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Go to any TABB (Testing, Adjusting and Balancing Bureau) lab or equivalent, and you will see that learning about airflow measurements is an involved procedure requiring years of practice, dedication, and specific schooling.
Simply, you cannot put the tools in the technician’s hands and have anything more than a qualified meter reader and button pusher on the job. TABB establishes and directs an international certification program for technicians, supervisors and contractors engaged in testing, adjusting, and balancing for all building environmental systems. TABB is the first program to combine all certification elements of a testing, adjusting, and balancing project.
Spend 20 minutes on any major grille/register manufacturer’s technical section of their Website and you will see there is more to understanding airflow than just measuring cubic feet per minute (cfm). Many white papers have detailed issues like flow, velocity, measurement, and noise. Issues regarding room stratification and delivery are explained in terms a residential or light commercial technician can understand.
Having an Ak factor may be part of a cost-effective solution for making a cfm measurement, but it does not provide all of the information the technician needs to know. Each Ak factor is different depending on the type of meter used to make the measurement. Additionally, if you do not know what the measurement should be what difference does it make what the measurement is?
To select a register, we need to consider not only air volume, but also the face velocity and throw. Residential and light commercial technicians are not often privy to information regarding the register selection such as room air requirements, intended application or design. Many times proper heat loss and gain calculations are not performed during the design phase in residential and light commercial situations. Even when calculations are performed, field modifications are often made to the ducting system and equipment. It is easy to imagine that grille/register selection is more often based on cost, color, and size.
MEASURING AIRFLOW IS EASYMeasuring airflow accurately can be very difficult. The trend to improve efficiency by installing 13 plus SEER equipment and high-efficiency furnaces is demanding more accurate airflow measurement. An airflow measurement that is not repeatable, accurate, and representative of mass flow will result in calculations of system operation that are not representative of the system’s efficiency, capacity or latent sensible split and resulting humidity removal. Airflow must be correct at the equipment and then delivery at the registers or terminal outlets must be verified to ensure proper capacity, velocity, and throw which will minimize stratification and/or noise problems. Without proper airflow, the system and/or equipment efficiency and operation are compromised.
Measuring cfm delivered at the register is only a small part of the equation; face velocity and throw must also be taken into consideration. BTUs of cooling or heat delivery cannot be determined without an accurate measure of the enthalpy or total heat in the air. Using the sensible heat formula and multipliers, such as the sensible split, to determine cooling capacity is less than an educated guess at system performance. Technicians should never form a habit of making an estimate where a true measurement can be made. With today’s cost-effective solutions in instrumentation, technicians not only need to make an investment in technology, but also in its application and proper use. Making measurements without knowledge of the expected results is nearly a valueless proposition.
Measuring airflow is a critical part of all service and sales calls. Before any system commissioning is complete, any evaluation of existing equipment is made, or during routine service, airflow should be measured and verified at the equipment and in problem areas of the home or building. When replacing existing equipment, a complete evaluation of the duct system including verification of proper airflow at the registers is warranted, but this information must be used in conjunction with additional information about the room’s load to be of value.
When verifying airflow, it is imperative that the technician understands some basic elements of system design, and basic principles of duct design and delivery to aid in understanding the measured parameters.
For a forced-air system to operate properly and as designed, airflow must first be set properly across the evaporator coil and furnace heat exchanger, and second the correct airflow must be delivered at the proper volume, velocity and throw - which are critical to optimal system performance and creature comfort.
The airflow must first be set according to the equipment design - not to the air delivered at the registers. While the design of the duct system is imperative for proper air distribution to the conditioned space, air measurements are only to be measured at the appliance for the equipment commissioning procedure. Due to leakage inherent with all ducting systems, airflow cannot be measured at the registers to verify correct airflow across an evaporator coil or heat exchanger.
After the airflow is properly set at the appliance and the equipment operation is verified to be correct, failure to heat or cool is usually not an equipment problem. The ducting system should be evaluated for excessive leakage, proper sizing, and proper design. A review of the heat load and heat loss calculation may be required to verify the equipment selection was correct if the system continues to perform poorly.
