Airflow Measurements: You Just Can’t Get Around Them

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The measurement of airflow is one of the most critical measurements you either make or avoid in the field of residential HVAC and home performance.

Unless you are using a capture device (capture hood) to measure airflow (volume flow or Cubic Feet per Minute (CFM)), you are actually measuring air velocity and averaging that over your target area. So both the measurement of air velocity (feet per minute or FPM) and air volume flow (CFM) are part of the discussion.

Engineers or technicians design residential heating and air conditioning systems to provide creature comfort. To make these designs work as planned, the installed system must deliver the proper amount of correctly conditioned air to the right place.

Let’s break this down in reverse order:

The right place.
That’s pretty straightforward; that’s about duct design. Nevertheless, hang on; don’t forget duct resistance (static pressures), duct leakage, and mixing in the room (e.g. register location, blockage, throw, and sufficient return airflow). These often forgotten factors can make all the difference between customer satisfaction and complaint.

Correctly conditioned air.
Now we need to look at the P-Word: psychrometrics or how we change the temperature and humidity of the air: effectively altering its energy content. Here’s where we think inside the box. What are the processed going on to warm, cool and dehumidify the air.

Delivering the proper amount.
Yeah! Finally, we are getting to the airflow measurement part; delivering the correct CFMs. Here’s where airflow measurements usually get most of the attention.

But if we back up, we’ll see that airflow is the connecting piece to the other two factors. The proper air velocity, definitely affects duct delivery, and it is in turn affected by duct design (increasing static pressure) and leakage (volume flow).

Also conditioning (heat, cooling or dehumidifying) of the air is affected by the mass flow (CFMs times density) of air through the heat exchanger or coil. If the airflow is too slow, the air may not take away enough heat and cause heat exchanger or other component breakdowns. Or in cooling mode, slow airflow can cause ice over of the evaporator coil. If airflow is too fast over the heat exchanger or coil it can cause system performance issues and customer complaints.

So it’s not about just one airflow measurement, multiple measurements are required at various key points to maintain proper system performance and customer satisfaction.

Since, airflow measurements are among the most daunting tasks in the industry we will dig into the science behind good airflow measurement, during my session at ACCA’s Building Performance Forum this October in Charlotte. I will cover every major airflow measurement tool and technique you have heard of and probably a few you have never heard of! We will explore and explain all methods and cover the ins and outs of picking the correct tool for the measurement task or application. Products will be on hand for your interaction.


Posted In: Building Performance, Residential Buildings

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