AIRHEAD: You can confidently state that the duct design method is effective if the measured air flow is ±10% of the calculated air flow.
Air ducts are one of the most important components of the ventilation and air conditioning system. High Performance HVAC Systems shows that 10 factors work together to determine duct performance. If one of these factors is neglected, the entire HVAC system may not provide the comfort and efficiency you expect for your customers. Let’s take a look at how these factors determine the performance of your duct system and how to make sure they’re correct.
Internal fans (blowers) are where the characteristics of air ducts begin. It determines the amount of air that can eventually circulate through the duct. If the duct size is too small or incorrectly installed, the fan will not be able to provide the required air flow to the system.
To make sure the fans are strong enough to move the required system airflow, you need to refer to the device’s fan chart. This information can usually be found in the manufacturer’s installation instructions or technical data. Refer to it to make sure the fan can overcome airflow resistance or pressure drop across coils, filters and ducts. You will be amazed at what you can learn from device information.
The internal coil and air filter are the two main components of the system through which the fan must pass air. Their resistance to air flow directly affects the performance of the duct. If they are too restrictive, they can drastically reduce the airflow before it leaves the ventilation unit.
You can reduce the chance of clipping coils and filters by doing a bit of work beforehand. Refer to the coil manufacturer’s information and select an indoor coil that will provide the required airflow with the lowest pressure drop when wet. Choose an air filter that meets your customers’ health and cleanliness needs while maintaining a low pressure drop and flow rate.
To help you correctly size your filter, I would like to suggest the National Comfort Institute (NCI) “Filter Sizing Program”. If you would like a PDF copy please send me an email request.
Proper piping design is the basis for piping installation. This is what the installed duct will look like if all the pieces fit together as expected. If the design is wrong from the start, the performance of the ductwork (and the entire HVAC system) can suffer due to improper airflow delivery.
Many professionals in our industry assume that proper duct design automatically equates to the performance of the duct system, but this is not the case. To ensure that your duct design approach is effective, no matter what it is, you must measure the actual airflow of your build system. If the measured airflow is ±10% of the calculated airflow, you can confidently state that your duct calculation method works.
Another consideration concerns the design of pipe fittings. Excessive turbulence due to poorly designed duct fittings reduces effective airflow and increases the resistance the fan must overcome.
Air duct fittings must provide a gradual and smooth removal of the air flow. Avoid sharp and limiting turns in pipe installations to improve their performance. A brief overview of ACCA Handbook D will help you decide which fitting configuration will work best. Fittings with the shortest equivalent length provide the most efficient air supply.
A dense duct system will keep the air circulating by the fan inside the ducts. Leaky piping can degrade system performance and cause a variety of problems, including IAQ and CO safety issues, and reduced system performance.
For simplicity, any mechanical connections in the piping system must be sealed. Putty works well when there is no need to tamper with a connection, such as a pipe or plumbing connection. If there is a component behind the mechanical joint that may require repair in the future, such as an internal coil, use an easily removable sealant. Do not glue work on panels of ventilation equipment.
Once the air is in the duct, you need a way to control it. Volumetric dampers allow you to control the airflow path and are critical to good system performance. Systems without bulk dampers allow air to follow the path of least resistance.
Unfortunately, many designers consider these accessories unnecessary and exclude them from many plumbing installations. The correct way to do this is to insert them into the supply and return duct branches so you can balance the flow of air in and out of the room or area.
So far, we have only focused on the air aspect. Temperature is another piping system performance factor that should not be ignored. Air ducts without insulation cannot provide the required amount of heat or cooling in air-conditioned rooms.
Duct insulation maintains the temperature of the air inside the duct in such a way that the temperature at the outlet of the unit is close to that which the consumer will feel at the checkout.
Insulation installed incorrectly or with a low R value will not prevent heat loss in the pipe. If the temperature difference between the unit outlet temperature and the furthest supply air temperature exceeds 3°F, additional piping insulation may be required.
Feed registers and return grills are an often overlooked part of the operation of a plumbing system. Usually designers use the cheapest registers and grilles. Many people think that their only purpose is to close rough openings in the supply and return lines, but they do much more.
The supply register controls the supply and mixing of conditioned air into the room. The return air grilles do not affect the air flow, but are important in terms of noise. Make sure they don’t hum or sing when the fans are running. Refer to the grate manufacturer’s information and select the register that best suits the airflow and room you wish to regulate.
The biggest variable in determining the performance of a piping system is how the piping is installed. Even an ideal system can fail if installed incorrectly.
Attention to detail and a little planning go a long way to getting the right installation technique. People will be amazed when they see how much air flow can be obtained from flexible ducting by simply removing the excess core and kinks and adding a hanger. The reflex reaction is that the product is to blame, not the installer being used. This brings us to the tenth factor.
To ensure successful design and installation of a piping system, it must be verified. This is done by comparing design data with data measured after the system has been installed. Individual room airflow measurements in conditioned rooms and temperature changes in ducts are the two main measurements that need to be collected. Use them to determine the amount of BTUs delivered to a building and to verify that design conditions are met.
This can come back to you if you rely on your design approach, assuming the system behaves as expected. Heat loss/gain, equipment selection and piping design calculations are never intended to guarantee performance – not out of context. Instead, use them as targets for field measurements of installed systems.
Without maintenance, the performance of your piping system will degrade over time. Consider how damage to air ducts from sofas or guy wires leaning against side walls disrupts airflow—how do you notice it?
Start measuring and recording your static pressure for every call. After verifying that the plumbing system is functioning properly, this iterative step allows you to monitor any changes. This allows you to stay connected to the ductwork and gives you a better understanding of issues that are degrading the performance of your ducting system.
This high-level view of how these 10 factors work together to determine the performance of a duct system is meant to make you think.
Ask yourself honestly: which of these factors are you paying attention to, and which should you pay attention to?
Work on these plumbing factors one at a time and you will gradually become a short seller. Incorporate them into your setup and you’ll get results that no one else can match.
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David Richardson is a Curriculum Developer and HVAC Industry Instructor at the National Comfort Institute, Inc. (NCI). NCI specializes in training to improve, measure and verify the performance of HVAC and buildings.
If you are an HVAC contractor or technician and would like to learn more about high precision pressure measurement, please contact Richardson at davidr@ncihvac.com. The NCI website, www.nationalcomfortinstitute.com, offers many free technical articles and downloads to help you grow professionally and strengthen your company.
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Post time: Apr-20-2023