19 Feb 2024

7 ductwork design mistakes to avoid
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Creating an efficient and effective ducting system in a building requires careful ductwork design. In case you don’t know already – ductwork carries conditioned air from a central system to the various spaces in the building to create optimal working conditions for employees, as well as keep materials and processes at the right temperatures.

However, poor ducting and design can lead to a whole host of problems for building owners and the employees working within them. Even small ductwork design flaws can decrease comfort, increase energy use, generate excessive noise, and reduce system lifespan. As you can imagine, this can be costly, too!

Unfortunately, common mistakes in layout, sizing, ductwork material selection and installation can undermine the performance of even a high-quality ducting system. We’ll explore seven of the most prevalent ductwork design mistakes made during the design and installation process. Understanding where designers and contractors can make ductwork design mistakes can help engineers, steel fabrication specialists and construction managers oversee quality control and avoid pitfalls on current and future projects. 

1. Poor load calculations

One of the most common ductwork design mistakes is improper calculation of the required duty volume of a system. Essentially, load calculations determine the amount of heating and cooling a system needs to provide –  based on factors like the size of the working space, current insulation (if any), number of windows, outdoor temperatures, and various other factors. By not performing accurate load calculations, you can get an improperly sized ductwork solution that’s just not fit for purpose.

The result? Oversized ductwork that results in energy and money wasted. When ducts are larger than necessary, air speeds slow down, which allows air to cool excessively before entering each room. Plus, the central HVAC system has to work even harder to reheat the air to the proper temperature. Plus, oversized ducts also cost more in materials and installation costs too.

On the flip side, undersized ductwork can lead to insufficient airflow that cannot adequately heat or cool a building or space. Restricting airflow causes excess static pressure, reduces equipment capacity, and strains HVAC components which can also cause them to overwork. Problems like hot and cold spots, short cycling, and employee discomfort occur. In the worst case – health issues can even arise due to the inevitable poor indoor air quality and ventilation from undersized ducting

The fix? Doing comprehensive, precision load calculations based on key details of the building is crucial before designing and sizing ductwork. Various load calculation methods exist, like RTS, CLTD/CLF, and transfer functions. Plus, properly sized ducting requires determining the exact cubic feet per minute (CFM) for each room – based on its size, usage, and heat load. Engineers must select the proper technique and meticulously consider all inputs to avoid the consequences of ducts that end up far too big or small.

2. Poor ductwork layout

Size isn’t just an issue. Proper ductwork layout is also crucial for duct efficiency and performance. Unfortunately, many duct designs contain layout issues that negatively impact the system. Some common ductwork layout mistakes include:

Unnecessary twists and turns

Too many twists and winds in a ductwork system can lead to turbulence that inhibits proper airflow. This added resistance forces the fan to work harder, increasing energy usage. Ductwork layouts should minimise directional changes where feasible.

Too many elbows

“Elbows”, or bends in ductwork significantly increase static pressure, as air must change direction. Each elbow adds resistance and requires the fan to expend more energy to push the air through the ducts. Designs should ideally limit elbows as much as possible.

Failure to minimise length and bends

Long duct runs and extra bends equate to higher static pressure and fan energy consumption. Ductwork layouts should be engineered to have the shortest feasible runs and the fewest number of directional changes. Long, winding runs with numerous elbows often indicate a poor design.

3. Ignoring friction loss

Friction loss is one of the most common yet overlooked factors in ductwork design. Unfortunately, it’s a rule of physics that can’t be completely nullified, yet there are factors which can reduce this. 

So how does friction loss occur as a common ductwork design mistake? Essentially, friction loss happens when air moving through the ductwork encounters resistance from the duct walls, fittings, grilles, and other components. The result is increased turbulence and pressure loss as the air travels through the system. Ignoring friction loss can lead to several issues:

  • Too much friction means the fans and blowers must work harder to push the air through the ducts. This wastes energy and drives up costs, as well as produces awful noise levels. It can also lead to uneven airflow in different parts of the system. Duct silencers can help, but shouldn’t be used to counteract poor design.
  • Size is a factor in friction loss. Duct sizes need to be increased to account for the reduction in airflow capacity from friction. Undersized ducts that ignore this can restrict airflow.
  • The longer the run, the more friction loss builds up. Generally speaking – runs over 15-20 feet need special consideration during design. Ignoring friction loss on long runs almost guarantees airflow problems.

The only way to prevent issues from excessive friction is to accurately calculate pressure losses during the design stage. This typically requires specialist ductwork sizing software and advanced calculations engineers acquire through specialist training.  

4. Poor material selection

Choosing the wrong ductwork materials or using poor-quality materials that don’t meet specifications is another common ductwork design mistake. This can lead to a variety of problems that negatively impact HVAC performance.

Some of the key issues that can occur from poor material selection include:

Using poor quality materials that are prone to leaks

Some types of ductwork (commonly flexible ducts) can be made from inferior metals or have poorly welded seals. This allows conditioned air to leak out, resulting in energy waste and difficulty maintaining stable temperatures. 

Choosing the wrong materials

Ductwork systems must withstand pressure, moisture and corrosion (among other things). For example, using the wrong metal gauge or material for the application can lead to premature failure, which can be costly.

By using stainless steel ducting materials, you can mitigate these issues and reap the benefits of long-lasting durability, hygiene, aesthetics and more.

5. Poor installation

Many signs point to replacing​​ ductwork once you know them, but one of the most common ductwork design mistakes is improper installation of the ductwork system. Even with a properly designed system, mistakes during the ductwork installation process can negate the system’s performance and efficiency. There are a few key installation issues to avoid:

Leaky connections and joints

If duct connections and joints are not properly sealed or welded, it can lead to substantial air leakage. Leaky joints allow conditioned air to escape the system before reaching the intended destination. This leads to comfort and efficiency issues over time.

Crushed or damaged ducts

Ducts can easily become dented, crushed, or damaged during installation if proper care is not taken when handling the ductwork. Any dents or deformations in the ductwork will restrict airflow and create turbulence that reduces system performance. 

6. No consideration for maintenance

One common ductwork design mistake is not allowing adequate space around ducts for maintenance access. Maintenance is required to ensure your ductwork has the improved airflow that may be required and functions as it should, but when ducts are packed tightly into small rooms or ceiling spaces, it can be extremely difficult for technicians to access them when repairs or maintenance are needed. 

Plus, technicians cannot physically fit into tight spaces to inspect or service the ductwork, meaning maintenance would take much longer to carry out, and may even involve demolishing the current ductwork system to get to where they need to be. This can cause serious downtime for business operations. 

7. Failing to coordinate with other building systems

A building contains many other systems like plumbing, fire sprinklers, electrical conduits, cable trays, and much more. Failing to properly coordinate ductwork design with these other systems often causes major issues. When a clash is found, the ducts typically need to be rerouted, since they are more flexible than other components.

However, last-minute duct rerouting leads to indirect duct paths, low duct clearances, tight bend radiuses, and other issues. All of these negatively impact airflow and pressure losses. Just as with structural elements, a lack of coordination also causes extra seams from duct splitting. This is why ductwork must be designed concurrently with other building systems to minimise unnecessary rerouting later on.

If you think your ductwork has been fitted properly or has suffered from some serious ductwork design mistakes from this list, then we can help. Contact Airmatic to find out more about our steel fabrication using DW114 sheet metal specifications. Be sure to bookmark our blog for all things ductwork too.

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