HVAC DuctWork – what is it and why is it important?

December 15, 2019

Ductwork or air ducts are the tubes that feed air from your home into the HVAC unit located in your attic or a closet where it is cooled or heated. Then exits the unit via the supply ducts to the grills located around your home. Having ductwork in good condition is key to your system efficiency, air quality, and your comfort.

What are the sub-components?

Return Sub Component


There is a return grill located in the ceiling near the location of the unit in a hallway. At a minimum, there is at least one in your home. If it’s not in the ceiling it will be in a wall.

The best practice is to have a return in each room, that way when the door is closed there is air being taken out of a room. Above the return register, there is a large diameter duct that takes back to attach to the return plenum.

Return Plenum

Return Plenum

The plenum is attached to the furnace and holds the filter and filter access door. The return air ducts if more than one is attached to the box.

Air handler

Typically located in a closet, will return air in a wall below the unit, the filter plenum is shallow 4-6 inches deep and holds the filter.

Supply Plenum

Supply PlenumThe supply plenum is attached to the evaporator unit, the conditioned air is fed into space and feeds air to the supply ducts. The supply plenum is typically made of metal, insulated to stop condensation from building up.

Supply Ducts

The supply ducts transfer the air from the supply plenum to the supply grills/ registers around your home. There will be at least one to each room in your home. The insulation around the ducts varies the best level of the insulated duct is R8 and code required in a number of cities.

R6 is available and is the standard in some cities, compared to R8 it has a lower insulation level and efficiency. Prior to R6, there are lower levels installed, in homes over 30 plus years metal ductwork is not uncommon. The ductwork is made up of sheet metal tubes which has insulation around it.


  • Collar- a metal ring that is used to attach ducts to the plenums
  • Damper – located in the collars of the supply there is used to adjust the level of airflow through the duct its located in
  • Zoning – where the design of the system has broken the home or parts of the home down into 2 or more zones to allow a thermostat to control an area of a home. For Picture, downstairs and upstairs allow the temperatures to be set a few degrees high or lower than the other
  • Zone controller – A unit which interfaces with the thermostats to direct airflow to the zones to achieve the target temperatures in the zones
  • Zone Damper

  • Zone damper – located in the duct is a motorized damper to control the volume of air through the duct directed by the zone controller
  • CFM (Cubic Feet Minute) – the metric used to calculate a target or measure actual airflow from a system or through a duct
  • Fresh air intake- A fresh air intake is a unit that pulls in air from outside the home when directed by the system. The purpose is to reduce the humidity and carbon dioxide level in the home
  • Boot – An insulated metal box that attaches to the grill above the sheetrock in the ceiling

What are the signs that my ductwork needs attention?

Signs That My Ductwork Needs AttentionAre there areas which have a different comfort level to the rest of your home. Does your bedroom feel hotter than the rest of the home or is the humidity higher? Do you need to adjust the thermostat at night so you can sleep?

This can be from the current configuration the ductwork from the unit to the room

  • It may be too small
  • The air may be unbalanced – the dampers may be adjusted incorrectly
  • There may be holes/leaks in the ductwork

Do you have high and low areas of flow inside your home?

Are there some rooms or grills in your home where you can feel or hear the air coming out of the grill with a higher velocity versus others where you cannot feel the air. This is caused by one or more issues:

  • A duct has fallen off
  • There is old leaky ductwork, holes in the ducts
  • Design of the ductwork system is not adequate
  • The system may be undersized, not big enough to heat and cool your space
  • The airflow needs to be balanced

Do you have any grills closed to try and improve the airflow to another room?

Do You Have Any Grills Closed to Try and Improve the Airflow to Another Room?A common sign there are ductwork issues is closed grills around a home. These have been closed typically to force cold air to another area of the home. This a band-aid as it is masking the issue and will make things worse in the long term. When an HVAC system is designed the heating and cooling needs are calculated for the whole home, this is then broken down for each of the areas, then the size of the duct to each of the grills.

Having a grill closed will cause issues due to low airflow and moisture if a grill is closed this is at the end of the duct so air is still going to be pushed from the HVAC unit to the grill so there will be hot or cold air in the duct behind the grill. If the system is in cooling mode cold air is going to be present in the duct. If the boot attaching the duct to the grill will condensate from temperature differential hot air meeting a cold surface and low or no airflow over time mildew can develop, metal will rust. Having a closed grill will increase the pressure in the ductwork system. This makes the blower work harder. ECM motors sense the pressure and will ramp up causing the unit to use more energy. The harder the motor runs the more air can leak out.

