All You Need to Know About Ductwork

We cannot leave the ventilation of our business at the mercy of windows and doors, since roughly 20% of natural air comes in through them. You also need ductwork to accomplish complete ventilation for systems and people.

Natural ventilation may serve you well if you have a one-room business space with less stuff and standby air conditioners. However, for businesses with plenty of rooms and extensions, surviving on natural ventilation could be a nightmare. And the problem may be doubled, if the business is situated in a region with sharp climate changes.

Ductwork can be likened to an air-extension that refines and supplies your home or business with clean-air. It’s simply the “Jack Bauer” of trapped air – it uses all means possible to get your ventilation back in order. 

What is Ductwork? 

Ductwork is a system of ducts that gives and takes air from a building. It can be used in commercial and living spaces too. The essence of a duct system is to keep the air in a building clean, free of germs, and of course, breathable. Ducts are used in both heating and cooling of work spaces, warehouses, office buildings, etc.

Components of a Ductwork 

The definition of a ductwork is not complete until we include other components like take-offs, vibration isolators, smoke, fire dampers, and turning vanes, etc. These components make the combination of ducts more efficient and durable. Below are the components of a ductwork and their distinct uses;

Vibration Isolators

The air handler at the beginning of most ductworks is always accompanied with blowers. Blowers aid the movement of air and in doing so, they cause noise pollution. To avoid the transmission of vibrations to the occupants of the business, the ductwork is designed to accommodate two vibration isolators (one at the beginning of the ductwork and the other at the end).

The vibration isolator can be best described as a rubberized material with a canvas-like structure. It allows the blowing of air while eliminating the unwanted vibrations. 

Stack boots and heads

A combination of stack boots and heads makes up the basic structure of a ductwork. Stack boot is the professional term for vertical duct which, as it name suggests, helps air flow vertically. On the other hand, a stack head connects a regular wide round or rectangular duct to the thin ducts on the walls. 

Unlike stack heads, register heads help the flow of air into a wall-attached air register. The stack heads link “to and fro” the ordinary ducts. 


Ductworks are designed in a way that the main duct branches into various ducts. The air source, which is placed below the air handler, gives air to various openings known as grilles, registers and diffusers. The movement of air from the mother duct into subsidiary ducts is aided by fittings termed take-offs.

The takeoffs are often stationed into circular or rectangular holes drilled into every subsidiary duct. They (takeoffs) are connected to the mother duct by little metal tabs. The metal tabs are bent to make the connection firm. Round take-offs are attached using the spin-in attachment method. Other versions are connected using the snap-in connection method. 

Smoke and fire dampers

Ducts that are positioned through or near a firewall require smoke and fire dampers. The smoke damper and the fire damper operate differently. They might be placed beside each other or farther based on the design of the HVAC-R contractor. 

Smoke dampers rely on electricity to function. They are triggered by a motor or actuator. The motor is linked with a probe inside the duct, so as to expose the fire damping system to the ventilation system. Smoke dampers are able to detect smoke coming from the supply air, or outgoing air. The actuator is programmed to close the air damper on detecting smoke.

Fire Dampers 

Fire dampers break the electricity barrier, since they are not reliant on a motor for activation. The inclusion of a manual trigger protects households and companies in case there is no electricity at the time of fire outbreak. Fire dampers are classified based on the mode of trigger. Vertical fire dampers exploit gravity while horizontal fire dampers operate on spring power. They are designed with a mechanical fusible link that is melted or destroyed at a particular temperature. The damper shuts the duct once the fusible link is destroyed, depriving the fire source of the required air to maintain combustion.

Volume control dampers

Unlike smoke dampers and fire dampers, volume control dampers (VCDs) are not triggered by a fire incident. They offer homeowners the means of regulating the bulk of air entering or exiting many parts of the ventilation system. Volume control dampers, manual or automatic, can be fitted in the locations of registers that help air transition from the parent ducts to the thin wall-mounted ducts. Simple duct systems often require zone dampers while the complex ones have variable air volume installed in them.

Turning vanes

Turning vanes are placed internally at the points where direction changes. They serve as the “wind breakers” of the duct system. They help reduce the pressure or turbulence caused by an approaching air flow. Also, turning vanes points the air flow where they ought to pass without causing the disruption of resisting ducts. 

