Who are we kidding? A variable that is not directly measured cannot be directly controlled. This is the simple fact of HVAC system airflow measuring/monitoring/controlling. It’s always good to pause to ponder if the way the airflow is being measured/monitored/controlled can indeed be defended in court if the design intent is not achieved. The cost of saving a few dollars on first cost can be far offset if shortcuts are taken when attempting to utilize and rationalize the use of indirect methods.
There are many types of indirect methods one may pursue to ensure the required airflow is being delivered or removed from a space or a building; however, are they all defendable, repeatable, and accurate all the time in all conditions? Of course it is possible to have algorithms within building automation software programming to calculate airflow based on temperature sensors using mixed flow calculations; however, this adds a greater degree of inaccuracy depending on the accuracy of the sensor at any given point in time. What risks are being taken to not measure/monitor/control airflow directly, and who is at risk?
Point to Ponder: What is the risk and liability that a professional engineer must deal with in today’s built environment when it comes to airflow measuring/monitoring/controlling?
As with any engineering decision, the starting point is identifying the required need to measure, monitor, and/or control airflow, and acknowledging how important it is to have accuracy in the measurement. Is the measurement needed to validate and prove the HVAC system is complying with codes and standards? Is that measurement needed to submeter tenants’ energy use if they decide they want a greater amount of outside air than the minimum required by their lease agreement? Is the measurement the control of building pressure? Is the measurement controlling the pressure of critical environment spaces in a hospital or lab? Is the measurement designated for energy savings?
Point to Ponder: Is understanding airflow measurement required and needed as an essential part of the design process in order to determine the method, technology, and products that can be used to accomplish the design intent and have a defendable installation?
When writing specifications and developing contract documents, it is important to use terminology that is clear and concise. It’s also important to not only understand the true definitions of the terms but also the industry-specific definitions. There are multiple sources for definitions in the HVAC industry that engineers need to be aware of. Whether the definition of a specific term be spelled out in the codes or standards, such as the IBC or ASHRAE or industry association publications from organizations such as AMCA and SMACNA, it’s good to delineate which definition source is being used in contract documents for defendable documentations and clarity of design intent.
There may also be variances among the sources of specific definitions. It may be wise to have a source of terminology statement in contract documents to make it clear which definition is being used. It’s essential to pay attention to marketing terminology in a manufacturer’s product literature that may have no industry-wide definition. These words and phrases should be avoided in contract document specifications if there is no definition given for manufacturers to comply with.
ASHRAE is a good starting source to understand some of the basic terminology in airflow measuring. “ASHRAE Terminology” is a free resource that includes a free online glossary. The terms in ASHRAE Terminology have been assembled and defined and are updated on an ongoing basis by ASHRAE Technical Committee (TC) 1.6, Terminology; however, there are some terms with regard to airflow measuring products, such as thermal dispersion and pitot arrays, that ASHRAE has not defined as of the publication of this article, so other sources may be needed for definition clarity.
When, where, and why is it necessary to provide an airflow measurement product? Professional engineers are challenged to find the answers to these fundamental questions on every HVAC system installation.
There may indeed be some systems where no airflow measurement products are needed to measure the airflow; however, as the codes and standards move forward, there are more reasons to include direct airflow measurement than ever before. There is no professional engineering reason to be the hero on a project by only looking at lowest first cost, taking on the risk of being non-defendable, and putting a client at risk at the same time.
In general, airflow measuring/monitoring/controlling is done in order to comply with one of a few codes and standards that relate to building or space pressure, ventilation, or energy. Also, more than ever, there is a focus on comfort that also gives a reason for airflow measuring/monitoring/controlling. The ongoing challenge for professional engineers is determining which takes precedence in a system design and product selection — energy, health, or comfort. Sometimes, energy is an easy answer to this debate, but is it really the most defendable answer?
