Wednesday, April 16, 2014
 
   
RED Calc Free Help Box Airflow
Box Airflow Tool User Guide

Box Airflow
Red Calc Tool User Guide

What this tool can do for you

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.

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Box used for airflow

Calculated values

  • Airflow rate.

Tips

  • 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). Read more.
  • 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.

Inputs and field measurements

  • Area of opening - the measured area of the square or rectangular opening you cut in the box surface.
  • Box pressure - the pressure difference between the inside and outside of the box during your measurement of airflow through the grille. Always enter this value as a positive number.
  • Altitude - the elevation below or above sea level at your location. This value affects the air density, and in turn, the airflow rate.
  • Air temperature - the temperature of the air flowing through the box. This value affects the air density, and in turn, the airflow rate.

Construction and use of 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.

References

  • RESNET. (2011). Mortgage industry National Home Energy Systems Standard. Residential Energy Services Network.
    Comment: See Section 804.1.2 for instructions for using "a rectangular user fabricated box" for measuring airflow at grilles.

Related tools

  • ASHRAE 62.2-2013 Ventilation: Determine whole-building ventilation requirements for new and existing dwellings, with the choice of using advanced blower door options and the alternative compliance path.
  • ASHRAE 62.2-2010 Ventilation: Determine whole-building ventilation requirements for new and existing dwellings, with the choice of using advanced blower door options and the alternative compliance path.
  • Depressurization Analysis: With this "solve-all" tool you can calculate house pressure, CFM50 tightness limit (Depressurization Tightness Limit), or total exhaust fan flow.
  • Pitot Tube Airflow: With this tool, a pitot tube, and a digital manometer, you can determine the airflow of a ducted range hood, a clothes dryer, or other appliance will an accessible duct.

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