Dirty vents in a home or office building is the first sign of dirty ductwork. When you look at the cold air return vents and see a buildup of dirt/dust that is the first sign that the ductwork is dirty. When you look at the heating duct vents (supply vents) and see the registers dirty that is the answer to your questions. Your ductwork is dirty and needs to get cleaned.
Make sure that you do your research and do not get taken advantage of like millions of homeowners a year. This is such a big deal that Chris Hanson did a special on Nightline about duct cleaning scams.
Check out the BBB (http://bbb.org) and see if the company you are looking into has had any complaints. If you can not even find the company on the site then you should think twice. Scammers like to change their names and start up different companies to run away from problems. Amistee has ZERO complaints with the BBB and has done over 5000 cleanings. Compair that with a company that might be $50 cheaper that does not have a record or might have a few complaints. Is it worth the risk?
Is the company you are looking into State Liscensed? Does the company have the correct equipment? What type of guarentee doe they offer? Will they show you before and after pictures? Are their employees actually their employees or do they sub out the work? How will they protect your home from being damaged from hoses? All of these are very important questions that you need to do a bit of research on.
When looking at Amistee you will feel at ease that we are obsessed with finding a better way to do everything from answering the phone to patching out holes. Call to find out why you should use Amistee. (877)-349-8877.