Space and building pressure control is one purpose for airflow measuring. This has been done successfully with direct differential pressure sensors and volume tracking using a variety of airflow measuring products and even linear damper positioning. When flow volume tracking is used, care must be taken that the absolute error of all the components are taken into consideration so the supply and exhaust airflow rates do not overlap due to product accuracy errors. This can be a challenge if the installation is not commissioned and maintained over time. When fan tracking is used to control a building or space pressure, there are other factors to consider, such as duct leakage.
Some spaces require more sensitive airflow measurement and differential pressure control than others. Health care spaces, such as operating rooms, isolation rooms, pharmaceutical spaces, research labs, etc., all will require a greater degree of design considerations than other spaces when it comes to space pressure control.
Points to Ponder: Does energy really matter when occupant health is the compared benchmark? What risk is worth taking in “value engineering” when it comes to comparing airflow measuring/monitoring/controlling strategies?
Whether it be to prove and document ventilation requirements to meet ASHRAE 62, to ensure adequate space airflow is provided to meet ASHRAE 55, for measurement to ensure energy use is minimized and in accordance to ASHRAE 90.1, or to meet basic code requirements, the risk of not providing airflow measurement and documentation is growing for engineers and facility operators and owners.
Airflow measurement/monitoring/controlling methods should be not only discussed but also documented at the start of every project. Why? So the project team and owner understand the commitment to have a level of airflow measurement that meets the required needs of the project. This may sound logical, but how many times is a design started with a good foundation and design intent only to be changed at the end, during bidding, or after the bids are in to devalue a project by cost cutting and sacrificing a required need based on a first-cost number given by someone with one main objective … to get the job for his or her company. If accuracy of airflow measurement is a required need, then devalued first cost is not negotiable nor defendable at the time of bidding. At what cost will accuracy be sacrificed and at who’s risk?
Minimum ventilation rates are identified in the code. The IMC has fairly clear language regarding ventilation systems as part of VAV systems:
Ventilation systems shall be designed to supply the required rate of ventilation air continuously during the period the building is occupied.
The occupant load utilized for design of the ventilation system shall not be less than the number determined from the estimated maximum occupant load rate indicated.
VAV air distribution systems, other than those designed to supply 100 percent outdoor air, shall be provided with controls to regulate the flow of outdoor air. Such control systems shall be designed to maintain an outdoor flow rate as required over the entire range of the supply air.
This means that any outside air damper that is part of a VAV system must have a control scheme that maintains the minimum ventilation cfm throughout the supply fan’s operating range during occupied hours.
One of the concerns with airflow measurement products is that, over time, accuracy suffers as the flow measuring device gets dirty and fouled up with debris in the sensors. This can be a real concern in some locations and with some products; however, this may be an unnecessary concern in other locations and with some products. It’s an important concern to address with every specific application and manufacturer to ensure the maintenance is clearly understood. Maintenance is a fact of any HVAC installation, so to say an airflow measuring product requires maintenance and use that as a sole decision not to have direct airflow measurement is a risky rationalization to do otherwise.
With indirect methods, it can be a false assumption that using only airflow offsets set by a balancing damper will ensure defendable airflow with no maintenance whatsoever. A lesser first-cost option is not always the best option even when maintenance costs are taken into consideration. There is a risk in any decision that is based on first-cost or maintenance requirements alone. The building owner and operator must be aware and clearly understand the maintenance requirements and commit to maintaining any product if indeed the airflow measurement is important and essential for the proper operation of the HVAC system.
Cost is a variable that is important in any engineering decision. Unfortunately, sometimes, which “cost” is not always clarified and more often than not the first cost is a primary focus. There are multiple costs associated with an HVAC installation — first cost, operating cost, maintenance cost or life-cycle cost, the cost to defend the decision if the decision is wrong, etc. Cost compared to what? Cost compared to the risk of not providing the appropriate products and taking the risk of having an un-defendable engineering decision that puts first cost over the cost of providing proper and verifiable airflow to ensure the required ventilation air is provided for the occupants in a building? What is the cost of the risk of improper ventilation in a school where students may not be able to learn properly due to insufficient ventilation air that allows the indoor air quality to impact learning?
