Your HVAC system is only as efficient as your ducts will allow it to be. Unfortunately, leaking ducts are very common, especially in older systems or poorly built ones. If you have ductwork that is coming apart, you might be paying to heat and cool your attic, crawlspaces or other areas that don’t need it. You could also be taking in unconditioned air from these places into your ducts, along with the dust and smells that you can find there.
To check for leaks, you should start closest to the unit. Since the pressure is higher when the air comes out it is also more likely that the seams could be coming apart.
Sometimes, injecting smoke into the ductwork will let you spot where you have leaks since the smoke will be coming out there.
If you are experiencing very high energy bills, and you aren’t sure why, it might be worth it to get in touch with a professional who will be able to locate where the problem is.