[Equest-users] Values choosing of the typical equipment &miscellaneous power densities.

[Nick Caton]

Bobby et al:

 

Important clarification: The building performance rating method does NOT require models ¡°closely match reality.¡±  This is an easy misperception which has landed many in hot water.  90.1 Appendix G is purposefully and necessarily arbitrary to reality in many regards (Consider:  it¡¯s made to be usable with buildings that are still only on paper), so it inherently follows the resulting rating and proposed model are often arbitrary to reality as well.  

 

Nearly every time I see a chart showing ¡°actual¡± vs. ¡°LEED modeled¡± utility bills, I roll my eyes (or heave a large imagined sigh if I¡¯m trying to be polite).  There¡¯s apparently money/litigation/business to be made perpetuating the fallacy, but I¡¯d rather not take part as this does a disservice to the modeling community at large.

 

To your actual query:  I have run into one reviewer demanding identical assumptions between load calculations and energy models (fair to note: not a LEED/USGBC reviewer).  This individual was unwilling to accept that an energy model and load calculations may be performed by different software and assembled by different persons, to start, and had some as-of-yet unexplained need to ¡°prove¡± our design was ¡°wrong,¡± so was trying his/her hardest to find a chink in my model¡¯s armor.  Long story short, hours upon hours spent syncing load calc assumptions/scheduling up had the net effect of improving our modeled performance rating (to this reviewers), and was altogether a pointless exercise from a ¡°reality matching¡± perspective regardless since the load calcs were made to use artificially inflated space occupancy/ventilation rates.  

 

That little scary anecdote aside, there are actually a number of good reasons for which it would be rational for a reviewer to request a comparison to ensure both load calcs and models are making reasonable and similar (not necessarily identical) assumptions for occupancies/loads/etc.  It doesn¡¯t take much creativity to figure out this is an area where one could artificially skew the results in their favor.   

 

If you need a reference:  90.1-2007 Table G3.1.4. ¡°Schedules.¡±  This section makes two important points:  (1) the rating authority has the designation to approve (or not) the scheduling used, and (2) such scheduling needs to agree with what the designer came up with.  The specific wording is ¡°typical of¡± which I personally don¡¯t think can/should be read as ¡°match exactly.¡±

 

Between matching proposed design in capacities & unmet hour restrictions, 90.1 further reinforces reasonable assumptions on items like occupancy density and scheduling, otherwise you¡¯ll find pushing the envelope will cause irresolvable unmet hours.  

 

All things said:  I¡¯ve found eQuest defaults and library entries tend to be reasonable for the most part, and when necessary can often be traced back to matching/similar guidance from ASHRAE and other industry authorities.  I rely on them for PRM modeling, and reference a corresponding set of HVAC equipment sizing calculations as a QC resource where available, to ensure similar ventilation/occupancy/load assumptions are being made.  This often resolves unmet hours before I even need to address them.  I¡¯ve found such QC measures are usually a small time investment returning a faster/easier workflow as you avoid repeating work like baseline sizing calculations down the road when something askew comes to light (or a reviewer decides this needs a closer look!).

 

~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: Bobby Sy [mailto:rsg4999 at gmail.com] 
Sent: Saturday, October 08, 2011 1:40 PM
To: Nick Caton
Cc: Jiao, Joey; Equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] Values choosing of the typical equipment &miscellaneous power densities.

 

Hi all,

 

Thank you guys for bringing this up. 

 

I'd like to ask a simple question about the occupancy assumption. Do we need to match the occupancy density to the assumed occupancy (usually peak or dense) by the HVAC designer, in the model? Often times HVAC designers assume more occupants for peak load.

 

I understand that simulation needs to be close to reality as possible but if I will reduce the occupancy close to reality, savings may be reduced due to increase in the HVAC equipment capacity difference between the baseline & proposed because the baselines equipment will be auto-sized (1.15 ratio for cooling, 1.25 for heating).

 

Have you experienced a comment from a reviewer picking on inconsistencies on assumed occupancy between other credit/prerequisites?

 

Thanks,

Bob

 

 

2011/9/29 Nick Caton <ncaton at smithboucher.com>

Hi Joey,

 

I thought to write about the 25% topic but thought it best to not bring it up¡­  Perhaps a quick recap is in order for your situation and others¡¯ benefit!

 

There has been much discussion regarding that ¡°25% rule¡± between myself and others on these lists ([bldg-rate], [bldg-sim], [equest-users]).  Re: archives.

 

I started out with the same conclusion you¡¯re stating: the baseline consumption is supposed to be 25% plug/process loads.  The LEED 2.2 handbook is unfortunately worded under EAc1 to suggest this as a prescriptive default in the absence of substantial documentation, but that ¡°documentation¡± can be as simple as a one-line reference as I described.  Note the LEED v3 handbook (under EAp2 I think) re-words those lines on process loads to emphasize the expectation is to simply match between the models ¨C no mention of 25%.  

