Ducts are conduits or passages used in heating, ventilation, and air conditioning (HVAC) to deliver and remove air. The needed airflows include, for example, supply air, return air, and exhaust air. Ducts commonly also deliver Ventilation air as part of the supply air. As such, air ducts are one method of ensuring acceptable indoor air quality as well as thermal comfort.

TYPES OF DUCT SYSTEMS

There are three types of duct systems. Namely, flexible ductwork, rigid ductwork, and semi-rigid ductwork. We’ve compiled information on each of these duct systems. 

1. FLEXIBLE DUCTWORK

These ducts are typically tube-shaped and made of a wire coil covered with a bendable, durable plastic, and surrounded by insulation. Flexible ducting is best in complicated and tricky spaces where it is impossible to install or use rigid ducts to attach non-flexible ductwork to an air supply outlet.

Like most of the central air-conditioning parts, flexible duct systems have specific installation requirements. For instance, flexible ductwork needs to be secured and supported correctly to eliminate little sagging or snaking. Bends turns and kinks also need to be minimized because these reduce airflow and hamper the air conditioning system’s efficiency and effectiveness. The advantages of using flexible ducts include quick and easy installation, and often cost less than the rigid ductwork.

2. RIGID DUCTWORK

Rigid duct systems come in a variety of materials and sizes and can be either rectangular or cylindrical. Often, these ducts are insulated. They are popularly in use because they are hardy, enduring, and reliable. 

For rigid duct systems, there are three common types:

Sheet metal ducts – If you like watching action movies, you most probably have seen sheet metal ducts. They are the ducts you see in films that usually have the main hero crawling through the air ducts. The most common materials for sheet metal ducts are galvanized-steel and aluminum. Aluminum is relatively light and relatively easy to install. These materials are also the least likely to harbor dangerous molds or growths because of their non-porous surfaces. 

Fiberglass lined ducts – Fiberglass lined ducts are also sheet metal ducts, but they differ in one thing: they have internal fiberglass lining. This duct system is more common in office and commercial buildings, as it helps dampen the air conditioner unit’s sound. Unfortunately, the fiberglass in these ducts is susceptible to deterioration and eventually releases fiberglass particles into the air, which can be a significant health concern, especially with long-term exposure. Moreover, fiberglass lined ducts are also challenging to clean for the same reason that the cleaning process can damage the lining and release fibers. Molds and bacteria can also contaminate these ducts.

Fiberboard ducts – Fiberboard ducts are composed of fiberglass strands compressed and bonded with resin and covered with a foil laminate sheet for moisture protection. Fiberboard ducts are suitable for cooling and heating systems because they are well insulated. However, we don’t recommend it when it comes to ventilation because they can become breeding grounds for mold and mildew in humid climates, just like fiberglass-lined ducts. They also have rough surfaces that can affect airflow and efficiency.

3. SEMI-RIGID DUCTS

Viewed as the best type of ventilation ducting available, semi-rigid ducting offers installers and gives some benefits. An excellent quality semi-rigid ducting helps a ventilation system operate at its optimal performance because it is a zero leakage ventilation system. On the other hand, high-quality semi-rigid duct systems also have high crushability levels. Additionally, some semi-rigid ducting systems offer the installer versatility by switching between round and oval ducting without losing any hydraulic pressure loss or system performance. It is also easier to maintain because many semi-rigid duct systems have anti-bacterial and anti-static linings.

The duct system is one of the most important components of an HVAC system. High-performing HVAC systems reveal there are 10 factors that work together to determine duct system performance. If one of these factors is ignored, the overall HVAC system may fail to deliver the comfort and efficiency you implied to your customer. Let’s look at how these factors determine duct system performance and how you can ensure they are correct.

1. Fan Capacity

The indoor fan (blower) is where duct system performance starts. It determines how much airflow can ultimately be circulated through the duct system. If the duct is undersized or poorly installed, the fan can’t deliver required system airflow.

To ensure the fan is strong enough to move required system airflow, you’ll need to refer to the equipment’s fan table. This information is typically found in the manufacturer’s installation instructions or engineering data. Reference it to verify the fan can overcome the resistance to airflow, or pressure drop over the coil, filter, and duct system. You’ll be surprised what you can learn from equipment engineering information.

