Re: cleaning duct work on gas, forced air furnace
If you're getting dust and dirt out the vents a filter isn't going to help. The filter cleans the air going into the furnace the dust and dirt is in the output ducts. For no more than it costs I would suggest you get it professionally done. They have the giant vacuum and the brushes needed for the job. It's also possible that you will need to have the furnace cleaned especially if you have an "A" coil for central air.
Be sure you live your life, because you are a long time dead.-Scottish Proverb