Pay particular attention to the joints where the ducts come together -- the tape used to seal the joints can come loose over time, allowing air leaks to occur that waste heat. Since the joints may be partially or completely covered with insulation, it's a good idea to have the heat on while you're making your inspection. Feel along the ducts for air leaks, and also listen for any sounds of escaping air.
Repair: The next step in the process is to repair any problems you discovered with the system, which typically means resecuring and resealing joints, and repairing or replacing duct strapping.
In hard ducting -- solid sheet metal as opposed to flexible ducts -- the joints can be secured using sheet metal screws or pop rivets. After the joints have been secured, seal them up using a foil HVAC (Heating, Ventilating and Air Conditioning ) tape that is specifically made for this purpose -- despite its name, do not use standard cloth-back duct tape, which loses its grip over time and comes loose, leaving you right back where you started. Foil HVAC tape has a tough silver aluminum face and an aggressive, heat- and water-resistant adhesive that's designed for long life.
Flex ducts typically use a clamp system to secure the flex duct to a hard duct. If a flex duct joint has come loose, check to see if you can reuse the original clamp. If you can't, you can typically use a large worm-drive clamp or flexible plastic clamp to secure the joint. After resecuring, wrap the duct's inner insulation blanket and outer shell back into place to cover and seal the joint.
If ducts need to be resupported, use duct support strapping that is made for this purpose. Attach the strapping to a joist, girder or other solid support, using nails or screws. Do not use wire or string for this purpose, as it does not provide adequate, long-term support and can also kink flex ducts.
Insulate: Since the ducts are running through an unheated space, duct insulation is a huge part of the system's ability to retain heated air within the ducts until it gets delivered into the house. So as part of your inspection, carefully check the insulation as well. See if the ducts are completely wrapped without any gaps, and that the insulation is of sufficient thickness to provide good insulating value. Look for a level of about R-8 for your ducts, which is approximately 2 1/2 inches of fiberglass.
Clean: When all of the ducts have been repaired and insulated, the final step in ensuring a healthy and efficient system is to clean them. This is a process that is best left to the pros, who have equipment with sufficient vacuum pressure to suck dust and debris from the system. At the time of the cleaning, they can also inspect the condition of the furnace itself and replace the filter.
Professional duct cleaning should typically be done every couple of years -- more if you live a very dusty environment, have a lot of pets, or have other factors that might contribute to a buildup in the system. Between professional cleanings, regular maintenance should include removing the registers and vacuuming out the first foot or two of the duct with a shop vacuum, and changing the filters in the spring and fall.