2. Indoor Coil and Air Filter

The indoor coil and air filter are two primary system components a fan must move air through. Their resistance to airflow has a direct impact on duct system performance. If they are too restrictive, they can drastically reduce airflow before it leaves the air-handling equipment.

You can reduce the chances of restrictive coils and filters with a little upfront work. Refer to the manufacturer’s coil information and select an indoor coil that delivers required airflow at the lowest pressure drop when wet. Select air filters that accommodate the health and cleanliness needs of your customers while maintaining a low pressure drop and face velocity.

To help you size your filters correctly, I would like to offer National Comfort Institute’s (NCI’s) “Filter Sizing Procedure.” If you would like a PDF copy, send me an email request.

3. Duct Design Methods

Proper duct design is the blueprint upon which a duct installation is built. It’s what the installed duct system should look like if all the pieces fit together as intended. If the design is incorrect from the start, duct system (and overall HVAC system) performance suffers due to improper airflow delivery.

Many professionals in our industry assume proper duct design automatically equals duct system performance — this simply isn’t the case. To verify your duct design method works, whatever it might be, you must measure the built system’s actual airflow. If measured airflow is ±10 percent of design airflow, you can confidently proclaim your duct design method works.

4. Duct Fitting Construction

Another consideration is duct fitting construction. Excessive turbulence from poorly constructed duct fittings reduces efficient airflow delivery and increases resistance the fan must overcome.

Duct fittings should allow gradual and smooth airflow turns. Avoid sharp and restrictive turns in your duct installations to improve their performance. A quick review of ACCA Manual D will help you decide which fitting configurations work best. The fittings with the lowest equivalent length allow for the most effective airflow delivery.

5. Duct Tightness

A tight duct system will keep the fan-circulated air inside the ducts. Leaky ducts decrease system performance and cause various issues, including IAQ and CO safety problems and decreased system capacity.

To keep it simple — any mechanical connection in a duct system should be sealed. If the connection doesn’t need to be tampered with, such as a duct or pipe connection, mastic works great. If the mechanical connection has a component behind it that might need future service, like an indoor coil, use a sealant that is easily removable. Don’t be the person who mastics access panels to the air-handling equipment.

6. Volume Dampers

Once air is contained inside the duct system, you need a way to control it. Volume dampers allow you to control the airflow’s path and are essential to a well-performing system. A system without volume dampers allows air to take the path of least resistance.

It’s unfortunate that many designers consider these accessories unnecessary and leave them out of many duct system installations. The right approach is to insert them in the supply and return duct branches, so you can balance airflow into and out of a room or area.

7. Duct Insulation

Up to this point, we have only looked at the air side. Temperature is another duct system performance factor that cannot be ignored. A duct system that lacks insulation cannot deliver the proper amount of heating or cooling to the conditioned space.

Duct insulation maintains air temperatures inside ducts so the temperature leaving the equipment is close to what the customer feels at the supply register.

Insulation that is installed incorrectly or has a low R-value will fail to prevent temperature-related duct losses. If there is more than a 3°F temperature difference between equipment leaving temperature and the farthest supply register temperature, the ducts probably need more insulation.

8. Supply Registers and Return Grilles

Supply registers and return grilles are an often forgotten part of duct system performance. Typically, designers use the most inexpensive registers and grilles. Many assume their only purpose is to cover the rough openings from the supply and return ducts, but they do so much more.

Supply registers control the delivery and mixing of conditioned air into a room. Return grilles don’t influence air movement but are important from a noise perspective. Make sure they don’t hum or sing when the fan is running. Refer to grille manufacturer information and choose registers that best fit the airflow and room you’re trying to condition.

9. Installation Techniques

The biggest variable that determines duct system performance is how well the ducts are installed. The perfect system will fail miserably if it isn’t installed correctly.

Attention to detail and a little planning go a long way to ensure installation techniques are correct. It blows guys away when they see how much airflow is gained from a flexible duct just by removing excess core and kinks and adding suspension. The knee-jerk reaction is to blame the product instead of the installation procedure used. This leads us to the 10th factor.

10. Verification

To guarantee successful duct system design and installation, it must be verified. This is done by comparing design data to data measured after the system is installed. Individual room airflow measurement into the conditioned space and duct system temperature change are two essential measurements you’ll need to gather. Use them to determine delivered Btu into the building and verify design conditions are met.

