[Equest-users] Basic stuff - Infiltration

John Bixler JBixler at Sebesta.com
Wed Aug 10 08:17:09 PDT 2011


Thanks for the response Nick.

As I recall from a recent foray into this subject in eQuest (in detailed mode), the cfm/sq ft entry is based on floor area.

It would be logical that if cfm/sq ft of exterior wall is an option in wizard mode, it would also be an option in detailed mode - I just haven't dug that far yet.

From: Nick Caton [mailto:ncaton at smithboucher.com]
Sent: Wednesday, August 10, 2011 10:08 AM
To: John Bixler; equest-users at lists.onebuilding.org
Subject: RE: [Equest-users] Basic stuff - Infiltration

Hi John - thanks for your thoughts!

I was relating some of the same concerns with others off-list ... I'm am no infiltration-auditing expert (though some lurk among us here on the lists ^_~), but one thing I can say based on my past attempts to build a better mousetrap regarding infiltration is that where ASHRAE may be generally vague on the topic - they are actually doing a lot to say (if not always directly) that whole building infiltration is a very difficult thing to quantify at best (sorry for excessive use of parentheticals (I mean it!)...).

The best guidance imaginable that would still be practical in day-to-day use would have to rely in some part on subjective observations (guesses) regarding envelope constructions.  Two brick walls of certain grout/masonry ratios weathered for the same period in the same climate may still have different leakiness because the two masons used slightly different grout mixes...  What I'm getting at is you couldn't realistically construct a table that covered every variable, and many variables are not "knowable."

That's not to say the residential ACH table isn't useful for subjective estimations, nor that research couldn't be undertaken to raise the bar a notch.  As John is alluding, a table providing representative commercial envelope constructions (with accompanying illustrations!) and/or layer combinations could be undertaken that would provide infiltration performance as a function of time.  Values could be given for new construction, and after weathering for 1/5/10 years.  While new constructions/layers could be assessed in a controlled environment, initial research on aged constructions would need to be done sampling within a single climate zone.  Separate/concurrent research could explore determining multipliers on the weathering effects based on varying climate and geography...  All things being equal, a beachside wall built in Miami, FL with lots of sun/salt/torrential rain seasons and the occasional hurricane will weather differently over a decade than the same wall in a milder climate.  The net result of such research could ultimately produce some really helpful tools in better assessing existing and new constructions for a variety of industries and purposes (energy modeling included).

Considering the growing presence and pressing need for better tools in the world of energy modeling, I would put forward prime candidates for whole construction assemblies would be ASHRAE 90.1 baseline constructions as defined in Appendix A.

For all I know, such research may be underway or completed years ago - my ear is not quite so close to the ground with the academic world... can anyone comment?


To another point you brought up - eQuest is quite capable of using your personally developed CFM/ft2 values - in the wizards even!  In detailed mode you'll find there are inputs for more involved estimations as well if you wish to pursue other methods:
[cid:image002.png at 01CC5746.A9886FC0]
[cid:image004.png at 01CC5746.A9886FC0]

[cid:489575314 at 22072009-0ABB]

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: equest-users-bounces at lists.onebuilding.org [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of John Bixler
Sent: Wednesday, August 10, 2011 9:23 AM
To: equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] Basic stuff - Infiltration

My own personal opinion is that ASHRAE Fundamentals has not rescued us here.

As Nick mentioned, the data presented there is for residential houses and relies on incredibly vague and subjective judgment calls by the modeler.

Furthermore, the values presented are in the unit's of air changes per hour.  To me, this is a terrible way to use the data.  Your entries for infiltration then rely on the volume of the room to determine the amount of infiltration.  The volume of the room (zone is probably a better term) has NOTHING to do with the infiltration.  What if I have a gym that's 150'x100'x50' tall, but it only has 10' wide by 50' tall of exterior wall???  If I use the ASHRAE method and rely on air changes per hour, the zone will be modeled with a HUGE GIGANTIC REALLY REALLY LARGE amount of infiltration.

Yet this seems to be the only recourse we have that is grounded in any sort of defendable data.

