Wx's long tradition of cheap & good enough diagnostics

Back in my old BSU days Dr. Hill made great efforts to get as many diagnostic tools into our hands as possible.  It was always fun to use a DIY tool.  The all time greatest DIY tool is the cardboard stack effect box.  It is about the only good use for a 150 W bulb!  Take a tall cardboard box with a light fixture at the bottom.  Give it a few minutes to heat the air in the box up, and with a little chemical smoke and some smallish holes at the top and bottom, you can clearly see the air moving into the box's 'foundation' and out the box's 'roof'! 

In a pinch a well taped cardboard box will work as a pressure pan.  Cut a hole in the cardboard pressure pan and now you've got an exhaust flow hood.  Then there is the calibrated air flow gauge.  Also known as a trash bag.  It is good to hear ACI can call back to some of its earlier, less refined roots!

There are approximately 1.7 cubic feet in 13 gallons.  So if you have an exhaust fan moving 30 cfm of air, it would take about three and a half seconds for a 13 gallon trash bag full of air to be completely empty if that fan were moving 30 cfm of air.  While the volume of the bag is well known, it must be full of air or the accuracy is reduced.  Another source of error is the timing of how long it takes to suck (or inflate) the bag.  The more air the fan is moving, the less time there is to measure. 

So if you happen to forget your TEC exhaust fan flow hood, ask for a large trash bag and a metal coat hanger if you are in a pinch.  Or, if you've got rooms with comfort issues, see how long it takes to fill the trash bag up.  That can give you a good feel for how many cfm the duct is delivering.