Terminal units

Terminal units are not present in single-zone constant air volume systems, but they are installed in the branch ducts of the multi-zone counterparts. Every thermal zone has a terminal unit. It might come with a heating or cooling coil.

Three types of terminal units exist, namely: fan-powered mixing boxes (in series or parallel order), VAV boxes (uni- or bi- duct), and induction terminal units.

Air terminals

Air terminals aid the exit or entry of air flow in the duct system. Types of air terminals are grilles, diffusers, and registers. 

A grille is an outlet of various slits sideways in a duct. They are placed at the terminals of ducts, allowing in-flow and outflow of air. Though entry or exit grilles don’t make any technical difference, advanced grilles/filter returns come with air filters. The air filter provides additional value to the grille.


Plenum is an intricate component of the HVAC system, so they ought to be carefully planned. They (plenums) are majorly used in collection and central distribution units.

The airflow coming from the return grilles or bell mouths passes through the return plenum before it is distributed to the central air handler. The central unit is not the final destination. The air in the central unit is redistributed to the final destinations (rooms or warehouse).

How to install a Ductwork

Getting an HVAC-R contractor to do your ductwork could cost $2,500 and up (based on $50 per hour). It’s best to get a professional to handle it for you, since you’re going to be occupying and operating the business for a good period of time.  However, if you can’t afford it, this section would guide you through all the details of ductwork installation.

What tools do I need to install ductwork?

The tools needed for ductwork installation are; hand seamers, tape measure, claw hammers, reciprocating saw, circular saw, straight bench snips, left hand snips and right hand snips, a drill with a quarter hex head driver, gloves and glasses.

Apart from making you look like an engineer, the gloves and glasses keeps you safe from the possible hazards you may encounter while installing the ductwork.

This guide can be applied to every design type but you have to adjust the parameters to suit the sizes of the registers, ducts and so on. 

The home or small business used as a case study;

  • Has 3 or 4 good-sized return registers.
  • Is built with a centrally situated mechanical room down the incomplete basement. 
  • Needs a rectangle-shaped trunkline installed in the plenum front.

Step 1: Map out the design

The first thing you need to map out is the ductwork design. It’s advisable to get someone to map out your HVAC ductwork design. Don’t just use a neighbor’s copy or any online design because every building has a unique design. The design would also specify the width and size of the ducts as well as the structure.

Step 2: measure and mark out the design on the walls

With the aid of the design, peruse the main floor. Then, pinpoint the supply and return registers positions on the floor. Mark out the specific points on the walls. You may want to keep the supplies 8 units away from the walls for easy curtain clearance. 

The holes should not be more than ¼ units, so as to keep some space for future works. Keep the registers in the middle of the floor joists, not above them. A pro trick to getting the accurate measurements is to drive a nail and view from below before cutting.

The returns have to be positioned horizontally between the walls. The measurement of the cut is ¾” but you can increase the front edge cut outside a bit, as it aids the airflow. The grille size should be larger than the wall depth. Doing so would reduce the noise pollution that comes from speeding airflow. For instance, a grille of 6” or 8” tall would warrant a 3.5” deep wall. While cutting through the walls, avoid tampering with the floor joists. 

Sometimes hot air is trapped in the ceiling. Installing a return at the zenith of your ceiling could contribute to the efficiency of the cooling system.  

Step 4: Install the b-vent flue before covering the wall with ductwork

A b-vent flue traps air from inside and transfers it for combustion. Then, dispose it to the environment. If the building was designed to have a b-vent flue, it's better to install it before moving forward with the ductwork installation.

Step 5: Position the AHU, returns and coil side. 

If the room has a central air conditioner, its evaporator coil can be installed on the air handler. The evaporator coil can either be cased or uncased. The cased coil can be easily accessed while the uncased coil has to be fitted inside the plenum, so as to ensure that incoming airflow is influenced by the refrigerating effects of the coil.

Step 6: Install the plenum 

As mentioned earlier, the plenum is an intricate part of the duct that provides refined air to the main parts of the building.  It can be installed on the air handler or the evaporator coil. At the top, the duct ends with a cap. 

Adjust the plenum by cutting until it is 1” below the ceiling. Attach rectangular or round takeoffs at the beginning and end of the plenum. Most takeoffs have a metallic lock that can be bent into the plenum, so as to stay intact. Position the plenum well and install with the aid of self-tapping screws.

