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.

I would assume you tested that high air leakage from the equipment cabinet and not on a flex connected to the return? This is super important.

Yes,  I was connected to the AH cabinet.  I find it easier in this area, given the common mechanical set ups.  Where I've tried going into a return, I usually have gravity to fight. <GRIN>

I did a guarded Blower Door Test with my QAD on a duplex unit.  He set up his duct blaster as well for the fun of it.  I went into the cabinet and he went into a ceiling return.  It worked for him.  

Ok I will bite especially as many parts of the country have a single return which is where testing should be done from - the exclusion for most comes with multiple returns. I have run the tests both ways & the numbers are almost identical - the only thing connecting to the AH has is cutting out the leakage from the door of the AH which if installed properly should be near 0 anyways

In day to day operation, the blower is the source of pressure in the supply system and relatively depressurized return.  The Static is slightly higher near the source of pressure then at the far end.

Hooking the test fan to the blower compartment is a better representation of the pressure at the point of leakage then at the return.  

The purpose of my question was somewhat different and doesn't apply to your particular test but can be a huge source of errors for many testers.

The foregoing does NOT apply when you attach the Blaster to the cabinet. 

When the Duct Blaster is attached to the return via the flex duct and you read over 650 CFM on Open Range, there is good chance the Blaster is well into the NO GO zone. The manual specifically states not to go over 100 Pa back pressure but when you get to 650 CFM you're usually up to around 250 Pa backpressure which is well above it calibrated range. In the last TEC Webinar they warned to measure the back pressure to ensure you don't go above 100 Pa.

Here is what happens when you do. Open range starts to read high by about 10% at 600 CFM and by the time you reach 750 CFM you might be at 100% error. When you get to around 840 CFM flow stops altogether and the backlpressure on your Blaster is around 450 Pa. To prove this to yourself, simply turn your Blaster up to full speed with no flex, on Open Fan with the back of the Blaster on the floor. You will read around 840 CFM where clearly the flow is zero. 

This is an extreme case but I have had several people swear they were reading 850 CFM on Open Fan on a flex.  

The solution to the dillemma is super simple. Always use Ring 1 which will provide almost exactly the same flow as Open anyway but is not subject to this problem. This is a training issue that should be demonstrated to ensure Blasters are used correctly. When they are, they work fine. I would suggest never using Open Fan on the Flex. Works fine on the equipment cabinet for all the experiments I have done but I have  not gone up to 1500 CFM although duct leakage at that flow rate is no longer a duct anyway. 

Got you - thanks for the explanation

Fortunately for most if anyone is anywhere close to that mark they have failed in a big way (or someone missed sealing a few covers)

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?

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