Dose anyone have experience testing high velocity duct systems?

I've been asked to duct test a Zhender HRV with comfoducts (2" homerun ducts).

I designed & installed a hydronic high velocity system a few years ago. It had 25' of 12" return flex duct, and 15-20' of 6+8" sheet metal ducting, 2 + 2-1/2" branch ducting. I sealed everything with mastic.

I installed the duct tester at the return, and the duct pressure at the nearest supply to the air hander, as I would on a low velocity system. I measured a high duct leakage, I was disappointed. I'm wondering if I should have taken my duct pressure at the supply trunk (6-8") rather than the 2 - 2-1/2" ducts, due to pressure drop.

Any thoughts?



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This duct system should be tight enough that it should not matter where you test it from.  I recommend you test under negative pressure to ensure you get good seals at the registers.  How high was your reading and what flow range were you on?

When measuring super low flow, do a field check of your duct tester (thank you for not calling it a Blaster) by cutting a 1 inch x 3 inch hole in thick card board and tape it to the flange on your duct tester. This hole should come in at around 16 cfm at 25 Pa with the pressure measured across the cardboard.  If it measures more than that, your duct testing system could be leaking or out of whack some how. 

if the duct leakage is relatively low, 200 cfm or less, the pressure in the duct system will be quite uniform and either location will give consistent results.  If the duct leakage is high, over 500 cfm, we recommend that you do two tests.  One with the probe in the supply register closest to the Duct Blaster setup and one with it in the farthest supply register and average the readings.  


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