Analysis of inefficient buildings and individual zones

[Skander Spies]

Our team was contracted to provide design consulting for a very inefficient house in the Chicago area.  In this case, we are making recommendations to ensure thermal comfort and help the GC avoid getting sued.  What can I say, we all gotta eat?  We are using PHPP to model the house because we know that it is accurate, and is the tool we are most familiar with for modeling residential buildings (almost pulled the trigger on modeling in eQUEST, but decided PHPP would be easier... still debating this decision).

Specifically:

1) Does anyone have reservations about modeling high energy use buildings (rather than low energy use) with PHPP?  Have you found concerns or issues in the calculations that might be troubling when the building has no inclinations towards energy efficiency whatsoever?  I've been modeling and digging through the PHPP guidebook and found a number of features that say things like "this tool has been especially designed for Passive Houses and is not suitable for buildings with higher energy consumptions" (p. 112 section 17.9 Evaluation of group heating capacity for individual rooms)
2) The HVAC contractor wants to know what the loads are going to be in specific rooms.  We modeled the building as a whole in PHPP, and as far as I am aware, there's no real way to get at specific rooms (with the except of the risk analysis tool I mention above, which does not apply to this case).  My initial thought is to start a new PHPP for each room, enter the geometry for the individual room, and see what we get, while making any partition walls ridiculously insulated to simulate being attached to the rest of the building.  Any thoughts? 

[Graham S. Wright]

I would guess PHPP is probably not the right tool for this job.

For peak load calcs in particular PHPP uses something like a coldest day 24-hr avg design temperature instead of coldest hour, that is, it presumes a slower-responding building due to high insulation and even factors in some solar gains, which coldest-hour-of-the-coldest-night calculation would not do.  

I would suggest to do at least one crossover calc in both eQuest and PHPP, see if they agree.

-gw

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[Rob Harrison AIA]

What Graham said. As I understand it PHPP gets a good bit of its accuracy by eliminating the variable of infiltration. You lose that with air tightness greater than the standard 0.6 ACH@50. 

Rob

Sent from my iPhone

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[Adam Cohen]

I would not use the PHPP for a std project

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Adam Cohen
Certified Passivhaus Consultant - North America and Europe

Registered Architect, LEED AP, NAHB Green Professional

Design/Builder of the First US Passivhaus Public School Building

Structures Design/Build, LLC
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[Sam Hagerman]

I asked Wolfgang this question and he said PHPP is good at all low-load buildings.  He was unclear on "normal" load buildings but my horse sense is that Rob has it exactly right.  Just screw around with the ACH cell and you can see why.



Sam Hagerman
Hammer and Hand, 503 232-2447
s...@hammerandhand.com
Passive House Alliance United States

[Mark Wilkerson]

While I assume the PHPP is much less accurate for 'standard' construction, can someone explain why adjusting the ACH to something typical for standard construction would not at least account for that difference in energy use between a 'standard' design and a passive house design?  

In my experience, the reality seems to be that there are no really accurate tools for 'predictive' energy modeling for standard construction.  Before PHPP, I only considered modeling with Energy Plus, for example, to be a tool for 'comparitive' energy analysis.  That is, to compare different designs and pick the best one.  I had little faith that the building would perform as modeled, only that it would be some degree better than the other scenarios.  

mark

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4826  45th  avenue  south

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[Albert Rooks]

Mark,

My favorite Adam Cohen quote is: 

"air tightness is the secrete sauce in Passive House".  Meaning... it's a dominant contributor to performance. The PHPP won't let you "toggle" the ACH from PH levels up to 7ACH. Therefore you have no way to PHPP model a "windy" envelope, only a tight one.

Most other non PHPP programs aren't really able to model incremental changes in air tightness. I've tried a couple and the results are not what I believe the real effects to be. Currently I'm not aware that there is an option for mid-range buildings that allow realistic estimates. They can work with insulation values, but they don't appear to have a "handle" on air-tightness effects. Sad but true.

albert.

[Mark Wilkerson]

Thanks Albert.  To me, the fundamental question posed by Skander is if the PHPP is more or less accurate in modeling standard construction than other software out there.  In his case, eQUEST.  I'd like to pose the question a bit differently, and will frame it as a statement for consideration:

The PHPP is an energy modeling tool that, when used appropriately, can predict the energy use of a Passive House within 10%.  Applied to home designs of standard construction, the PHPP is much less accurate, but may still be just as accurate as other mainstream energy modeling software, such as eQUEST.  

What do you think?

[Rob Harrison AIA]

On Jun 20, 2012, at 1:39 PM, Albert Rooks wrote:

The PHPP won't let you "toggle" the ACH from PH levels up to 7ACH. 

You can...but that doesn't mean you should. :) Cell G48 on the Ventilation page. 

Rob

Rob Harrison AIA

HARRISON architects

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Seattle, WA  98101-2120

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lyrical sustainable design  ::  passive house plus

[Albert Rooks]

See... It's already past what my little head can handle...

albert.

[Sam Hagerman]

Albert, I told you to stop thinking with the little head. 

Tsk tsk...

Sent using right index finger.

[Graham S. Wright]

A couple of other thoughts:

Software that is good for relative comparison only can work for you, if you have even a little data to hang your hat on.  Like if you know "this room is good" then you can tweak the model so it works for that case and have more confidence extrapolating to other places.

Also if you know any radiant-floor guys you might see what they use.  I think they are used to doing room by room calculations for their systems.

-gw

On Jun 19, 2012, at 6:35 PM, Skander Spies wrote:

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[Graham S. Wright]

You might be on safe ground in a case like this, with an ACCA Manual J calculation.

-gw