@jcboy,
You should consider engaging the services of Animal Control professional.
Squirrels are crafty little buggers. They're not easy to catch and they're hard to keep from getting back in your house. You have got to find out where they are getting in and close it off. Otherwise the effort you spend catching the critters will have been wasted.
Pros have done this before (a large chuck of their business involves squirrels) and they know where to look to find the entry point(s).
Be reasonably sure you've captured all of the squirrels before you close the entry points(s). We didn't do this one year and when the time came to get the Christmas decorations out of the attic we found a large squirrel curled up and laying on a wreath...unfortunately it had died some time before. Another reason to use a Pro.
Consider wearing gloves and a surgical mask while you are in the process of evicting the rodents. They can spread some nasty diseases, particularly through their feces. Yet another reason to hire a Pro.
Once you've caught the bugger(s) and closed off all entry points don't just release the squirrel in the woods behind your home. He may not be able to get back into your place but I presume you have neighbors. You'll have to take him someplace where he can't readily attempt to invade another home. Guess who will take care of this for you? A Pro! Here you may need to do some due diligence to make certain that you find a Pro who will actually go through the bother of taking the squirrel for a ride. Many will simply let the creature go when they get to the end of your street or worse...drown them.
I'm sure you've seen or heard this next bit before, but you will do wise to pay attention to it.
Don't procrastinate. Beside the fact that you are likely to hear the pitter patter of lots of additional squirrel feet if you wait long, the biggest problem with squirrels in your house is the damage they can do. You've already seen evidence of them chewing on your duct work (that needs to get fixed or your energy bills will climb) and they won't limit themselve to that one area. Squirrels chewing through electrical wires cause quite a few fires every year.
I'm neither in the animal control business nor know anyone, personally, who is, but I've tackled animal problems on my own and through the use of a Pro and using the Pro is infinitely easier.