Duct Work | Heating and Cooling Mechanical

Duct WorkMost air conditioning and heating systems require some form of duct work to channel or direct the air to places in the dwelling where the conditioned air is needed. There are many types of ductwork available and often times the ductwork can make a big difference in your utility bills. For that reason, it is important that the ductwork is installed correctly by qualified installation technicians. A poor installation job will result in poor performance, bad air flow, leaky ductwork systems, and higher than usual utility bills. Another important factor in the installation process is to make sure the duct work is sized properly. Over sizing systems cost more and does not maintain the desired air flow and undersized ductwork causes the system to strain mechanically and can be noisy. It is important that you have qualified people design and install your system from start to finish so that you get the best and highest performing duct work system money can buy. It is also critical that the air conditioning ductwork system be designed for the air conditioning load.

Sizing Duct Work

When an estimator goes over your house or business for sizing the air conditioning ductwork he or she is looking at several factors. These factors will determine the tonnage and or BTUh requirements of the system and the size of the duct work. These factors for sizing duct work come from Manual D which is a guide used by contractors to size residential duct work. It is important to size the air conditioning ductwork for air conditioning systems. Since air conditioning requires more CFM’s than heating air conditioning ductwork should be sized for air conditioning and not heating if you have a combination air conditioning and heating system that uses the same ductwork. Among the many factors considered are:

  • shading around the house
  • color of the roof
  • type of materials used in construction of the dwelling and insulation “R” values
  • average degree day for your geographical location
  • square feet of the dwelling
  • number of occupants
  • mathematical area of windows and doors
  • construction and insulation values of windows and doors
  • size of equipment in kitchen and size of the kitchen
  • which direction the house is facing

These are some of the factors considered when sizing systems and are very important to the design of the entire system. Each room of the house should be evaluated and a determination of how much air flow will be required to ensure that each room remains at a desirable and comfortable temperature. Additionally, if you have a large home or commercial property HVAC zoning systems should be considered for comfort and efficiency purposes. HVAC zoning systems, if engineered correctly, can offer you more comfort and efficiency than just one single large system.

DuctWork and Zoning

In some cases, to ensure particular areas remain comfortable, an HVAC ductwork zoning system will be required to maintain comfort and ideal conditions. In single story residential it is normal for the system to be a single zone. In some instances, two story dwellings have single HVAC zones but it is difficult to maintain optimum comfort from the first floor to the second floor for obvious reasons. Because warm air rises the upstairs of the dwelling will always be warm and the downstairs will always be cooler. A two story dwelling will keep each floor comfortable by HVAC zoning the systems or by having two separate systems, one for upstairs and one for downstairs. Large commercial buildings usually use a VAV (Variable Air Volume) zoning system to keep spaces conditioned. For more information on VAV systems and how the work please see the DDC pages.

HVAC Duct Work Airtight Integrity

There are many HVAC systems which have been evaluated by inspectors or HVAC technicians and a recommendation made to repair or replace the duct work because it leaks air and doesn’t have airtight integrity. I myself have made these recommendations only to return six months to a year later to find the recommendation was never followed. Sometimes I hear the people complain about their high energy bills and what I can do to make the system more efficient. I look the system over and if the ductwork is bad and leaking I make the recommendation to repair or replace it. I can also do other things to make the system electrically and mechanically efficient but if the duct work is bad and leaky and my recommendation is not followed then the things I do electrically and mechanically will be negated by the leaking ductwork.

The people who do not listen to the recommendations could pay for new duct work with the money they save in energy costs throughout the year. Instead they choose to pay this money to the power companies for wasted finite energy. Energy is not cheap nor is it getting any cheaper and it is ever more important to maintain or systems which consume energy and in HVAC this is our air conditioners and heating systems.

I wrote a very good article on ductwork on High Performance Air Conditioning and Heating Page 2 and would be remiss for not steering you to that article for a final say on ductwork. Of course I don’t believe there is a final say on ductwork as I am constantly updating the site to bring you good information about your HVAC systems so that you stay informed and knowledgeable about your systems.

The Mechanical Code and HVAC DuctWork

The Mechanical Code states that HVAC ductwork should made of durable construction so that it is rugged and will not fall apart or leak too easily. As a part of that durability it should also be supported properly. It is important that when you hire an HVAC contractor to replace your duct system that you have the proper permits for inspection. This is a kind of way to protect yourself because the inspector in many cases will find problems with the duct work installation. California takes this process a step further than other States and they require new duct work to be pressure tested by a specialty Duct Work testing company. This ensures the duct work installation is tight and will deliver the air as the duct work was designed to do. There are other specialty companies which test commercial duct work systems for this same thing plus they balance the system according to engineering duct work design parameters. Typically, it is in all commercial contracts that the mechanical contractor who installed the duct work will not get final payment until the duct work system has passed all these tests. A good contractor will not be weary of obtaining permits or inspections for their work. Many invite this check up after the ductwork is completed because if it is a good job the inspector will usually say it is a great job and this validates the skills and workmanship of the ductwork installer.

