Air Capture Hoods: Uses, Features, and Buying Advice

Air capture hoods offer technicians quick and accurate answers on how well a forced air system is functioning. Proper airflow will determine occupant comfort, system energy efficiency, and equipment lifespan. An air capture hood speeds the process of testing, adjusting, and balancing (TAB) by providing a snapshot of supply or return airflow directly at the opening. They are widely used by mechanical contractors, facilities maintenance personnel, plant engineers, and indoor air quality specialists.
TSI Alnor 6200 LoFlo Balometer Capture Hood

Measuring Airflow

First let’s take a look at the broad principles of how an air capture hood works, assuming measurement of a supply register. The capture hood is placed such that it completely covers the register opening to be measured, and a button is pressed to begin the test. As the air stream enters the hood area from the register it is often turbulent, erratic, or swirling, so every flow hood uses a mechanism (some more elaborate than others) to smooth the air jet. Then the stream passes through a measuring device to determine its air velocity (more on this below). After the measurement device, the air flow passes through the outlet at the bottom of the flow hood. Because this outlet is of a known size, and air velocity has already been determined, the air capture hood can now calculate the air flow volume.

Air capture hoods measure air velocity (and ultimately air flow) in one of two ways, either through thermal anemometer sensors (as in a hot wire anemometer) or through a pitot tube array. When measuring through thermal anemometry, air passes over a thin wire that is kept at a constant condition (current, voltage or temperature) through electrical resistance. As air passes over the wire it is cooled and thus requires more electricity to maintain its constant condition. The amount of electricity required to keep the hot wire in its constant state is proportional to the air velocity. Air capture hoods using hot wire anemometers are often targeted at low flow measurement applications, measuring air volumes up to 500 CFM.

The second method used by capture hoods to determine velocity is through the use of a pitot tube array (also known as a wilson grid, or true flow grid). In this arrangement two grids of tubes connected to a differential manometer measure the total pressure and the static pressure of the air flow. The difference between these pressures is the dynamic pressure or velocity pressure, which correlates to the air velocity. Velocity measurement through a pitot array is well suited to medium and high velocity applications, with air volume measurement up to 2500 CFM.

Kanomax 6710 Air Capture HoodBack Pressure Compensation

One factor all air capture hoods must account for is the effect of the instrument itself on the flow being measured. The resistance caused by the hood, internal flow conditioners, and outlet size can cause a drop in pressure, yielding inaccurate measurements. Accounting for this pressure drop is handled through two main techniques, generally known as "back pressure compensation" and "zero pressure compensation.”

Back pressure compensation is most often handled through a built in flap that remains closed as the instrument makes one measurement, then open as a second measurement is taken. Each measurement takes only a few seconds. The difference between these two results allows the air capture hood to calculate how much back pressure is present. The meter will then show a final CFM result that will be more accurate in most conditions than a one-point measurement. This is the most common technique of accounting for the pressure change caused by the flow hood. When industry literature indicates that an instrument has “back pressure compensation,” it is usually referring to this method.

Energy conservatory FlowBlaster Capture HoodPowered Flow Hoods

Another way to compensate for the presence of resistance is through what is known as a powered flow hood (or an active flow hood). In this method the air capture hood is equipped with a variable speed fan that equalizes the pressure inside the hood to the pressure of the surrounding room. This “zero pressure compensation” allows the instrument to measure the pressure from the register as if the hood were not present. Consequently the powered flow hood is often more accurate than a non-powered flow hood. Because the fan must compensate for the pressure difference, the powered flow hood is limited to lower flow rates of about 300 CFM; beyond this threshold the fan cannot zero the pressures and provide the same accuracy. Even so, powered flow hoods are sufficient for most residential conditions and offer the greatest accuracy. These instruments generally cost a bit more and are clearly identified in industry literature as a “powered capture hood” or “active capture hood.”

The powered flow hood and its zero pressure compensation method have received much attention in recent years. This is in part due to several papers published by Lawrence Berkeley National Laboratory (LBNL), authored by Walker, Wray, et. al., evaluating flow hood accuracy in residential applications. These publications (LBNL-5983E, LBNL-49697, LBNL-47382) find that "powered flow hoods yield more accurate measurements than non-powered flow hoods” (LBNL-5983E). The report continues, "This does not mean that active hoods are the only ones that can work...some passive hoods can obtain good results when they are used with appropriate care….” Manufacturers have heeded this call for appropriate care, and in recent years have given more direction in the proper use of flow hoods, and now offer a greater number of options in flow conditioning screens and interchangeable hood sizes.

TSI EBT731 Balometer Capture Hood with StandOptions and Accessories

Air capture hoods are available with many options and accessories to simplify work flow, extend the usefulness of the instrument, or improve accuracy under certain conditions. Some flow hoods measure relative humidity and air temperature as they measure air flow. This can be helpful for air density corrections if needed, or to quickly calculate dewpoint or wet bulb temperature. Another useful option offered by some manufacturers is the ability to add a different flow conditioning screen. This is most important when measuring supply flows, since not all registers or grilles distribute air in the same patterns. Most air capture hoods are designed to measure a standard 2 ft x 2 ft 4-way throw diffuser. Flow conditioners are available to give more accurate measurements on smaller residential diffusers or swirl diffusers. Interchangeable hoods can also offer better results on registers of varying size. Accuracy is usually best when the hood size most closely matches the register size.

If testing, adjusting, and balancing a very large facility, or if record keeping is called for, it may be desirable to have a flow hood with the ability to capture and export data. Several hoods can store thousands of measurements in different test groups for easy download, analysis, and reporting. Bluetooth or WiFi communications allow some models to connect to a PC, tablet, or smart phone. This can be a great option when paired with a telescoping stand, allowing the user to monitor the capture hood measurement without needing a ladder. Wireless communications also allow a technician to view live measurements on a phone while adjusting a damper for optimum air flow. Wireless printers are also available for some models.

TSI EBT731 ManometerSome balometers feature a detachable manometer that is compatible with a variety of probes. The same micro-manometer that measures pressures in the flow hood can be removed and used to measure duct airflow with a pitot tube or hot wire anemometer probe. The manometer can also be used for zonal pressure diagnostics to determine how areas of the building are connected to one another or to the outside. Velocity matrix probes are offered which can measure laboratory hood face velocity, test filter face velocity, or measure exhaust flow on a kitchen hood.

Selecting an Air Capture Hood

The first consideration when buying an air capture hood should be its airflow range. If testing will be performed on 1000 CFM commercial diffusers, a low flow hood simply cannot do the job. Hoods are typically offered in residential (up to 500 CFM) and commercial ranges (up to 2500 CFM). The next consideration involves balancing accuracy and cost. Used under the right circumstance with the correct flow conditioners, most air capture hoods will yield satisfactory accuracy. If measuring residential or light commercial flows, however, greater accuracy may be obtained through the use of a powered flow hood. Hood size is another consideration, with most instruments being offered in multiple hood size configurations from 16 in x 16 in square to 1 ft x 5 ft rectangular. Choose the hood or hoods that most closely match the opening size. Finally the buyer should weigh other potential uses for the tool. The detachable manometer available with some capture hoods can become the backbone of several high quality airflow and pressure measuring instruments, justifying a slightly higher price.

Air capture hoods offer the ability to more accurately understand how air is delivered to and from a building. Whether commissioning, spot checking, or balancing, flow hoods give the testing professional much needed information on how an HVAC system is actually performing. They measure air flow volume faster than any other means, have a simple user interface, and offer an accurate means of evaluating system performance.

Please let us know if you have any questions about the applications, features, or operation of air capture hoods. You can reach us at [email protected]