We heat it, and cool it, humidify it dehumidify it, clean it, move it, supply it, return it, and monitor it, yet somehow we have forgotten, never learned, or never been taught how to measure and set airflow. Billions of dollars are wasted each year on improperly commissioned and serviced HVAC equipment. Improper equipment performance, pressurization and depressurization problems can all be linked to design or performance issues. Compressor failures, heat exchanger failures, component failures, high utility bills, allergies, CO poisonings, poor IAQ, are the short list of common problems linked directly to poor airflow.
Measuring air velocity at the register
When making an airflow measurement, what should it be, and how are you going to measure it? Are you selecting a blower speed from four possible choices, or quantifying equipment or system performance? Measurements made without knowledge of their expected outcome is a valueless proposition. For years technicians have been using standard air equations and formulas, making non-corrected airflow measurements, or making no measurement at all, simply leaving the airflow at factory settings or selecting high for cooling and medium or low for heat. In reality they were never really measuring airflow they were simply estimating airflow at best. It’s not what we consider when measuring airflow that matters its what we don’t consider that causes problems.
Airflow is the key to proper system performance. Refrigerant charge cannot be verified until it is correct, an accurate or final combustion analysis cannot be performed, system performance cannot be measured, and really no part of the commissioning process will remain uncompromised or unaffected if airflow is anything but correct. Measuring airflow should be the first step in the commissioning process and the first step when servicing equipment, it is the key component for proper equipment operation.
Measuring airflow is easy. Measuring airflow accurately can be very difficult. The trend to improve efficiency by installing 13-seer equipment and higher and high efficiency furnaces is in turn requiring 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 systems 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 assure proper capacity, velocity, and throw assuring stratification or noise problems do not exist. Without proper airflow the system and or equipment efficiency and operation are compromised resulting in unsatisfactory equipment operation, customer dissatisfaction and utility waste.
So what does airflow measurement encompass? • Airflow across the heat exchanger or evaporator coil
• Delivered airflow to each space (supply air)
• Air returned to the appliance from the conditioned space for conditioning (return air)
• Acceptable room pressures
• Sufficient pressure at the combustion air zone of an appliance
Total HVAC performance testing is principally based upon airflow, enthalpy and their accurate measure. Not only is total air volume considered, but also the proper velocity, quantity and throw of the conditioned air to each part of the conditioned space, along with the delivered air conditioned and its impact on the surrounding surfaces. Depressurization or pressurization of the combustion air zone or outlying rooms is also considered part of the total system performance evaluation as more powerful blowers can negatively impact these parameters.
The Airflow Objective
Our objectives have changed. We are no longer making an airflow measurement to estimate airflow, we are also trying to calculate duct leakage, measure equipment performance, and measure delivered BTU’s and determine if system performance is affecting operation of equipment in the combustion air zone. Simply what was good enough to get you in the ball park will get you thrown out today if you are not careful. You can spend days chasing a problem that doesn’t exist, and loose a lot of money in the process.
Lets start with the basics. I apologize in advance for the basics not seeming so basic, but different standards organizations have established a number of definitions for the standard conditions of temperature and pressure under which air is measured. Before we can get anywhere we have to start on common ground, so lets start with a common standard, standard air. Standard air has a very specific properties 68.4F, and 0% relative humidity. No, that is not a typo; there is no moisture in what we consider standard air! And….. remember, standard air is measured at a pressure equivalent to sea level, which means in most cases, above sea level, the actual pressure will be lower. What really makes standard air intriguing is it is not standard at all considering the air technicians’ work with always contains moisture, is measured often at pressures below that of sea level, and at temperatures above or below 68F. For common testing of fans and their capacity in the HVAC industry, NTP is used. NTP (Normal Temperature and Pressure) is defined as air at 68F, 29.92 in Hg, and 0% relative humidity. At precisely 68.4F and 0% Rh, NTP air has a specific density of .075 lb/cf and a specific volume of 13.333. From this point on, we will refer to this as Standard air, which it is for the HVAC industry. Each standard has different reference conditions, so care must be taken when looking at specifications especially when evaluating performance of many non-domestic made fans or measuring equipment.
