Measuring airflow w/handheld anemometer

In summary, a building contractor is searching for a way to measure the airflow (in CFM) of typical vent fans in homes. The tool available at the moment is an item from Extech. However, the contractor is concerned about the accuracy of the reading due to the shape of the inlet and the difficulty of placing the vane directly within it. While researching, the contractor found that flow hoods are uncommon and expensive. They are seeking advice on how to get a reasonably accurate reading of airflow, taking into consideration the impact of ductwork on actual flow. A balancing contractor suggests measuring at the end of a long, straight section of duct or at the outlet on the roof, as it is common for residential ductwork to be undersized
  • #1
David Meiland
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I'm a building contractor looking for a way to measure the airflow (in CFM) of the vent fans that are typical in homes. The usual suspect is in the range of 80-150 CFM, has a squirrelcage fan installed in a metal housing that's installed in the ceiling, and a 3" or 4" round outlet that ducted to the exterior (hopefully) with either rigid metal pipe or some sort of flexible pipe. The tool I have available at the moment is this item: http://www.extech.com/instruments/product.asp?catid=1&prodid=2

My hunch is that the specific spot in which the vane is held will have a big effect on the accuracy of the reading. Some fans (when the ceiling grille is removed) have a nice round inlet, but more often than not it's a more complex shape. In addition, it's often not possible to hold the vane directly within the shape, but rather right in front of it.

While googling around I've noticed a few manufacturers of flow hoods, but these seem to be both uncommon for HVAC diagnosis, and very expensive.

I'd be grateful for any comments or advice from those with experience and understanding of this type of issue. My goal is to be able to get a reasonably accurate reading (+/- 10%) of actual airflow. The reason I don't just read the equipment nameplate is that the quality of the ductwork install has a major impact on actual flow.

Thanks!
 
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  • #2
Welcome to PF.

A balancing contractor will certainly have a flow hood - it is their most important tool.

Measuring airflow at the inlet will be problematic because the air isn't going to be pulled into the fan in a coherent column and will have a tendency to swirl as it enters. Plus, removing the grille covering it will have an effect on the flow. You're better off measuring at the end of a long, straight section of duct or just at the outlet on the roof.

It is very common to see residential ductwork undersized. The fact that the outlet of the fan is 3" does not imply that the duct should be 3". Fans need a high outlet velocity to properly generate pressure, but once out in the duct, the velocity needs to be lower due to frictional losses...plus, the flex-duct typically used is vastly rougher than hard duct. A 3" flex-duct will only pass 50 CFM (minimum code required toilet exhaust) if it isn't too long and there is a little pressure behind it.
 

1. How does a handheld anemometer measure airflow?

A handheld anemometer measures airflow by using a small propeller or impeller that rotates when wind passes through it. The speed of rotation is then converted into a measurement of air velocity, which can be displayed in different units such as meters per second, miles per hour, or knots.

2. What factors can affect the accuracy of an anemometer's measurements?

The accuracy of an anemometer's measurements can be affected by several factors, including the quality and calibration of the device, the location and orientation of the anemometer, and the presence of obstructions or turbulence in the airflow being measured. It is important to carefully follow the manufacturer's instructions and regularly calibrate the anemometer for accurate readings.

3. Can a handheld anemometer measure both wind speed and direction?

Yes, some handheld anemometers are equipped with additional sensors that allow them to measure both wind speed and direction. These anemometers typically have a vane or wind vane attached to the device, which can rotate and indicate the direction of the wind.

4. How do you convert the units of measurement for an anemometer?

The units of measurement for an anemometer can be converted using conversion factors or formulas. For example, to convert meters per second to miles per hour, you can multiply the measurement in meters per second by 2.237. It is important to carefully check the units shown on the anemometer and use the appropriate conversion factor or formula to ensure accurate results.

5. Is a handheld anemometer suitable for all types of airflow measurement?

No, handheld anemometers are best suited for measuring relatively low and constant airflow speeds, such as in indoor environments or gentle outdoor breezes. They may not be as accurate for measuring high-speed or turbulent airflow, such as during storms or near large obstructions. In these cases, it is recommended to use more specialized anemometers or other measuring equipment.

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