 

I have since that time done an about face on that viewpoint and believe that figure to be entirely arbitrary.  ¡°Forcing¡± 25% plug load consumption in the baseline is in many cases arbitrarily detrimental to the overall performance rating, is rarely realistic, and the resulting internal gains can cause irresolvable unmet hours for proposed equipment of a specified capacity (remembering the baseline and proposed should normally have matching process loads).  I came to the lists seeking to share and learn better approaches to ¡°fairly¡± mitigate these artificial internal loads, and in doing so learned I was far off the path from others.

 

I could re-iterate my perception of the history and intent behind the 25% figure, but you¡¯re probably best advised to seek out those discussions in the archives to come to a fuller understanding.  I would presently advise referencing and using something reasonable for your proposed building in both models, and not treating the 25% figure as a mandate, but do inform yourself by reading those archived discussions so that you can defend your position against any argumentative v2.2 reviewer.  

 

~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 <http://www.smithboucher.com>  

 

From: Jiao, Joey [mailto:Joey.Jiao at WSPGroup.com.cn] 
Sent: Wednesday, September 28, 2011 9:56 PM
To: Nick Caton
Subject: ´ð¸´: [Equest-users] Values choosing of the typical equipment &miscellaneous power densities.

 

Hi, Nick:

Thank you for your suggestion , it¡¯s really helpful!

I have found the TABLE GB, in ASHRAE 90.1-2007 Manual. See it below. I think it¡¯s what¡¯s your refer to.

As an addition,  I get another tip which should be a limitation for making a baseline model. 

On the LEED RATING SYSTEM , there is a rule that the process energy cost should be 25% of the total baseline energy cost. 

Hope it helpful for others.

Thanks for your help again.

 

Joey

 

·¢¼þÈË: Nick Caton [mailto:ncaton at smithboucher.com] 
·¢ËÍʱ¼ä: 2011Äê9ÔÂ28ÈÕ 22:48
ÊÕ¼þÈË: Jiao, Joey; Equest-users at lists.onebuilding.org
Ö÷Ìâ: RE: [Equest-users] Values choosing of the typical equipment &miscellaneous power densities.

 

Generally, the ¡°correct¡± baseline model values for these entries (misc/plug loads and occupancy/ventilation) are to match whatever is in the proposed model.

 

There is more than one ¡°correct¡± approach regarding misc load quantities for the proposed model.  In any case you should be prepared to reproduce what decision you made and back it up for baseline/proposed documentation:

-          Reference tables in the 90.1 users manual (also found in certain state energy codes) with representative W/SF figures for various ¡®whole building¡¯ types and assign that figure to each space.  I¡¯ve found this approach to be time-efficient and so far totally accepted by the USGBC/LEED reviewers.

-          Stick with the eQuest defaults per occupancy.  I¡¯m unsure of where they¡¯re all derived from exactly (Title 24?  Some other standard/reference?), but they will vary as you change space activity types and appear sensible from what I¡¯ve observed.  These values, default or otherwise, are rolled into a weighted average for each zone group based on the % distribution in the zone group screen ¨C which is why the resulting W/SF coming out of the wizards can appear somewhat random at first glance.

-          Flex your spreadsheet muscles and do a space-by-space plug load takeoff to come up with an ¡°actual¡± number.  I have never pursued this degree of detail myself, but I understand others have specifically to account for gains from providing efficient plug load equipment with the project over ¡°standard¡± equipment¡­ resulting in a deliberate divergence between the baseline/proposed models. 

 

I¡¯ve found eQuest default values for occupant density/ventilation/loads are pretty much on the mark as to what I¡¯d define from ASHRAE references, so I leave the defaults alone in wizards and only focus on tweaking the calculated zone occupant quantities in detailed mode as necessary for model QC and matching loads.  I do make a point to assign the appropriate ¡°space type percentages¡± for each zone in the following wizard screens however.

 

~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 <http://www.smithboucher.com>  

 

From: equest-users-bounces at lists.onebuilding.org [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Jiao, Joey
Sent: Wednesday, September 28, 2011 4:59 AM
To: Equest-users at lists.onebuilding.org
Subject: [Equest-users] Values choosing of the typical equipment &miscellaneous power densities.

 

Hi, Everyone:

I¡¯m confused with the value choosing for the equipment & miscellaneous power densities like the illustrate below.

The table G 3.1 no.12 said it should be based on the building and space type, but there isn¡¯t other information. So do we need to add this load part  when we make a baseline model? 

Maybe this isn¡¯t very important , but I want to make sure about it.

I know we can choosing the lighting power densities in Section 9, but I can¡¯t find the table for other equipment. So what should you do?

 

 

By the way , about the occupancy density and ventilation inputs shown below, I found  the reference information from ASHRAE 62.1, is that a common way when we make a baseline? 

 

Thank you for your kindly help.

Best wishes.

 

Joey

---------------------------------------------------------------------------

Joey Jiao

Graduate Engineer         

 


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