On another note for inspections for mechanical and electrical systems including duct work be weary yourself if the inspector comes and doesn’t perform a good thorough inspection. It is very important that you get a good thorough inspection of the ductwork and other work performed by the HVAC contractor. If this happened to you call the inspection department and ask if this is how they usually perform inspections. You could also think about hiring an independent HVAC contractor to inspect the ductwork installation.

If you live in a flood plain or flood zone the mechanical code requires the the duct work be sealed to prevent water from getting inside the duct work. This would typically be hard duct work made of sheet metal and sealed very well at all the seams and joints. It is a sure bet though that if you do experiencing flooding of the ductwork that the ductwork insulation will have to be replaced at a minimum. Wet insulation provides little insulation value and will fall apart exposing your ductwork to moisture. This moisture from uninsulated duct work can cause moisture damage and mold and mildew problems. Mold and mildew problems can be harmful to your health so it is important to address any ductwork insulation which is not providing the ductwork with a good seal or vapor barrier.

The History of Duct Tape in HVAC

Books have been written about the uses of duct tape and has led others to inspiring levels like writing a book about duct tape art, duct shui (a book about integrating duct tape with feng shui), and the thousands of uses of duct tape including fun with duct tape. Where did all this originate from in the history of duct tape? To figure that out we must look into the origins of this amazing invention made by Richard Drew who worked for Minnesota Mining and Manufacturing or as we know it today as 3M Corporation who invented masking tape or the first adhesive tape. The original use of duct tape as we know it today was in WW2 when Johnson and Johnson created the modern version of tape to help the military protect ammunition from moisture. The tape was used to seal ammunition cases from moisture which was a problem in WW2. The duct tape sealed the lids of the ammunition cases and kept the ammunition dry under the most extreme conditions. The original name was Duck Tape because it kept the ammunition dry like water on a ducks back. That name changed after the war in the housing boom.

Duct Tape Use Morphs

From using it to protect the ammunition from moisture to using for a multitude of other uses began in WW2 when soldiers and sailors discovered they could use it for other things like making repairs on airplanes, firearms, jeeps, tanks, and even on themselves when they needed to make a quick fix for a battlefield wound. After the war duct tape came home and was used on HVAC duct work to seal seams and keep them airtight. Many people then started calling it duct tape because they thought is was specifically made for HVAC duct work. Today duct tape is manufactured in many various forms and many are specifically made for sealing ductwork. This modern duct tape is a much better sealant than the older original duct tape. The old fabric mesh for sealing duct work is now an aluminum backing and the adhesive used for duct tape has also improved making it more durable and resistant to moisture. Duct tape specifically engineered for HVAC ductwork has saved thousands of dollars in energy by sealing ductwork and preventing air leaks. Duct tape has come a long ways since it protected the ammunition containers in WW2 and today it saves money by sealing duct work from energy draining air leaks in your HVAC system.

An additional resource for duct work can be found here.

High Performance HVAC

Your Resource for HVAC DuctWork Information

  1. Our heating (electrical bills have gone up greatly -about doubled – compared to the same months last year. The heat pump system seems to be operating as usual but is using a lot more electricity. Can you recommend someone in the Seneca SC area who can trouble shoot the system? Thank you.

    • Sorry, I have been to SC but do not know any AC companies there that I know of. Simply do a Google search and you will find someone to take look at that problem for you. I have found when your electric bills rise with a heat pump it is possible you have a refrigerant leak in the system somewhere. You don’t get heating from the actual heat pump itself but off of the electric heat strips in the air handler. It is more expensive that way to operate a heat pump.

  2. what type of duct material is the best for effeciency ,, live in florida.
    thanks,mark holguin

    • Round hard duct properly designed and installed is the best. On installation it should be fitted together properly and sealed with duct sealant before it is insulation. In many cases a sealant is also applied to the insulation to create a good vapor barrier on the insulation to prevent moisture from getting under the insulation which will cause condensation problem especially when the air conditioner is running and making things cold. When the job is complete, if you want to be through, a Duct Air Leak Test is done to make sure the duct work is not leaking anywhere. In a few states this is required on new installations (namely California). On many government and commercial projects a DALT test is required to pass inspection to get LEED (not residential) points.

  3. Hi,
    I am upgrading to a 5 ton AC unit in my house, the previous AC unit was a 3.5 ton. The intake opening for that unit measured 20″ x 30″. Will I have to increase the intake size once I install the new 5 ton AC unit.I have the space in the ceiling to increase the intake size, if it is needed. I live in Riverside CA. This is a one story house.
    Thank you.