In homes with forced air heating the ductwork is your lifeline to warmer air, so you want it to operate efficiently. Unfortunately many times these pipelines have leaks that cost you comfort and money, especially if they run through unheated spaces like the crawlspace, basement or attic.
Inspect the ducts with the system running to find any areas where warm air is escaping. Seams and joints where two or more pieces meet are the most likely culprits.
When you find a leak, seal it with foil backed duct tape. You don’t want the typical cloth backed stuff for this job. Then, paint over the tape with a generous coat of duct mastic. This will dry to form a flexible, but permanent seal, keeping the warm air where it belongs.