What is Ductwork?

Commercial ductwork functions as the arteries and veins of the HVAC system. It consists of a network of ducts through which air flows around a building. Supply ducts facilitate the movement of heated and cooled air from the HVAC system to the various indoor areas, and return ducts facilitate the return of expended air back to the HVAC equipment where it can be filtered, reconditioned, and redistributed. The ducting is usually either round or rectangular and is most often mounted near the ceiling.

Air Flow

The HVAC system depends on the effective flow of air from one area to another as a result of differences in air pressure.

The amount of air that flows through a duct is regulated by pressure differences and by system resistance. The higher the difference in pressure, the greater the air velocity, and therefore the greater the amount of air that flows from the duct.

While ducts are usually made of lightweight aluminium, they can also be manufactured from steel, flexible plastic, polyurethane, fibreglass or fabric.

Ductwork Designs

The type of ductwork design employed in a building is usually influenced by the size and layout of the building. The general types of supply duct systems are; radial system, extended plenum system, reducing plenum system, reducing trunk system, and perimeter loop system.

The radial duct system includes a central supply plenum which feeds a number of individual branch ducts arranged in a radial pattern. This system can be designed so that each run leaving the plenum can feed two or more supply outlets. The radial ductwork system can also be used with different air handlers and furnaces.

The extended plenum duct system consists of boxlike sections of ductwork extending from a main plenum. The extended plenum maintains the same size from the starting collar to the end of the run. The maximum length of the extended plenum should not be greater than 7.5m from the air handler or furnace. However, if two plenums are combined, the total length can be extended to 15m. The main issue with the extended plenum system is caused by the higher velocities in the plenum, resulting in the branches closest to the indoor blower not being fed the desired amount of air.

The reducing plenum duct system can be used if the size or layout of the building requires greater distances than the restricted lengths of the extended plenum. The reducing plenum system works by reducing the size of the plenum at the halfway point to restore the velocity in the second part of the plenum. The effect of this size reduction is also to improve the air flow characteristics at the branch ducts closest to the air-handling unit.

The reducing trunk duct system is a system similar to the reducing plenum system, only in this case the plenum run reduces in size after each branch of the take-off. Through these repeated reductions in size, it is possible to maintain a constant air velocity in the plenum, despite the fact that the total air volume is being reduced as each branch is supplied.

The perimeter loop duct system is circulated around the perimeter of the building with the same size ducting. This loop is fed by ducts that radiate from the central plenum and are usually the same size as the loop duct. Boot boxes are sized in order to deliver the appropriate air pressure to each area of the building.

Potential Design Problems

If the ductwork is too small in size, it can cause the system to run too hot and the excess heat created can cause the entire system to wear out. The exchangers and the sensors will overheat, triggering safety shutoffs. When the system shuts off as a result of overheating, it may not cool down straight away. Although the system has cooled, the safety shutoffs will return to baseline levels after the system has been shut off. This reset function will therefore allow the overheating to begin again.

Another problem is created if ducts are either too long or if there are too many bends, resulting in air having to travel too great a distance, thus reducing efficiency. An experienced technician will be able to identify ways to improve the design of ductwork, resulting in a more efficient performance. Sharp turns, for example, can be modified using elbows, which will ease the sharpness of the turn.

Sufficient supply vents are needed to provide for airflow together with enough return vents to draw the air back into the system for conditioning. If there is an insufficient number of either of these vents, the result will be an unbalanced pressure in ducts, which will reduce comfort and efficiency.

Maintenance Issues

Air exfiltration can cause your system to run less efficiently and occurs if the pressure inside the air-duct system causes air to leak out through cracks, holes, and other openings. This leakage will reduce air circulation throughout a building and cause the system to work harder and run longer; the implications being both physically and financially inefficient.

Another sign that there may be problems in the ducting is a lack of air blowing from the vents, which will result in noticeably stuffy air from insufficient air circulating. If, having cleaned the system, the problem persists, a commercial HVAC technician can inspect ducts for cracks, holes and missing pieces, and seal any found.

Changes in sound can also be an indication of a problem. Air escaping through leakage points can produce a whistling sound, while rectangular sheet metal ducts can generate a popping sound when an air pressure problem develops.

Leaks and damage to the system can result in harmful particles accumulating inside your building as dirt from outside enter circulation. If the HVAC unit is smelly, the ductwork might be the source of these odours. Sometimes moisture that has built up becomes a breeding ground for mould, mildew and other microorganisms. Rodent and insect vermin deposits can also find their way into the ductwork system, creating bad odours.

If filters often clog up, it may well be a sign of duct problems. Ducts that are full of debris and other particles can clog filters very quickly. The problem can often be resolved by calling in an HVAC service company to clean and unclog the ducts and service the HVAC system.

There can be a problem with flexible ducts made from plastic getting crushed, twisted or torn, and these should be checked for kinks and tears. Supports can help to prevent twisting and alignment problems.

Learn More with Therma-Mech

Ductwork is an essential part of the HVAC system and can extend for many metres around a building. Failures in ducting caused by leakage, dirt and debris, or simply bad design can create an uncomfortable environment for workers, affecting their productivity and result in unnecessarily high running costs.

If you are looking for help on HVAC ducting installation, adaption or servicing, contact us today at Therma-Mech for expert advice.