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"Air can leak in and out of duct systems in several places," Wiggins noted. He listed 11 places:
1. Where the actual duct joins the starting collar;
2. Where the duct joins the boot box;
3. Where the duct connects to a wye;
4. Where the starting collar connects to the plenum;
5. Joints in the plenum;
6. Where the plenum joins the equipment;
7. Joints in the equipment;
8. Equipment door panels;
9. Holes in the door panels for tubing, etc.;
10. Joints in the boot box; and
11. Where the boot box meets the sheet rock.
"There are more places air can leak, but let's consider these 11 for now," Wiggins said. "Typically air leaks more in some areas than others. Empirically speaking, the worst of the air leaks are in and between the equipment door panels, where the equipment connects to the plenum(s), and where the starting collars connect to the plenum.
"Comparatively, even without mastic, very little if any air leaks at the duct connections themselves when the inner liner of a flex duct is secured with a zip tie. Now we have taken the area that leaks the least and focused the most attention to it. How does this make sense? There are city officials going around all over the state [Texas] looking for this mastic and UL 181 tape to verify the system is sealed substantially airtight.
"Inspectors do not pressure test the system, yet they are able to tell a contractor to correct something that may very well be a benefit to the building envelope as a whole. According to Manual D, page 14-1, "It is possible for duct leakage to have a positive effect on health and comfort. If duct leakage increases the infiltration rate, a tight structure benefits from the leakage, provided that the infiltrating air is of good quality. In this case a duct sealing project might degrade the indoor air quality (unless mechanical ventilation is used to maintain an adequate fresh air exchange rate)."
"My point is if the duct system leaks a minor amount here and there it is not going to be the end of all that is good. The licensed contractor (design professional) should have the freedom to make field judgments based on experience while following the general guidelines of the International Mechanical Code. City officials are taking this duct sealing way too literally and it causes a tremendous burden on the contracting professionals that are trying their best to comply with all the new rules.
"As a licensed contractor I am in favor of "substantially airtight" systems, but when the manufacturers are exempt from the rules and the inspectors are zeroing in with a magnifying glass on such a small section of where air leaks I have a problem with that. A chain is only as strong as its weakest link and the powers that be are focused on the strongest link they have - the permit-pulling licensed contractor. Why? Because they are the most vulnerable and least protected part of the entire process.
"City building officials are important to the inspection process when people such as homeowners and property manager employees attempt their own HVAC installations. They also are very much needed when a license holder cannot or will not inspect the work that his employees or subs are doing."