Box Airflow Tool Guide

What this tool does – [Tool]

This tool, along with a cardboard box and a digital manometer, allows you to accurately measure exhaust airflow at a grille. This custom-made airflow measurement device can be made on site to accommodate tight fitting spaces or exhaust fan grilles that are too large for other measurement methods.

  • Clicking the label for any input or result will cause a popup help box to appear. This help box includes the allowed and normal values (for inputs).
    Boxes used for airflow measurements
  • This box will work in a manner similar to the Exhaust Fan Flow Meter® manufactured by the Energy Conservatory. However, this home-made box does not have a readily adjustable opening. The photograph above shows two boxes made for measuring exhaust fan airflow with the RED Calc Box Airflow Tool.
  • Do not attempt to use this RED Calc tool or the cardboard box for supply fan airflow or at supply registers.

Making and Using the Box

  • It is recommended that this device be used only for exhaust flows between 10 and 200 CFM (5 and 100 L/s).
  • The box must be tight at all joints with a known hole size, usually a rectangle.
  • The area of the opening you cut in the box should not be more than 50 percent of the area of that surface. “…the size of the hole [should not be] greater than half the size of the box in each direction and the distance from the hole to the grille [should be] at least as large as the larger dimension of the hole. (RESNET 804.1.2, see references section below.)
  • You can reduce the opening size with duct tape.
  • Install the hose attached to your digital manometer in a corner of the box that is away from the opening you cut in the box. The end of the hose should not protrude into the box more than 1/2 inch. Use tape to hold the hose in place.
  • Adjust the opening size in the box so that the pressure difference is between 1 and 8 pascals (0.008 and 0.040 iwc). Four to 5 pascals gives the best accuracy (0.016 to 0.020 iwc).
  • The open side of the box must be positioned so that it is tight against the surface around the grille so that all the air entering the grille of the duct or exhaust fan flows through the hole in the box.