Residential duct systems leak. Depending on the complexity, the design and location of your duct work the impacts on efficiency can change dramatically. Mechanical duct work joints and fittings are much leakier than even your heating man believes! Ducts are typically located in either or both of the following locations:
1. ALL DUCTS INSIDE THE THERMAL BOUNDARY
With all the duct work inside the heated spaces of your home, the energy losses are reduced considerably. Leakage will still be contained within the conditioned area but will still result in rooms or spaces that are varying in temperature. Duct leakage is a cause of that "colder than most" rooms that you have at the end of the hvac system. Take time to seal all accessible joints with UL 181 Duct tapes and/or listed mastics. A tight duct system will then allow your dampers and registers to better control air flow. The bottom line? Tightening your ducts will get conditioned air to where you want it!
2. ALL/PARTIAL DUCTS OUTSIDE THE THERMAL BOUNDARY
A design condition that requires much more attention. In this part of the country it is common to see all or part of the hvac duct system in an attic or in a ventilated crawlspace. Sealing these systems are much more critical as lossed conditioned air moves out to the atmosphere. The maximum duct leakage to outdoors should not exceed 8cfm/100sq.ft. of floor area when tested with diagnostic equipment. Ducts outside the conditioned space should be insulated as well to a minimum R-8. It is not uncommon in this part of the country in older homes to find completely uninsulated supply or return ducts in attics!