HVAC components are the backbone of any building. Comfortable, consistent temperatures, good air quality and energy efficiency are just a couple of rewards you get when your ductwork design is done right.
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Most business and property owners probably don’t realize that HVAC systems and ductwork installation need to be planned even before the building plans are completed by architects. HVACs are complicated systems. Even a small glitch somewhere means that the consequences will be time-consuming, messy and expensive. The wise plan ahead. And part of this advanced planning is to create a schematic through the right duct shop drawings which allow contractors in figuring out the right place to install the duct systems and related equipment. Rightly installed ductwork provides comfort and ease to the space occupants.
But, what happens with poor ductwork design?
The US Department of Energy reports that the average HVAC duct systems inside buildings and hotels are only 60 percent efficient. This means that 40 properties out of a 100 either do not have the air flowing inside them correctly or did not have the right duct design detailing done before their construction.
Poorly constructed duct designs lead to:
- Drafts, stuffy air, hot and cold spots due to impeded air flow.
- Additional load on air conditioners since they need to work harder and run longer for compensating ductwork design flaws. This leads to shorter appliance life and more malfunctions.
- Below par air quality with more dust, fumes, pollutants and humidity which can lead to mold and fungal growth.
- Unbalanced air pressure which can cause doors slamming shut by themselves, odors to linger and heightened noise levels inside the house.
Accurate HVAC duct shop drawings ensure efficient air flow inside the building to give optimum comfort to their occupants. Some of the most common mistakes in designing ductwork that the BluEntCAD team has seen over the years include air leaks, under-sizing, protracted runs and sharply bent ends. All of these errors can be rectified with careful HVAC design drafting.
What are the common duct design mistakes?
- Non-compatible components
Without the right ductwork shop drawings, contractors can record wrong requirements for the air-conditioning equipment and the construction material. As a result, ducts may be under-sized. Load calculations should be done individually to cater to the varying cooling and heating needs of every room.
- Insufficient return vents
A duct system needs both outlets and return vents for air to be sent back inside the HVAC for balanced air movement and pressure. Lack of returns causes discomfort and extra load on existing AC vents. Ideally, every room’s supply vent should be accompanied with a dedicated return duct for the best possible balanced airflow. In spaces where there is a single central return, the air pathway should be clear and obstruction-free.
- Air leakage
Incorrectly sealed ducts make the cooled air to leak in walls. As much as 20 percent conditioned air can leak if the duct joins are not sealed correctly. The ducts should be sealed at installation and fastened mechanically with mastic and not duct tapes. The joints can be further secured with sheet metal screws.
- Runs which are too long
If the HVAC system is not optimized during the planning phase, the entire network ends up being far away from spaces where the cooling has to be done. To facilitate cooling this way, longer ductwork runs are required which decrease the overall cooling efficiency of the system.
- Sharply bent turns
Just as longer runs impede air flow, bends that are too sharp along the edges decrease the air that reaches its final destination.
So, what to do?
How to get your ductwork installation right?
Before you even begin designing a duct system, do your homework.
- Calculate the heating and cooling load (BTU/hr) that each room requires. These units can then be translated to room-by-room air flow in cubic feet per minute.
- After this, you must select the right product. Make sure that the appliance you pick adjusts to the outdoor and indoor design conditions of your space. Take help of the manufacturer’s data to get this right.
Done? You’re now ready to build a duct design system.
HVAC systems duct design – How to do it right?
- Consider the weight of the air
Basics first – air has weight.
For example, if you install a 2.5 ton AC, the nominal air flow would be around 1,000 cfm. This means that nearly 81 pounds of air has to be pushed by the blower every minute and to move this weight around, it takes work.
- The physics of air flow and using it to our advantage
We know that a weight can be moved freely in the horizontal position without any resistance. However, to move it upward against gravity or friction, now that would require work. Let’s understand this with a real-life example.
A fan gives you the maximum air flow if there are no obstacles in between. However, if you introduce a cardboard tube and blow through it, the air has to flow against the pressure that builds inside. The smaller the size, the higher the pressure, and lower the air flow.
This is the basic principle when designing a ductwork. Friction and turbulence are two factors which obstruct air flow inside a duct. As the air passes through a duct, it interacts with surfaces and fittings. Smoother surfaces lead to better air flow and vice versa. Every fitting and duct-section adds resistance to the air, along with grilles, filters, dampers and registers.
So, the air which starts at a higher pressure near the blower almost drops to zero near the supply vent. To ensure that the right amount of air is delivered to every room, the duct system needs to take pressure drops into account and create a balance from the blower to the supply vents. Know more about our Shop Drawings
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Lesson learnt here?
The ductwork design should be smooth and obstruction-free to deliver the optimum quantity of air in every room.
Good duct design tips
- Keep it simple, stupid!
Start with a simple sketch. Add obstructions along the framework. Add notations everywhere to explain different components such as outside air and duct-routing.
- Document, document, document
If you document the entire HVAC system, you will not miss any area. For example, if you’re planning to zone the system, note down the thermostat locations and connect these with branch lines to the main HVAC duct drawing. This will help you shortlist damper locations.
- Follow industry standards
To keep your equipment and ductwork the right size, follow the accepted ACCA (Air Conditioning Contractors of America) industry standards for heating, ventilation, air-conditioning and building performance.
- Location, location, location
Ideally, HVAC equipment should be located centrally to ensure the shortest duct runs. The ducts should be installed within the internal walls for losing as little conditioned air as possible. To ensure maximum efficiency, do not install ducts in unconditioned crawl spaces and attics. Keep elbows and bends limited to restrict the static pressure and air friction.
Request an architectural walkthrough of your building’s HVAC components from your duct shop drawing company. You will be able to see how your HVAC system fits with your property layout.
- What kind of equipment and supporting systems do you need?
Heat pumps require larger ducts. So do air purifiers with activated charcoal filters. Your shop drawings should be properly annotated to include these details and make sure your contractor is aware of these specifications.
Of course, it goes without saying that you should use the right material, supports and fittings. The ductwork materials vary with your budget and requirement. Sure plastic flex ducts are cheaper, but they are not as durable and sturdy as sheet metal. Look for long term returns rather than cheaper alternatives. This will allow your design professional to create the most appropriate and industry approved duct sizes and framework for your property.
As a novice or beginner, designing ductwork can be a hair-pulling experience. To get the best bang for your buck, get your designs done by professionals like BluEntCAD. We provide consultation for renovations, makeovers and new property construction from our team of expert designers and drafters. We also provide architectural walkthrough of your designs and 3D renders of your plans. BluEntCAD’s experts can provide you with the skills and prowess to produce the results you want. Request a demo today!