When making any airflow/air quantity measurements for cooling or heating, all dampers must be in their normal open position, and all equipment panels and doors must be in place. If the duct system is designed properly, the quantity of air delivered to the register will be dictated by the fan performance, duct design and branch size, and the throw and face velocity by register selection.
HOW TO MEASUREIt is my opinion that the best and most cost-effective solution for a quick and highly accurate airflow measurement in residential and light commercial systems is the rotating vane anemometer. Vane anemometers have several advantages over any other method. The primary advantages are cost, speed, accuracy, and ease of use. Pitot tubes and hot wires both require calculation of air density for correction of the velocity measurement, and many hot wires are calibrated only for standard air. To measure velocity or volume of air, vane anemometers do not require air density compensation due to variations in air temperature, humidity, or atmospheric pressure. Vane anemometers simply measure the air speed directly. Different size vanes are available for each step in the commissioning procedure.
Remember, if you are using air measurements obtained with a vane to calculate cooling or heating capacity, air density will still have to be considered for accurate calculation involving the true mass flow rate across the heat exchanger or coil.
Many commercially available capture hoods are specifically designed for standard 2x2 registers that produce fairly laminar flow. Smaller residential registers with 1-, 2-, or 3-way throw and low cfm requirements do not always provide the kind of laminar flow required by most hood manufacturers for accurate measurement of velocity or volume.
Some manufacturers require a flow conditioner to straighten the flow as it goes through the measurement array, creating additional backpressure at the register. There are hoods that can do the job, but like anything else you get what you pay for and the additional cost may not provide enough benefit for a residential application.
The mini-vane anemometer is the ideal tool for measuring airflow in a duct, across a heat exchanger, or evaporator coil as required in the commissioning process. The mini vane allows for a full duct traverse with an automatic calculation of the cfm in the duct (as the duct dimensions and the free area are input into the instrument before the measurement is taken). If done carefully, the measurement error will be less than 3 percent, and often less than 1 percent error. Changes in yaw and pitch of the probe head in the duct by as much as 10 percent will result in less than 1 percent error in the measurement, making the mini-vane an ideal probe for field in-duct air measurement.
At the grilles/registers, a 4-inch diameter vane anemometer may be used to determine air velocity leaving at each terminal outlet. Again, no air density compensation is required, it is a simple one-hand operation, and most are easy to carry and operate. Another advantage is a more accurate average of true airflow over the sample area. The 4-inch vane does not respond to local stray eddy air currents; a hot wire probe or Pitot tube may. Many times 4-6 trucks can be equipped with a large vane anemometer for the equivalent cost of one of the more expensive options previously mentioned.
For residential and light commercial technicians, balancing of air velocity will be adequate due to the limited resources on hand and limited knowledge of original system design available to them in the field. Duct leakage should be determined using alternate methods, as studies have shown most of the methods mentioned above are not accurate enough to measure duct system leakage.
For residential applications, grille/register face velocity should be 400-600 feet per minute. Higher air velocities may be noisy and lower velocities may not provide proper spread and throw, leading to problems with air stratification and comfort issues. The quantity of air delivered is a function of the blower performance, duct and branch size and, finally, the register performance. A branch can be delivering the correct cfm, but if the register selection is incorrect, and the throw and spread are not adequate, the air will not mix with the room, leading to problems with space comfort.
If the duct system is designed properly, equal velocity balancing of the system will ensure proper air delivery to the space with only minor adjustments of the dampers required to balance out the system.
As far as printing Ak on the backs of registers, the Ak factor is nice to know if you have the instrument the Ak factor was calibrated with. Ak factor is only part of the equation. For what it’s worth, if you’re asking, you may as well ask for an owner’s manual for each register detailing flow, throw, spread, Ak factor, free area and delivery along with back pressures for each possible application and cfm.
Maybe it can be printed in real small type on the back of the grille. For now though, I think I’ll just reference the manual, and measure face velocity - it may not tell the whole story, but given the limited information I have about most residential and light commercial jobs, it’s all I really need to know.
Publication date: 04/16/2007