Points to Ponder: Is the rationalization of the “cost” evaluation truly defendable if something would go wrong and a lawsuit surfaces challenging the engineering decision? Whose decision should it be? Whose risk should it be?
Accuracy will indeed vary by product and installation. Understanding the definitions of accuracy is important. Always read the fine print in a manufacturer’s marketing and technical literature. It may be claimed that a product has a certain accuracy, yet in the fine print the accuracy claim may not be over the entire range of operation that the product is advertised to cover. Written documentation is essential.
Some products may claim an accuracy of as low as 2 percent or lower while others are 3 percent or higher depending upon various installation conditions. Keep in mind that many TAB specs allow up to 10 percent variance in documented airflow, and this is with airflow measuring instruments that have their own inaccuracies allowed in calibration.
Point to Ponder: What is the standard of care when it comes to the accuracy of airflow measurement/monitoring/control in today’s buildings?
The below definitions are from the ASHRAE Terminology resource and must be taken into consideration in understanding the accuracy of a product as documented in writing by a manufacturer. It’s always good to ensure the manufacturers’ literature is using the same definitions as the contract documents imply.
(1) Degree of freedom from error, that is, the degree of conformity to truth or to a rule. Accuracy is contrasted with precision (e.g., four place numbers are less precise than six place numbers). (2) the ability of an instrument to indicate or record the true value of a measured quantity. The error of indication, which is the difference between the indicated value and the true value of the measured quantity, expresses the accuracy of an instrument.
The accuracy of an instrument testing against a primary or calibrated instrument.
HVAC system end-to-end accuracy
The combined end-to-end accuracy of the EMCS (energy monitoring and control system) and the accuracy with which the EMCS sensors represent the HVAC process.
If the TAB process is done correctly, is it really necessary to directly measure airflow in every system? A good answer is, “It depends.” Maybe or maybe not may also be the answer. Is the accuracy of a TAB process sufficient for defendable operation to ensure the required airflow is provided? Is the TAB company liable for the measurements over time? Right or wrong, a TAB report is sometimes taken as exactly what is happening without a healthy dose of reality of the inaccuracies of a TAB process and measurement.
A well-trained and experienced TAB technician can indeed use a pitot static pressure tube traverse to set damper positions and verify the performance of an airflow measuring device; however, this is not an ongoing measuring/monitoring/controlling process over time. Is there always a good place to take measurements such that the measurements are indeed accurate?
Even if dampers are set to provide a certain amount of air at the time of a TAB setup, this is not necessarily defendable since even a damper position can change over time due to wear and tear on linkages. Also, pressures within a system can vary with variables, such as filter loading and other changes that may occur due to a damper being changed downstream for some reason to a zone or space.
Beyond the normal TAB process and documentation, some projects require commissioning. There are indeed varying degrees and types of commissioning. It’s important to understand to what level the commissioning process is such that the TAB report is indeed conducted, verified, and documented properly.
There are a variety of products on the market by many manufacturers that use varying technologies for air measuring. Engineers certainly have a lot of options to consider and a lot of specifiable performance characteristics to ensure the product provided meets the required needs of each specific project.
One source several years ago delineated nine different methods of controlling outside air. Although these methods would likely not all be defendable for every application, there may be some engineers who rationalize that a particular method has worked for years so why do anything different since they’ve never had any problems (that they know of).
Without getting into details of the pros, cons, and risks associated with the methods, it’s good to acknowledge them since some of them fall under the sometimes false sense of security provided courtesy of the Keep It Simple (KIS) principle. The methods may not necessarily fall under the Keep It Defendable (KID) principle (yes, you heard it here first). Some of these involve complete airflow measurement/monitoring/controlling, while others do not do all three.
- Fixed damper percentage;
- OA and RA temperature comparison and mixed air temp calculation;
- Constant pressure mixed air plenum;
- Fan controlled outside air injection;
- RA and total SA difference method;
- Exhaust air measurement;
- Minimum OA measurement;
- Flow measurement outside air damper; and
- Carbon dioxide measurement/monitoring/control.