If there is low airflow to one duct, then it’s related to ductwork:

Low Airflow in Duct

  • There is a leak hole in the duct
  • Design of the duct is not adequate
  • The airflow is unbalanced
Alternatively, if there are several ducts it can be:
  • A duct has fallen off
  • There is old leaky ductwork, holes in the ducts
  • Design of the ductwork system is not adequate
  • The system may be undersized, not big enough to heat and cool your space
  • The airflow needs to be balanced
If all your home is impacted the root cause:
  • A problem with the blower motor or wheel not moving enough air
  • The filter is dirty and restricting the airflow
  • The evaporator coil is restricted from dirt build-up in the fins

Water Dripping on a Grill

Do you have water dripping or have condensation on a grill?

This scenario will occur in the summer when the system is running in cooling mode the air coming out of your duct will be around 60 degrees F, the air in your attic will typically be 120 degrees plus.

If there are any areas of poor or no insulation around the boot attaching the duct to the grill, then it will cause condensation.

Mildew on Grills

Do you have mildew or black spots in your grills?

This scenario will occur in the summer when the system is running in cooling mode the air coming out of your duct will be around 60 degrees F, the air in your attic will typically be 120 degrees plus. If there are any areas of poor or no insulation around the boot attaching the duct to the grill, then it will cause condensation.

Do you have dust building up on surfaces around your home?

Do you clean surfaces in your home to see the dust return a day later? even if you have the doors and windows closed during that time and have had your HVAC system running it coming from your system.

Dust Building Up On Surfaces

Potential Causes Are:
  • Dust build up inside your ductwork which has built up over years and is breaking loose or there was someone in the attic and agitated the ductwork breaking dust-free to blow into your home
  • There is attic air getting into your ductwork, attic air is dusty a combination of outside air and insulation
  • Holes in aging ductwork
  • A collar may have come loose
  • If someone was in the attic and put pressure on a collar or duct it can deform a connection and created a hole
  • Rodents in an attic can chew holes

Do you have noisy grill?

If there is high airflow through a grill or part of a grill it can cause it to make a humming or whistling sound. It can be annoying you don’t have to live with it. It can be addressed by:

  • Normally adjusting the grill
  • Reconfiguration of the duct or boot
  • Upsizing the grill or the duct

Do you see lines around your grills/registers?

Discoloring on Grills/RegistersIf you see dark or discoloring on a ceiling around a grill or multiple grills, this is from attic air being pulled through the vent. Checking in the attic above the vent you have dark or black areas of insulation. Insulation turns back when exposed to moisture.

The moisture turning the insulation black is from hot humid moist attic being pulled through it, due to a gap in the connection between the ceiling boot and drywall. The black mark across the ceiling is insulation that has built up over time. This is resolved from sealing the boot to the drywall if there is one often multiple around your home that needs to be sealed.

Sealing these improves your indoor air quality, eliminates passageways for the dust to enter your home. Your system becomes more efficient as it is not having to cool the attic air entering your home, reduces the humidity that is being pulled into your home and improves comfort.

Is there high humidity in your home?

In the Texas summer, the air is hot, the hotter the air the higher the level of moisture that is in the air. Similar to temperature, there are differing levels of comfort some like it higher 60% other prefer it at 50% or lower. With new functions being added to HVAC to target humidity levels for cooling and heating to control temperature and humidity to set point.

If you have high humidity in your home based on preference more than 55 or 60% there are a range of causes:

Why Does Ductwork Deteriorate?

  • Attic air entering the system with moist air
  • Ductwork issues
  • Low level of attic insulation
  • Outside air coming to the home, around windows, doors, the ceiling can light, ceiling fan mounts, attic door
  • System not running long enough to pull humidity out of the conditioned air
  • Airflow may be too high across the evaporator to give time to remove moisture
  • There can be an issue with the ac system that is reducing the cooling capacity and efficiency i.e. the refrigerant level may be low

Why does ductwork deteriorate?

Torn DuctLike most things as ductwork gets older it does not perform like it did when it was new. The ducts are going through multiple heating and cooling cycles a day which can lead to cracking of joints, sealers from expanding and contracting. The flex duct inside and out will fall apart over time depending on the quality of the original material.

The moisture in the environment and temperature will take a toll on adhesives over time especially if they are under tension. Rodents like rats and mice can chew holes, squirrels and larger animals can destroy sections of ducts. Years of strain on joints over extended time will cause failure. Duct hanging strips will tear or snap, putting more load on the next strap.

The first step will be for a technician to understand your issue then investigate what is causing the issue, then to propose a solution

Remove Dust With Duct Cleaning

  • If related to the furnace or evaporator what the repair is, Cleaning the evaporator coil or blower wheel if there is an airflow restriction
  • If there are holes in the ductwork, leaky ductwork finding the areas at fault documenting and replacing parts or sections of ductwork
  • If there is an imbalance of airflow – adding and adjusting dampers to balance the air between ducts
  • Sealing and or insulating boots
  • Redesigning of a section or all the ductwork to improve the whole home airflow and comfort
  • Duct cleaning to remove dust and dirt from the system

If you notice any of these issues or would like to find out how you can improve the comfort of your home, contact Southern Comfort Mechanical with any questions!

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