Error! Filename not specified.
>  
> NICK CATON, E.I.T.
> PROJECT ENGINEER
> 25501 west valley parkway
> olathe ks 66061
> direct 913 344.0036
> fax 913 345.0617
> Check out our new web-site @ www.smithboucher.com
>  
> From: equest-users-bounces at lists.onebuilding.org [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of John Aulbach
> Sent: Wednesday, August 04, 2010 12:15 PM
> To: Nijenmanting, F.C.; equest-users at lists.onebuilding.org
> Subject: Re: [Equest-users] area of (internal) partitions
>  
> Filique:
>  
> Can you rephrase your question, please?  I don't understand how doubling the size of your internal partitions decreases your floor area. The eQuest floor areas are (to my undestandinding), from the outside of outside walls to the "outside" of any internal partitions IN THE SPACE THAT PARTITION RESIDES. You can have only one internal wall separating two adjacent spaces.
>  
> Others may comment.
>  
> John R. Aulbach, PE, CEM
> Senior Energy Engineer
> Partner Energy
> 1990 E. Grand Avenue, El Segundo, CA 90245
> W: 888-826-1216, X254| D: 310-765-7295 | F: 310-817-2745
> www.ptrenergy.com | jaulbach at ptrenergy.com
>  
>  
> From: "Nijenmanting, F.C." <F.C.Nijenmanting at student.tue.nl>
> To: "equest-users at lists.onebuilding.org" <equest-users at lists.onebuilding.org>
> Sent: Wed, August 4, 2010 12:15:19 AM
> Subject: [Equest-users] area of (internal) partitions
> 
> Hello all,
> 
> I have a question about the area which is calculated by equest.
> Does it take into account the width of internal partitions?
> In reality, if I would double the size of my internal walls, my actual floor area decreases.
> But I could not find differences in the floor area calculated by Equest if I change the thickness of my walls.
> 
> A similar question accounts for the external partitions. For drawing the ground plan, should I therefore take the internal boundary, external boundary or middle line?
> 
> Kind regards,
> Filique
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>  
>  
> 
> 
> 
> -- 
> with regards,
> Praveen K. Jain
> Roorkee
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