Common air standards:
• U.S. Standard Atmosphere (used by ASHREA)
• STP – Standard Temperature and Pressure
• NTP – Normal Temperature and Pressure
• SATP Standard Ambient Temperature and Pressure
• ISA International Standard Atmosphere
Properties of Standard Air
Air has mass and takes up space, it has a specific heat, specific density, and. specific volume. Standard air weighs .075 lb/ft3, (Specific Density) and 1 lb of it occupies 13.33 ft3 of space. (Specific Volume) The amount of heat required to raise the temperature of 1 lb or air 1 degree F is .24 btu. (Specific heat)
SCFM
SCFM is used to compare performance between fans in HVAC equipment. It is a standard set of conditions that defines the parameters under which performance will be measured. It is purely used to design and to produce apples to apples performance within the HVAC industry.
Standard cubic feet per minute, SCFM or scfm is flow of free air measured at some reference point and converted to a standard set of reference conditions (e.g., 14.7 psia, 68° F, and 0% relative humidity.) Again, scfm means CFM at a standard set of conditions. Here the problem lies. Unfortunately real-life conditions “actual conditions” are never standard conditions and often never considered by technicians making measurement in the field. Simply technicians are trying to quantify factory performance under field conditions without correcting for the conditions or using measurement techniques that that yield results independent of the air density. This problem does not rest solely on the shoulders of contractors; manufacturers, manufacturers reps and engineers alike also misunderstand it and misapply standard air equations.
Fans move a constant CFM independent of the air density. As the density changes, the mass flow rate changes under the varying conditions. Restated, a fan is a constant volume flow machine with a variable mass flow rate.
Mass flow rate varies under three common conditions which directly affect air density:
• atmospheric pressure
• temperature
• humidity
Density Matters
Pressure, temperature and humidity all directly impact air density. When striving for an accurate airflow measurement, you must consider the measurement technique, or tools you are using or mathematically correct for conditions outside of standard air. Density has a large impact on many of the measurement process you might be performing. The temperature rise method, most capture hoods, pitot tubes, pressure drop across fixed orifices, many hotwire anemometers and the Truflow® grid are all density dependant methods of measurement, and will require correction if an accurate result is required. If air density is not considered and corrected for, there will be a substantial amount of error in the computation of the final results especially at the grills and registers of the air being measured is conditioned (Heated or cooled).
Some manufacturers of airflow measurement instrumentation provide a means for either adjusting the air density directly, or allow the user to input temperature, relative humidity and air pressure to compensate for conditions outside of standard air, some manufacturers use dynamic processes and real-time measurement of the temperature, relative humidity and input of elevation to calculate air density as part of a dynamic process, and some measurement techniques measure airflow accurately independent of the air density.
The Underlying Issue
74% of residential systems installed today have improper airflow according to a recent EPA study pertaining to Energy Star. While our industry is in need of repair, I am afraid there is no quick fix will to provide the needed answers. Furthermore, neither will capture hoods, vane anemometers, hotwires or pitot tubes, high-resolution Magnehelics or monometers. The key lies in technician and consumer education.
Airflow Measurement
If you do not know what the airflow measurement should be what difference does it make what it is? For example, to select a register, we need to consider not only air volume, but also the face velocity and throw. Technicians are not often privy to information regarding the register selection such as room air requirements, intended application or design. As can be seen from past experience, many times a proper heat loss and gain are not performed during the design phase, when modifications are made to the ducting system, and when replacement equipment is selected in the field. I would care to bet register selection is more often based on cost, color and size. Measuring CFM delivered at the register is only a small part of the equation; face velocity and throw are just as integral parts of information. If the heat never gets off of the ceiling or the cooling never gets off of the floor, a correct quantity will not provide comfort do to stratification of the air in the room.
Technicians should not make a habit of ever making estimations where true measurements can be made. With the cost effective solutions in instrumentation that are available, 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 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 office. When replacing existing equipment, a complete evaluation of the ducting system including verification of proper airflow at the registers is warranted.
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 is 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. Equipment can perform at its optimal capacity and efficiency on a poorly designed duct system. The overall system efficiency and air delivery will be substandard, but it is not indicative of the equipment performance. This is of particular interest to the manufacturer, as equipment will almost without exception perform as listed on a properly designed system. When referring to equipment, it should be understood that equipment can operate properly independent of the duct system, which is required for air delivery to the space. A poorly designed duct system can be the root cause of poor equipment performance, however, poor equipment performance is usually not the cause of poor performance of the system if the equipment is properly selected, commissioned and maintained. The technician, designer and or salesman can be confident that listed equipment will perform as stated in the manufacturer’s literature if the duct system is properly designed for the blower performance and the addition of external airside devices is limited to the available static pressure. Third party verification of equipment performance through the listing agency validates the manufacturer’s performance claims.
