How to Measure CFM w The Testo 420 Flow Hood

The Testo 420 is a very lightweight flow hood and contains a manometer that you can use to take pitot tube measurements. You can take spot-to-spot readings or use the flow hood to make a detailed report for an entire structure.

The flow hood has flow straighteners in a grid configuration, which lowers turbulence as air flows through the hood and stabilizes your reading. Assembly is quite easy, as there are lines that help guide you when it comes to inserting the poles. The manometer display is easy to see and access. To take a reading, you hit the red button on the hood, and the display will hold the reading.

This flow hood will read accurately down to around 50 CFM. When you have those low-flow applications, you may want to use a vane anemometer. However, the Testo 420 is good for measuring airflow up to 2000 CFM.

When you initially set up the manometer, you’ll want to make sure you set your testing units to the appropriate units for your application and region. (For example, in the USA, we would use CFM for air volume, “wc for differential pressure, and degrees Fahrenheit for temperature.)

The Testo app can show you tables of all the readings you take and allow you to view trends and see the relationship between your readings.

When making residential CFM measurements, we find that the best way to measure CFM is to put the system into high-stage cooling for maximum airflow. We also recommend disconnecting the condenser, if possible.

Place the flow hood right up to the vent you want to measure; you may have to give the flow hood a few seconds to sit in and start reading the CFM accurately. Once you hold a reading by pressing the red button, you can save the test value into the report by pressing the “document” button.

When making commercial CFM measurements, we do pretty much the same thing. However, since it’s a bit more difficult to press the buttons on the flow hood, we decide to use the Bluetooth measurements and controls. The app displays the exact same data as the manometer, and we can start, stop, and save readings remotely.