redsky49
Well-known member
Duct leakage has been mentioned by other posters in separate threads, and I think that some further discussion may be helpful.
Most residential systems utilize a fabricated sheetmetal duct system. While this is typically fabricated by a dedicated sheetmetal contractor, many installing HVAC contractors will frequently bend their own ductwork.
Ducts typically leak at two locations: the longitudinal seam for each duct section, and the joint where two duct sections are joined together.
As a rule, unless the Contractor is required to the contrary, the standard of residential duct sealing is as determined by the sheet metal standards published by SMACNA, the sheetmetal Contractor's organization. This establishes that ductwork of a residential nature (1" pressure class) will only be sealed at the joints - none of the seams will be sealed. Since most installers do not have supply air measuring devices, or choose not to use them, the sealing performance standard is 'no leakage discernable to the senses'. That is, you should not be able to hear or feel any air leakage as you examine the operating duct system. As to the percentage of air that may be "lost" as a result of this level of duct sealing, anywhere from 10-30% would be typical.
In a higher performing system, better seam and joint designs would be employed, and both seams and joints would be sealed, as well as all penetrations of the duct. A well sealed system, as evidenced by testing, would display less than 10% leakage. 3-5% confirmed leakage would indicate a very conscientious contractor employing the best materials and methods.
How does this apply to the typical homeowner? Excessive leakage will definitely impact the performance of your HVAC system - especially at peak load periods. Rooms at the end of long duct runs will be especially vulnerable to this.
So what is a properly sealed duct? Well, in most cases it is not sealed with duct tape, particularly the cloth backed type. The adhesives used in these materials are not permanent and frequently will fail in short order. Contractors use these materials since they are quick (read cheap) for the installer, and require little expertise to install.
If duct tape is not the answer, what is? The current standard is a putty like sealant such as Hardcast or Foster's Fostex. These are typically oil based though some manufacturers are providing water based low odor products. This is as permanent, and as effective, as the industry can currently provide. However this product takes time to apply and even more time for the product to properly dry.
Okay, you have gone the extra mile and applied a high quality duct installation. How do you know that you have gotten your money's worth? You must have the system "balanced". This is the term for the testing and adjusting of your system. The total supply air will be measured at the furnace or air handler. This number will then be compared to the measured air supply at all the air outlets (registers or grilles). If you have lost 10% or less of supply air, you have a good installation. If you have higher losses, more effort will be required. If you have sealed the ductwork, and leakage is still evident, look at the flexible duct connector(s) at the connections to the furnace. These frequently are a source of leakage. Another area of attention is the filter holder. Also check the plenum at the top (usually) of the unit where the cooling coil is located.
Even in systems where there are no complaints over room temperatures inside the house, substantial money can be saved by solving this leakage, just as you will improve operating costs by sealing the doors and windows of your home from the outside.
Hope this was helpful.
As always, offered only as opinion
Most residential systems utilize a fabricated sheetmetal duct system. While this is typically fabricated by a dedicated sheetmetal contractor, many installing HVAC contractors will frequently bend their own ductwork.
Ducts typically leak at two locations: the longitudinal seam for each duct section, and the joint where two duct sections are joined together.
As a rule, unless the Contractor is required to the contrary, the standard of residential duct sealing is as determined by the sheet metal standards published by SMACNA, the sheetmetal Contractor's organization. This establishes that ductwork of a residential nature (1" pressure class) will only be sealed at the joints - none of the seams will be sealed. Since most installers do not have supply air measuring devices, or choose not to use them, the sealing performance standard is 'no leakage discernable to the senses'. That is, you should not be able to hear or feel any air leakage as you examine the operating duct system. As to the percentage of air that may be "lost" as a result of this level of duct sealing, anywhere from 10-30% would be typical.
In a higher performing system, better seam and joint designs would be employed, and both seams and joints would be sealed, as well as all penetrations of the duct. A well sealed system, as evidenced by testing, would display less than 10% leakage. 3-5% confirmed leakage would indicate a very conscientious contractor employing the best materials and methods.
How does this apply to the typical homeowner? Excessive leakage will definitely impact the performance of your HVAC system - especially at peak load periods. Rooms at the end of long duct runs will be especially vulnerable to this.
So what is a properly sealed duct? Well, in most cases it is not sealed with duct tape, particularly the cloth backed type. The adhesives used in these materials are not permanent and frequently will fail in short order. Contractors use these materials since they are quick (read cheap) for the installer, and require little expertise to install.
If duct tape is not the answer, what is? The current standard is a putty like sealant such as Hardcast or Foster's Fostex. These are typically oil based though some manufacturers are providing water based low odor products. This is as permanent, and as effective, as the industry can currently provide. However this product takes time to apply and even more time for the product to properly dry.
Okay, you have gone the extra mile and applied a high quality duct installation. How do you know that you have gotten your money's worth? You must have the system "balanced". This is the term for the testing and adjusting of your system. The total supply air will be measured at the furnace or air handler. This number will then be compared to the measured air supply at all the air outlets (registers or grilles). If you have lost 10% or less of supply air, you have a good installation. If you have higher losses, more effort will be required. If you have sealed the ductwork, and leakage is still evident, look at the flexible duct connector(s) at the connections to the furnace. These frequently are a source of leakage. Another area of attention is the filter holder. Also check the plenum at the top (usually) of the unit where the cooling coil is located.
Even in systems where there are no complaints over room temperatures inside the house, substantial money can be saved by solving this leakage, just as you will improve operating costs by sealing the doors and windows of your home from the outside.
Hope this was helpful.
As always, offered only as opinion