Step 7: Arrange the supply air ducts

The supply air ducts ought to be arranged on the ground before installing them. Fit in S-cleats at the beginning and end of each joint. Then, attach them together using drives. Designate the point where each subsidiary duct would have to be attached to the top of the primary trunk.

Find the hole that needs to be drilled for the top take offs to be attached. Usually, top take-offs are designed with dovetails, which serve as a means of attachment to the parent trunk. Place the tabs in opposite directions, in a way that half of the tabs would key in and close the diverting line that leads to the primary trunk.

Follow the same method until every part of the supply air ducts is complete, as stipulated by the designer. Ensure all outlets are attached with takeoffs and seal the primary trunk with a terminal cap.

Once the branch ducts are properly attached on the ground. Raise the structure up. Fit the galvanized branch ducts into the joist spaces. The supply ducts end with stack boots and heads. Seal all joints with a recommended duct tape and three screws each. Then, install the ducts within the joist spaces with a “2 by 4” or fashion sheet metal bracket. 

Step 8: Install the return ducts

The return ducts can be installed on the right or left sides of the supply ducts. There’s no recommended side for the installation; your discretion matters here. 

Step 9: Install the return air drop

The return airdrop is vertically situated beside the air handler. Its function is to link the return trunk line to the fan area of the air handler. Place a takeoff where the return air drop meets the rear of the return air trunk. There’s a space between the return air drop and air handler that can contain a filter. Advanced filters may require a 7” - 10” width for the compartment.

Seal the fitting using the hammer lock flange. The return air drop extends to the floor and may end with a terminal cap. 

Step 10: Seal the openings for the remaining parts of the duct structure.

This step is necessary, so as to protect the ductwork from contaminants and debris. You can use pieces of sheet metal to cover the openings. Crosscheck the whole structure to ensure no stone is left unturned. 

How to inspect Ductwork 

Whether you hire a ductwork inspector, or you plan to inspect it, the two factors to look out for are leakages and contamination (dirt and germs). Commercial spaces should be inspected every 1-2 years while living spaces may not be inspected at all, unless there’s a risk of air borne disease or a family member is suffering from allergies. 

The U.S Environmental protection Agency (EPA) maintains that there’s no need for duct cleaning in homes except if there are cases of air borne diseases or allergies among the members of the household. The agency further states that “duct cleaning may be irrelevant if the result of a visual inspection does not show the indications that the ductwork has been infected with sizable quantities of molds or dusts. 

In this section, we would discuss two types of inspection, namely; inspection for leakage and inspection for contamination.

How to inspect your ductwork for leakage 

Once you notice a change in air quality indoors, your best guess is a duct leakage. However, it’s just a guess. To ascertain your theory, you have to carry out a test to determine if the change in the air quality was caused by a duct leakage. 

There’s no one way to inspect a duct leakage, but below is the best way to run a duct leakage test.

Using a smoke puffer and blower door, you can easily detect a duct leakage that’s causing air to enter from external microenvironments, such as the fireplace, garages, attics and crawl places. However, a more efficient instrument is the duct air tightness tester. The combination of infiltrometer blower door and airflow capture hood is also a great alternative. 

The test for duct leakage depends on two factors, namely:
  • the input pressure of the calibrated blower used during the procedure
  • the output pressure given by the ductwork. 

If the output pressure is considerably lower than the input pressure, then, a duct leakage is the most likely explanation for the loss in air pressure. 

The rule is that each cubic foot of air that is given through the ducts must be balanced with the fan input. We use a value which is close to the normal functional pressure of an average duct system. The commonest value is 25 Pascals.  

There are two types of duct leaks – mild and chronic. The mild leak occurs inside the conditioned area and they do not pose an energy threat. On the other hand, the chronic leak helps conditioned air escape into an unconditioned environment and invites unrefined air into your house. The chronic leaks are the ones that reduce the efficiency of the duct system and pollute your home.

Things to expect after a thorough duct cleaning

Proper duct cleaning should get rid of the dust, pet hair, debris, cobwebs, droppings, calcium deposits, and other substances present inside the ducts. The end result would be shiny and bright ducts. 


The two major consequences of duct leakage are the pollution of air and reduction of ductwork efficiency. The amount of energy lost due to a leakage depends on the area of the leakage and size. As we’ve mentioned earlier, some leakages might need urgent repair while others may not pose any threat. Give B.I.M.S., Inc. a call today if you have questions in the greater Texas area.

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