Airflow Across the Evaporator Coil
Airflow across the evaporator is one of the most overlooked yet the most important parts of verifying proper operation of air conditioning systems. Low evaporator airflow can cause symptoms like evaporator freezing, low system capacity, poor distribution and high-energy consumption. High airflow can cause symptoms of poor humidity removal; higher energy costs, noise, drafts and water/equipment damage due to water droplets blowing from the evaporator coil do to excessive air velocity. Air conditioners are designed for a nominal 400 CFM (450 for heat pumps) of airflow per ton.
To operate air conditioning with the designed capacity the airflow has to be set to the manufacturer’s design criteria at the evaporator coil. Temperature drop across a coil will vary with the latent load (humidity) the more humidity, the more cooling energy goes to converting water vapor to water. The temperature drop across the evaporator can easily be between 16 to 24 F. Therefore it is imperative to set the airflow to the proper range and not to rely on the temperature drop across the coil to verify system performance. It is important to understand that the conditions of the air entering the coil will not normally affect the designed temperature difference of the coil. It will however affect the temperature of the air leaving the coil.
Airflow across the heat exchanger Low airflow is “a” if not “the” primary cause of premature heat exchanger failure. Low airflow due to plugged filters; incorrect blower speed selection, improper duct design or equipment over sizing can all be detrimental to the heat exchanger. Using the sensible heat formula and determining the center of the desired temperature rise, the correct airflow across the heat exchanger can be set in a matter of minutes with a mini vane anemometer without ever firing the furnace. The furnace still must be operated to verify correct temperature rise, but normally no further adjustment will be necessary, and the time required to set the blower speed will be greatly reduced.
Airflow measurement, selecting the measurement process
Before a technician measures airflow, a conscious decision must be made. Specifically, what is the purpose of the airflow measurement? There are many measurements that can be made to “estimate” the airflow for the equipment commissioning procedure. Not every airflow measurement requires a high degree of accuracy. We don’t use inside outside calipers to measure the length of a fish, we would obviously use a standard ruler. On a standard furnace or air handler, the airflow measurement need only be accurate enough to determine the proper fan speed selection as usually there are only 3 to 4 speeds that can be selected. An exact airflow (400 CFM/ton) cannot be often set across the appliance evaporator coil or heat exchanger. However of the technician desires to determine duct leakage, verify equipment performance or measure actual BTUH delivery an airflow measurement with a higher degree of accuracy is required. When it comes to measuring airflow, the degree of accuracy only possible in the lab a few years ago is now possible for every field installation. Equipment can be setup with laboratory accuracy guaranteeing equipment operating efficiencies and capacities that can be verified with a high degree of accuracy
Measurement Techniques
Technicians and manufactures have long struggled with issues of airflow and airflow measurement. Due to the time-intensive nature of many measurement procedures and the limited tool selection of the technician, it has been common to use gross airflow estimation methods that are uncorrected. Many of the methods discussed are those methods. The temperature rise method, total external static method, pressure drop across filters or coils all examples of gross airflow estimation methods and many times are adequate for the equipment commissioning procedure, however if the desire is to evaluate equipment performance, a more accurate method is required. 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 manufactures 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 most common and easiest ways to measure or estimate and set airflow at the equipment or registers is one of the following methods:
1) Rotating Vane Anemometer (Large or small)
2) Pressure drop across the dry evaporator coil
3) Total external static pressure method
4) Pitot tube and digital manometer
5) Velocity Stick (Hot Wire Anemometer)
6) The temperature rise method (Sensible heat formula)
7) RPM and manufacturers fan curve (Belt or VF Drive)
8) The capture hood
When making any air flow/quantity measurements for cooling or heating all dampers must be in their normal open position, 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 branch size, and the throw and face velocity by register selection.
Rotating Vane Anemometer
The mini-vane anemometer is the ideal tool to 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%, and often less than 1% error. Changes in yaw and pitch of the probe head in the duct by as much as 10% will result in less than 1% error in the measurement making the mini-vane an ideal probe for field in-duct air measurement.
At the grilles/registers
A 4” 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” vane does not respond to local stray eddy air currents, that 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 (specifically capture hoods) are not accurate enough to measure duct system leakage. For residential applications, grille/register face velocity should be 400-600 FPM. 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 assure proper air delivery to the space with only minor adjustments of the dampers required to balance out the system.
