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 think each type of home is different.  Trunk/branch have advantages, spider has advantages.

Smaller more compact area for distribution is benefited by a spider system. Think 1000 sf

Longer, less compact areas are benefitted from a trunk/branch. Think 2500 sf ranch

If your trunk is formed sheet metal, less leakage then flex.  Not much sense in penalizing this type for size when it will probably leak less.

We need to have a better  understanding of air flow in the field.  Too many guys pick equipment by system air flow.  I need 4 tons of air flow.  Then everything flows from that choice.

I agree, what is the point of this change?  

For that matter, what is the point of setting standards that have no common reference?  It's like changing our MPG to teaspoons per yard.  How many teaspoons per yard does YOUR car get?

How much surface area does a duct system HAVE?  I have no idea.  How would you accurately measure this?  What would it cost to measure?  How is that juice worth the squeeze?!

 787 CFM @ 25 leakage total.  2 ton system.

Thanks John!!!   Some common reference there most of us can find MEANING in!  787 cfm25 is a number we can easily understand, and explain to pretty much anyone without them taking a nap.  That quickly communicates there is a huge amount of leakage (what is leakage to outdoors?)  2 ton tells me the house is probably less than 2000 sf.  It is not a meaningless, obscure reference that requires crib notes to decipher. 

SF tends to be a language that needs little conversion or translation.  Is it perfect?  No, but isn't it better to have some easily translatable information than information that has requires a lot of detail to understand? 

Give us the total, and the sf.  We are already building schema's around sf:

Cost per sf

Tax per sf

Energy use per sf

Energy cost per sf

Enclosure leakage per sf

BTU per sf

Hey, Duct Leakage per SF?  What a concept!!

All these things are just proxies for comparative cost.  Let's keep them simple so they stay relevant. 

Oh Ted, when you keep things revilent and on point it takes all the fun out of it.

In my opinion there is only one way to measure duct leakage and it is as a percentage of fan flow. It can be measured directly, based on nominal airflow or using static pressure and fan curves. Pick one. Nominal would be the easiest, x% of 800 cfm = the maximum allowable duct leakage for that unit.

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

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 

Duct surface area clearly gives the more precise assessment of the quality of the installed duct system. But that precision is only precise in appearance, due to the difficulties of getting the correct surface area number. This is similar to blower door testing and reporting of floor area or volume normalized values (cfm50/ft2 or ACH50) vs. normalized by surface area (cfm50/ft2SA). The latter better characterizes the quality of the envelope, but it is harder to assess (easier than duct surface area), and the other values actually relate better to other isseus we care about, like infiltration rates and energy use. 

That being said, the real reason I think that duct leakage normalized to the home's floor area is acceptable, is that as the requirements for duct leakage improve and approach 0, it no longer matters what you normalize to. Tight ducts will have low % leakage by either metric. 

As for encouraging certain design and installation behaviors, I don't honestly think that very many contractors are selecting their duct materials or layout design, based on a strategy to game duct airtightness requirements. 

Cheers.

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