When it comes to direct airflow measurement/monitoring/control, there are basically two types of products on the market for airflow measuring devices today: The thermal dispersion type (Figure 1) and the velocity pressure array type (also known as pitot array or self-averaging array). At least one type of thermal dispersion sensor is designed to fit on a fan scroll inlet, which can be great for retrofit applications and locations where there is insufficient straight duct length for other types of sensors. (Figure 2). The pros and cons of each type must be taken into consideration in each application and clearly, concisely, completely, and correctly specified.
At any given time, one manufacturer may have the “best” product whereas another day another manufacturer may have a new-and-improved product that takes care of some of the “cons” a competitor or their own previous model may have had. It’s important to take a fresh look on each project as to what products, features, and specifiable performance characteristics are available to get the best value based on the required needs of the application. (Note: This is a plug for considering attendance to the AHR Expo in Atlanta during the ASHRAE Winter Meeting in January of 2019, where many airflow measuring product manufacturers will have products on display and factory engineers to explain their performance features and benefits.)
A simple airflow measuring/monitoring/controlling device that has been used for years is an air terminal unit. Whether it be a VAV or CV box, there is a device to allow a pressure reading that can be then used to measure/monitor/control the airflow indirectly. Although the pressure sensors in a terminal unit are not directly measuring the airflow, the signal can be and is taken as an input into a pneumatic-to-electric transducer and allows the information to be used in a BAS to measure and monitor and control the airflow. In other more critical applications, an “air valve” is used, which provides theoretically more accurate measuring; however, many times, these are not needed if proper offsets are used with a standard air terminal unit for space pressure control if the speed of response of an air valve is not needed to maintain the pressure relationship in the space. This is where good engineering discretion is needed so as not to provide a more expensive system than is really needed and adding more complexity to the system than is necessary.
Although not an airflow measuring device, a somewhat new device on the market is an inline automatic balancing damper that is pressure independent within a certain range of pressure that can be used similar to a self-regulating waterflow device (Figure 3). These ensure a certain level of airflow and minimize or eliminate the cost of TAB, which can be difficult in low-flow applications. These are available from multiple manufacturers and arguably provide low airflow with better accuracy than can be achieved by a TAB process. These will function reliably in VAV-type supply, return, and/or exhaust applications, such as multitenant housing, dormitories, and even smaller applications that are in need of this type of constant flow control of air.
Another type of pneumatic sensing device that has been used by many engineers is a round damper that is sometimes called an iris damper, since it opens and closes somewhat like the iris in a human eye or lens of a camera. These are not used as often as they probably could be, but they are good products worth consideration.
Some airflow measuring stations and products are AMCA-tested, but some are not. When selecting products and writing specifications it’s good to know how the advertised numbers are achieved. Airflow measurement station accuracy and pressure drop are part of AMCA’s test methods. It would be good to become familiar with AMCA test standards for airflow measuring products before deciding what method and products are good enough for any specific application. AMCA does have a listing of manufacturers and products on its website for certified products.
Not a lot needs to be discussed here. The fundamental essential to any product functioning correctly is the installation. Most HVAC products have an installation, operation, and maintenance (IOM) manual. The starting point is the installation. Only then can the operation be achieved as the manufacturer guarantees and/or warrants. This is generally a requirement of every contract document and in each product specification section. It’s absolutely necessary to install a product in accordance with the manufacturer’s recommendations and per the IOM, such that they can be assessed and maintained.
Whether it be for measuring, monitoring, or controlling airflow, today’s engineers are challenged more than ever to look at the requirements of each and every cfm of air being circulated into, throughout, and out of a building. This can be for energy, pressure control, ventilation, or comfort requirements of the HVAC system. There is no quick and easy answer that applies to every building. There is not one single right way that worked years ago or even a year ago that may still be the best way for a current project. Codes change. Standards change. Products change. The only constant in airflow measuring, monitoring, and controlling is — you guessed it — change itself. Remember one thing from this article if nothing else: KID.