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