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 can see the advantages of duct surface area!  I also disagree with leaving the equipment out of the equation.  

The only way to get the surface area will require reporting to the AHJ.

BTW:  I tested a duct system 3 weeks ago.  787 CFM @ 25 leakage total.  2 ton system. 

Anything is better than this!!!!

Ouch that has got to take the cake for worst performance - where was the disconnected duct(s) at?

Along those lines, I was interested to see he did not mention testing using a % based on unit size which is a good thing IMO as I see too many problems with that, but I know it has been batted around by others.

Basing based on unit size encourages oversizing.

I don't know at this time.  Will have to see what develops on this one!

Follow Up on 787 cfm at 25 total leakage. Ted, 1425 sf conditioned space.

The systems were installed with no tape or mastic used. This model had sheet metal trunks with sheet metal take offs; in the basement ceiling / main floor truss system.  I reported this to the owner and the next week reviewed it with the owner and the HVAC Company.  Two days later the owner had permission from the tenant to retest.  Lots of weird looks and questions from the HVAC guys.  Then I fogged the system.  When the mech room filled with smoke the owners Reps left.

The HVAC pulled the air handler and sealed it to the duct system. They also put mastic on anything they could find in the mech room.  We retested and got it down to 500.  The IR showed all the elbows from take offs to supplies with hot spots.  We tested a 550 cfm @ 25 leakage on the same system in a slab on grade, ducts in the attic, 716 sf.  Smoke showed the same problem. Same fix retest came in at 100 cfm @ 25. Pretty close to the 10% of 80.  Leakage to the outside was 38 cfm at 25.  Blower door on that unit was 2.9 ACH at 50.

Showed the owner's Rep the Goodman manual statement requiring a tight connection to both sides of the duct system.  He had seen the smoke, he knew. The HVAC guys tried to down play the requirement.

They would not have had to pull the air handler in these units if they had taped them at rough-in.  They are voluntarily pulling the remaining 32 units and sealing the connections from the air handler to the duct system.

Basing it on duct area encourages oversized systems and inefficient duct design. A trunk/branch has much less surface area than a "spider" system. Short runs are encouraged with current method.

Correct ManJ is a HUGE issue. Systems should be less than 1/2 the size of what they install in typical 1970's homes, yet they are not. If heat gain/loss is less than 1/2 of an older home, why aren't HVAC systems less than 1/2 the size?

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.

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