measuring air flow

zacmobile

Recently I have been installing the odd HRV and other make-up air devices to add to my hydronics repertoire and I've taken a government sanctioned ventilation course so I am now certified for this kind of work however I am having trouble measuring airflow. I just got a Dwyyer magnahelic gauge and a pitot tube and yesterday I used it on a kitchen range hood to try it out but it barely flickers on the scale. The hood is definitely moving some air, it is rated for 450CFM and it's blowing over some bushes about 15 feet away from the discharge but by my calculations: .02" static pressure (that's kind of a guess because the needle hardly moves much at all) on the chart that comes with the pitot tube says 600 feet per minute velocity X .20 sq ft. for 6" round duct area gives me 120CFM, but that can't be right can it? I know i'm not going to get 450CFM from a 6" duct but I was expecting more than that. The gauge I have is a Dwyer 2001D with a range of 0-1" water column, with a 160-12 pitot tube.

BillW

Magnahelic

It sounds like you are doing the test correctly.  For my purposes I used an anemometer type instrument.  Contact the manufacturer technical support, or post the question on the "Wall".  Somebody who has more experience with magnahelics could be out there.  Sorry I couldn't give a better answer.

JStar

Well...

The area of 6" round pipe is 28.26 sq. inches.

That makes 2.35 sq ft.



600FPM x 2.35 sq ft = 1410 CFM



If your gauge barely moved, maybe it was reading 0.01" instead of 0.02? Then...



300FPM x 2.35 = 710CFM



I've attached a document that does a good job of explaining the test.

njwebdevguy

I would also very much like the answer to this question

I called the manufacturer on my HRV and they recommended using pitot tubes (one in each duct) and an electronic manometer . But the cost of that setup was a lot higher than I can afford.



One way Ive heard its done cheaply is with a large trash can liner and a stopwatch, but to do that you need access to both the fans with the duct as part of the circuit. that requires access to the outdoor exhaust of your HRV which in my case is hard to get to without climbing a ladder.

njwebdevguy

Multiple speeds

If your HRV has multiple speeds, be aware that the balancing will only be accurate for the speed its done at.



If you go to www.hvi.org, somewhere, buried on their site there is a huge PDF file that has the CFM curves for all the major HRV manufacturers models.



My HRV has a tendency to exhaust more than pull air in at the lowest speed. Not a great deal, but enough to change the direction a smoke candle blows in an open window on the ground floor when its an equal temp indoors and outdoors.



For obvious reasons I think the measurement is only valid when the temps indoor and oudoors are the same and the measurement is taken at the middle of the habitable space in terms of height. Otherwise, the stack effect may throw everything off.



Also be aware that the filters have to be clean - if the filers get dirty, (and its always he intake filter that gets the dirt) at some point the intake is going to be reduced proportionately to the amount of dust/dirt on the filter.