David Richardson posted an interesting piece on changing from % based testing based on conditioned floor area versus the duct area

http://ncidavid.blogspot.com/2013/10/should-duct-surface-area-be-us... 

Personally I see quite a few problems with this based on the way things are done now, but I am curious what you all think.

As for some of my concerns - most installers have no clue what the duct area is because we cant even get them to do a Manual J much less a D (sorry I also have no desire to run a tape against them either)

Leaving the air handler out means more sloppy installs of them & manufacturers wouldn't have to concentrate on making them by default tighter

With that he does make some very valid points & I would love to hear your thoughts

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I never could understand duct leakage per sqft of conditioned floor area.

Hmmm.  Guess I didn't think that one through, good catch.  787 cfm25 2000 sf house is MUCH more informative than .3935 duct leakage per sf. 

Yep, you're right, good catch.  Can't conflate that - get's confusing and loses meaning.  I'd want total sf anyway, and that wouldn't give it. 

 Want the same for enclosure leakage.  Among the reasons, these things don't scale proportionally.  

My understanding of duct leakage per sqft of conditioned floor area came out in Energy Star 3.0.

The size adjustment factor, the penalty for oversizing homes with the use of a Target HERS Score were ways of making builders look at downsizing homes.  So perhaps the metric of pa/sq ft was another nudge.

Then It got picked up by IECC.  That means it will probably take moving the mountain to somewhere for change.

The ACCA have it right -  Their standard is 6% of system flow for systems with any of the duct work outside conditioned space and 10% for systems 100% within conditioned space.  I believe that is total leakage.  That would be a comfort thing.

Interesting thought but the issue with that is contractors bumping up manual J's for bigger units - the 99 occupants, leaky house, etc... tricks we have all seen used to get to there favorite 400 or 500 SF per ton. Granted commercial never sees this issue but that is because they require a mechanical engineer in many cases & also have higher loads than residential due to ventilation requirements

I think John has it nailed - 6% total (at final) & 10% for inside conditioned space (especially for "balanced" systems where they have actual returns in each room) with maybe a max based on nominal airflow. Shoot to make it fair for smaller houses  to promote them maybe even go 8% for homes under 1200 SF. As for reporting, I think it should be both the % but also as Ted points out - actual leakage numbers 

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