Digital Zeus™ HVAC Tool & Instruments Journal

Measure Duct Loss in Five Minutes
Rob Falke 

“We’ve all heard our customers describe how the air conditioning system works well in the morning, but doesn’t deliver any cooling in the afternoon. We send out a service tech, he services the unit and reports that it’s working pretty well. End of story, right? Well, not for the customer. He or she is still uncomfortable every afternoon until the weather cools down in the fall. Why does this happen? The most likely reason is duct loss.
Ideally, the temperature of the air leaving the equipment should be the same as the air leaving the farthest supply grille. But if the duct is running through a 140F attic, it’s not uncommon for supply air to lose up to half of its Btus of cooling.

Let’s take a look and discover how a simple five minute test can quantify the duct loss and help us solve a problem that may have been overlooked for years. But you won’t find the answer to this question at the unit. You’ve got to pull your head out of the box and check the duct system.

Check the Delta T
Delta T is the difference between any two related temperatures. Checking it is easy and quick: simply take two temperature readings, and subtract one from the other.”

Lets take a closer look at that idea and plot it out on a psychrometric chart
The first photo is a plot of the conditions as stated.

Click the Image for more Detail

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Air Velocity Measurement

Sensing Static Pressure

In air conditioning, heating and ventilating work, it is helpful to understand the techniques used to determine air velocity. In this field, air velocity (distance traveled per unit of time) is usually expressed in feet per minute (FPM). By multiplying air velocity by the cross section area of a duct, you can determine the air volume flowing past a point in the duct per unit of time. Volume flow is usually measured in cubic feet per minute (CFM).
Velocity or volume measurements can often be used with engineering handbook or design information to reveal proper or improper performance of an airflow system. The same principles used to determine velocity are also valuable in working with pneumatic conveying, flue gas low and process gas systems. However, in these fields the common units of velocity and volume are sometimes different from those used in air conditioning work.

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Airflow- A Critical Component

Introduction

Airflow measurement is a critical component when assessing an HVAC system. It helps ensure balance, energy efficiency and cost effectiveness of the system. The most effective way to test and verify these components is with airflow measurement instrumentation. These test instruments are called anemometers or thermoanemometers (if they measure temperature). Many anemometers measure airflow, air volume and temperature, while other parameters may include humidity, dew point, and static/differential pressure. Anemometers are available in two technologies: vane anemometers and hot-wire anemometers. Each has its application limitations, but hot-wire anemometers are more common due to the wider airflow range measured. Manometers are another test instrument that can measure air velocity and volume, but is most commonly used to measure the static and differential pressure of a system. Finally, capture hoods provide direct measurements of air volume through supply and exhaust readings of grilles and diffusers.

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