Ductwork Design Challenges and Recommendations

In the AEC industry, there is a constant requirement for MEP engineering design to provide better comfort while achieving higher energy efficiency and cost effectiveness from HVAC system design. As manufacturers, in their effort to improve efficiency, are reducing equipment size while increasing air flow at the same time, this poses a major challenge for HVAC design engineers when creating duct system designs. In an ideal air distribution system, ductwork must be designed to provide the right amount air flow at the right speed to the right conditioned space while improving energy efficiency, maintaining air quality and providing comfort.

While designing ducts for air distribution systems, heat load calculations must be completed accurately to determine the required air flow rates. Based on the ease of construction and the space available, the duct layout should then be designed. In any given air distribution system, only one set of combinations is ideal for distributing the required amount of air flow. Design parameters must be identified to determine the best combination and optimal design. Poor ducting system designs can negatively impact indoor air quality (IAQ), comfort levels, waste energy resources and could result in higher operating costs.

This article discusses the challenges of ductwork design which includes incomplete information that the owners, architects or MEP consultants should provide, incorrect calculations of heat load and duct size, design challenges related to achieving energy efficiency and comfort levels and external considerations such as budgetary constraints and adherence to standards and guidelines.

Challenges of Ductwork Design

1. Incomplete information from owners, architects and MEP consultants – HVAC designers must often address the issue of non-availability of complete information which impedes the design process. Before starting the ductwork design process all information must be provided such as the type of application, the number of people using the conditioned space, the activity or operations that will be conducted, building orientation and technical specifications such as the method of duct design and type of equipment and material.

• In addition to this, information on the method of duct design, maximum air quality of supply and return air ducts, material and equipment specifications and budgetary constraints must be specified. A detailed account of inputs required is described in a previous article titled, ‘Decoding the Ductwork Design Process, Methods and Standards’.

2. Design challenges related to achieving energy efficiency and comfort – One of the main challenges for the ducting system design is to achieve energy efficiency standards, comfort and indoor air quality levels and other design elements which affect the performance of air distribution systems such as leakage, air tightness, duct shape and size, insulation, noise control and acoustics.

• Duct shape and size – Improper duct lengths will negatively influence air flow and the shape and layout of ducts could reduce air flow leading to air tightness. The shape and size of ducts need to be determined based on the heating and cooling load requirements of the conditioned space.
• Leakage/air tightness – Faulty duct design invariably results in air leakage from ducts, accessories and equipment such as air handling units (AHUs) and variable air volume (VAV) boxes. The cost incurred due to air leaks has a high impact on overall energy costs as it pressurises system performance. Relevant leakage tests must be conducted and allowance for accessories should be provided to prevent leakage.
• Insulation – As the air flows through ducts, improper insulation leads to heat loss/gain, affecting the performance of the air conditioning system adversely. Using internal liners, consulting the manufacturer’s specifications and using installed valves will help in improving performance.
• Noise, vibration control and acoustics – Some designers use high air velocity in ducts to reduce initial costs. Using high air velocity not only results in higher operating costs it results in acoustic problems as well. Air flow must be kept at a low or medium velocity based on the activity, the number of people and application, to minimise vibration and noise levels. Insulating ducts and pipes could also help in reducing noise transmission and maintaining acoustics.

3. External considerations – There are also external considerations in the duct design process such as budgetary constraints and adherence to standards and guidelines based on which calculations should be done and duct sizes and layout must be designed.

• Budgetary constraints - When budgets for projects are tight, there is an increasing pressure to obtain highly cost effective designs. The challenge in duct design is to design a layout, use a method and select the material and equipment that will be energy efficient and reduce overall energy and operating costs. Making the right calculations, selecting the right equipment and developing designs that are cost effective, energy efficient and per industry standards is one way of keeping controlling costs.
• Adherence to standards and guidelines - Incorrect calculations of heat load and duct sizes lead to poor ductwork designs resulting in lower comfort, higher operating costs for maintaining equipment and an increase in energy costs. HVAC designers must refer to HVAC industry standards and handbooks when making calculations, selecting equipment and choosing duct design and size.
• Given the challenges to be addressed, there are common guidelines applicable to the ducting and HVAC design, from using standards and manuals for duct design and load calculations to general rules of ductwork design.

Recommendations for Ductwork Design

HVAC design and sizing vary depending on energy consumption, the number of occupants in the building, material and orientation of the building and preferences of temperature levels. While many parameters must be considered when selecting duct designs, sizes and layout and method, there are common guidelines that duct designers could consider when designing to achieve energy efficiency, cost effectiveness and comfort.

1. Use of Industry Standards and Manuals

• HVAC design engineers must use industry standards and manuals when designing and duct and HVAC systems. While different countries have various HVAC guidelines, standards and manuals for ducting system designs, the handbooks widely used are those published by the Air Conditioning Contractors of America (ACCA) and American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE).
• Heating and Cooling Load Calculation - Manual J – Different conditioned spaces have varying heating and cooling requirements. This manual is used to measure heating and cooling load calculation and provide accurate air flow rates so that every conditioned space gets the exact amount of air flow required.
• Equipment Selection – Manual S – Choosing the right type and size of equipment is essential in ensuring that heating and cooling is provided efficiently. Oversized equipment could increase installation, operating and maintenance costs, while undersized equipment may not be able to service the heating and cooling requirements of the conditioned space. This manual helps to select sizes of different type of heating and cooling equipment and explains how manufacturer’s performance data can be used.
• Ductwork Design and Size – Manual D – To ensure air circulation is balanced throughout the conditioned space, the ductwork design must have a simple layout, the supply and return ducts must be sized accurately and the system should be insulated and installed correctly. This manual is used as the industry standard for residential duct design and provides principles of duct design and fundamentals of duct sizing.
• Air Distribution Basics – Manual T – In many instances, incorrect selection of equipment and improper duct sizing results in stagnation and air drafts. To prevent such faults due to miscalculations, this manual provides details on how to select and size supply air diffusers, return grilles and registers. It also explains how to calculate pressure loss, control noise and use manufacturer’s performance data.