Pressure drop across the dry evaporator coil
An easy way to quickly verify airflow is to measure the static pressure drop across the evaporator coil, and compare the reading to that specific evaporator coil in the manufacturer’s literature. With a digital manometer, and a pressure drop vs. CFM chart, airflow can be set close to specification across a dry coil in a matter of minutes. The positive probe should be inserted ahead of the air entering the coil and the negative probe immediately downstream of the coil. The reading obtained will be the pressure drop in inches of water column or Pascal. NOTE: While this measurement is accurate enough for setting up equipment, it is not accurate enough to make a field measurement of the system capacity. This method is air density dependant and can be influenced by the point of measurement.
Total external static pressure method
The total external pressure method is performed in the same manner by measuring the pressure difference across the furnace (supply to return) and using the manufacturer’s chart. The CFM can also be set quickly and accurately using this method, but again, the measurement process is not precise enough to use for verification of the system capacity. This method is air density dependant and can be influenced by the point of measurement.
Pitot tube and digital manometer
If the return airdrop is tall and straight enough the airflow into the appliance can also be very accurately verified using a Pitot tube and a digital manometer. However, this method is very time consuming. By traversing the duct, (making several pressure measurements in predefined locations) and performing a couple of simple calculations to convert velocity pressure to speed in feet per minute, the air flow is determined by multiplying the average air velocity by the cross sectional area of the duct to obtain CFM. It is imperative that the ducting attached to the appliance, and the base pan, if side returned is used, is sealed. Air leaks up-stream of where the measurements are made will significantly alter the actual reading obtained with this method. This method is air density dependant and can be influenced by the point of measurement.
Hot Wire Anemometer
A hot wire anemometer can also be used in the return air duct to verify flow. Using this method, it is important to carefully traverse the duct in order to get accurate results. Until the development of the mini-vane anemometer, the Pitot tube and hot wire were the most precise field measurement of airflow in a duct. Both however are sensitive to changes in air density outside of standard air conditions. If done carefully most technicians can achieve accuracy within 20 CFM per ton or 5%.
The temperature rise method (Sensible heat formula)
Because the heat content of natural gas varies from day to day and hour to hour, and fuel oil from gallon to gallon, the temperature rise method should only be used to get the airflow close to the manufacturer’s recommendation, and cannot be used for AC system capacity verification. Even with electric heat it is only estimation if the constants used for the equation are not adjusted for air density. It is important to remember, for the temperature rise method to be accurate, the BTUH output of the appliance must be known. To verify CFM in a natural gas furnace, first let the furnace run for ten minutes or until the stack temperature stabilizes, allowing the appliance to reach steady state efficiency. Using a combustion analyzer determine the steady state operating efficiency of the appliance and multiply it times the BTUH input to get the output BTUH of the furnace. (Remember, if the heat is not going up the stack, it is going into the house.) If a combustion analyzer is not available, alternatively, the manufacturer’s literature could be used to determine the output BTUH’s of the furnace provided the manifold pressure is correctly set and the BTU content of the fuel used is consistent. (The manufacturer’s tag is a good place to look for this.) Do not use efficiency information from the yellow energy guide label, as this is AFUE, (Annual Fuel Utilization Efficiency) and takes into account the efficiency losses at start-up of the equipment. Second measure the temperature rise across the heat exchanger. It is important that your probe be out of the line of sight of the heat exchanger when making these measurements as the temperature probe can be affected by radiant heat from the heat exchanger. If the furnace has a bypass humidifier, make sure the bypass is closed. Next enter your results into the sensible heat formula (shown below). This is an approximate method as additional heat added from the blower motor. Heat added by the motor can be as much as 300 watts or 1024 Btu.
CFM = BTUH Output/(1.08 x ΔT)
Because of the complexity of accurate field measurement and inaccuracies associated with traditional measurement procedures particularly in airflow the measurement obtained with a precision vane anemometer should be considered the reference measurement against which others, field and lab methods can be compared. In real world applications, again, air is never “standard air” (68F, 0%Rh, 29.92” Hg) the standard air equations like those used in the temperature rise method cannot be applied to obtain the high degree of accuracy often desired and achievable with a vane. If using the standard air equations to compare measurements made with a vane, the constants in the standard air equations must be recalculated and the air measurement be compensated for fan air inlet density. Air measurements made via a Pitot tube, hotwire, or flow grid will have significant error if the air density is not considered and corrected. Because of the complexity of accurately using these instruments to measure airflow the time required, and the low velocities often encountered, the vane is a perfect choice for residential air measurement.
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