I have looked and looked for a reliable report or other source for real world commercial/institutional construction infiltration values to no avail.  It would be so incredibly useful.

I have, over the course of my energy modeling career, developed a set of seemingly practical infiltration values to use, using the units of "CFM per sq ft of gross external wall area" available in Trane Trace (I don't believe these units are an option in eQuest).  These values were developed by taking a number of buildings with no infiltration and arbitrarily adding infiltration in, until I get a reasonable utility consumption value.  Hardly scientific and no way could I defend these values if they came under scrutiny, other than to say "well, you got a better idea????"

Some may say "Eh, who cares about infiltration anyway?".  Well, it makes a bigger difference then you'd think.

A novice user who relies on the ASHRAE air changes per hour is likely significantly oversizing their cooling equipment in large rooms (ie conference, assembly, gyms, etc) which is exactly where you don't want to be oversizing cooling equipment.

Think about the components of a heating load - envelope conduction losses, taking in cold OA...and infiltration.  Envelope losses are generally small, the design community likes to temper their OA (rightfully so), so where is a major component of the heating (consumption) load coming from?  Infiltration really adds up.

How do you justify replacing leaky, wood sash windows?  How bout making a switch to spray foam insulation?  How about modeling door seals?

I've rambled enough.  Point being, we all are forced to use arbitrary numbers for something that is a significant component of both equipment sizing and energy modeling and it just makes me mad and embarrassed when I have to explain to a client or colleague "well those are really important, but completely imaginary, numbers..."

From: equest-users-bounces at lists.onebuilding.org [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Bruce Easterbrook
Sent: Tuesday, August 09, 2011 5:41 PM
To: John Bixler
Subject: Re: [Equest-users] Basic stuff - Infiltration

Since 1922!
Bruce Easterbrook P.Eng.
Abode Engineering
ASHRAE Member

On 09/08/2011 06:11 PM, lawrence Lile wrote:
Good ol ASHRAE Fundamentals!  Why didn't I think of looking there?  Thanks!


On Mon, Aug 8, 2011 at 11:36 AM, Nick Caton <ncaton at smithboucher.com<mailto:ncaton at smithboucher.com>> wrote:
Hi Lawrence,

My copy of ASHRAE Principles of HVAC includes a table (5-1) excerpted from ASHRAE Fundamentals, 2001 (Table 7, Ch 28).  This table provides air change rates as a function of subjective envelope airtightness ("tight" / "medium" / "loose") and as a function of the outdoor design temperature.  Upon reviewing the referenced Fundamentals chapter, I learned this table is built from research surveying residential homes of various vintages, so it helps to know that these are "tight" to "loose" residential constructions.

In any case, I've used and cited this resource before when modeling infiltration and calculating sizing loads for non-residential projects as well.  I've searched, but have yet to come up with an equivalent table based on surveying and measuring commercial constructions from a subjective/objective standpoint... That might be handier, but in the meantime this is a good tool for "converting" your subjective observations into the right ballpark.

~Nick

[cid:489575314 at 22072009-0ABB]

NICK CATON, P.E.
SENIOR ENGINEER

Smith & Boucher Engineers
25501 west valley parkway, suite 200
olathe, ks 66061
direct 913.344.0036<tel:913.344.0036>
fax 913.345.0617<tel: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> [mailto:equest-users-bounces at lists.onebuilding.org<mailto:equest-users-bounces at lists.onebuilding.org>] On Behalf Of lawrence Lile
Sent: Sunday, August 07, 2011 12:50 PM

To: equest-users at lists.onebuilding.org<mailto:equest-users at lists.onebuilding.org>
Subject: [Equest-users] Basic stuff - Infiltration

In building modeling programs one always has to provide precise values for infiltration.  In the real world, I will know one of two things:  almost nothing (The building appears to be kinda leaky with old windows), or I will have a blower door test done at a specific pressure.  How do I convert subjective ("kinda leaky") or objective (Blower door test) leakage into numbers that make sense in the program?  Is there a guide one can use?


--Lawrence



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