Introduction

When you change something in your mechanical ventilation system, it’s a good idea to double-check that all vents still deliver enough airflow for a healthy indoor climate.

Pro measuring tools are pricey, and the DIY versions I found didn’t seem like they would work well. So, I ordered a cheap anemometer from AliExpress and designed a capture hood. My first attempt used friction to hold the anemometer, but it fell apart when measuring ceiling vents. This final version uses three screws and works reliably.

Required parts

• Anemometer: HoldPeak HP-866B or similar (see drawing)
• 3 screws (choose one type):
  • Woodscrews 4×50 mm (countersunk preferred), or
  • Socket head cap screws M4×45 mm
• 3D-printed parts in PLA or PET
  • Funnel that fits your vent
  • Lockring
  • (Optional) Flex-ring printed in flexible filament

Printing parameters

• The parts are designed to print without supports using standard settings.
• Use a smaller layer height (0.15–0.20 mm) for a smoother surface.
• The files are already oriented for printing. The lockring holes have built-in single-line support.
• (Optional) For bathroom (or other high humidity rooms) use: consider printing the lower 5–10 mm of the funnel in antibacterial filament (to reduce mold risk). I swapped filament after 7.25 mm to cheaper recycled PLA.

Assembly

1. (Optional) Place the flex-ring in the funnel groove.
2. Remove the small built-in supports from the lockring.
3. Thread the screws into the lockring from the flat side until the tips stick out.
4. Attach the anemometer to the lockring (orientation doesn’t matter).
5. Fit the anemometer + lockring onto the funnel.
6. Tighten the screws.
7. Start measuring!

Converting Air Speed to Flow

The anemometer shows velocity, but you want volume flow.

Formula:

Flow=Velocity×Area

Area of the fan opening:

A=π×(D²−d²)/4

where D = outer fan blade diameter, d = hub diameter.

Shortcut for HoldPeak HP-866B:

Flow [m³/hr]=Velocity [m/s]×10.17