If you rely on your design methods to assume the system operates as it should, it will probably come back to haunt you. Heat loss/gain, equipment selection, and duct design calculations were never intended to guarantee performance — don’t take them out of context. Instead, use them as a target for what your installed system field measurements should be.

Proper duct design is the blueprint upon which a duct installation is built. It’s what the installed duct system should look like if all the pieces fit together as intended. If the design is incorrect from the start, duct system (and overall HVAC system) performance suffers due to improper airflow delivery.

Many professionals in our industry assume proper duct design automatically equals duct system performance — this simply isn’t the case. To verify your duct design method works, whatever it might be, you must measure the built system’s actual airflow. If measured airflow is ±10 percent of design airflow, you can confidently proclaim your duct design method works.

PRESSURE LOSSES IN AIR DISTRIBUTION SYSTEM The system resistance in ductwork has three components:

1. Friction loss (resistance to air flow caused by duct size, roughness of duct walls, and air velocity).

2. Dynamic loss (resistance to air flow caused by changes in air velocity and direction).
3. .Equipment pressure loss (resistance to air flow caused by components such as diffusers, coils, and filters).

Duct Friction Losses Any type of duct system offers frictional resistance to the movement of air. Resistance to air flow produces certain friction losses that vary with: a. Velocity of air b. Size of duct (smaller diameter duct has more friction) c. Roughness of the material d. Length of the duct The frictional resistance of a supply duct varies in proportion to the square of the ratio of the velocity, and the fan power varies as the cube of this ratio. For example, if a supply duct is carrying 5,000 cfm of air at 1000 fpm, and a second smaller supply duct is carrying 5,000 cfm of air at 2,000 fpm, the frictional resistance of the second duct per foot of duct length will be four times higher than that of the first duct: (2,000/1,000)²; and the power required to overcome this frictional resistance will be eight times as much: (2,000/1,000)³.

The need of building occupants for ventilation has been recognized many centuries ago; however, since the early 1970s, ventilation systems for buildings and transport systems have considerably evolved. This was invigorated by researchers who demonstrated the requirements for buildings to provide comfort and good air quality indoors (e.g., Fanger, 1972; Fanger and Christensen, 1986; Fanger, 1988; European Collaborative Action, 1992). Later on, this need evolved to address the additional energy requirement for buildings to achieve the indoor environment quality levels stipulated by those previous researchers (Awbi, 2003, 2007; Karimipanah et al., 2007, 2008).

Energy consumption for heating, cooling, and ventilating buildings often accounts for the largest part of a country’s energy usage, which is still mainly based on fossil fuels. There is a great global emphasis on reducing the reliance of buildings on fossil fuel energy and a move toward Nearly Zero Carbon Buildings (NZCB). This requires a major shift in the way buildings and their integrated heating, cooling, and ventilation systems are designed, operated, and maintained. Achieving this goal will require a rethink of the traditional designs of and types of systems currently in use. The proportion of ventilation energy in comparison with the total energy use in a building is expected to increase as the building fabric energy performance improves and ventilation standards recommend higher ventilation rates for improving indoor air quality (IAQ). At the same time, new building regulations (Directive 2010/31/EC, 2010; Building Regulation, 2010) are imposing air-tight construction, which will inevitably impact on IAQ, health (e.g., sick building syndrome), and human productivity in some future buildings (Seppänen, 2012).

Despite recent advances in building ventilation (Nielsen, 1993; Etheridge and Sandberg, 1996; Skistad et al., 2004; Awbi, 2011; Müller et al., 2013), it is evident that complaints about poor IAQ have increased in recent years (Gunnarsen and Fanger, 1992; Fisk, 2000, Bakó-Biró, 2004; Fanger, 2006; Boestra and van Dijken, 2010). There is a need therefore for assessing current methods of building ventilation and developing ventilation systems that are capable of providing good IAQ and energy performance to satisfy building occupants and meet new building energy codes.

This article gives a brief overview of the various types of mechanical ventilation and air distribution systems that are used for buildings; highlighting those systems that are capable of providing better IAQ and energy efficiency. The aim is to provide some insight to those building professionals whose tasks are selecting ventilation systems for low energy buildings that can provide the necessary levels of IAQ for the occupants; and for the research community to continue research in this area in order to develop new ventilation concepts and deliver the desired performance.