Rodents are a real pain in the neck!
Find out if Terminix will return until they are gone.
Rodents make noise.
listen for movement at night.
Avoid poisons. If you poison them they may die in your house's ductwork. Not good.
Best to trap them. Tie a string onto a mouse trap of proper size for your particular rodents and slide it into the vent.
Be careful with the droppings. Some rodents may carry the Hanta Virus. Let the Terminix guys deal with the droppings.
Find out how they are gaining access to your house and seal it up with steel wire.
You mention insulation, that means they are out of the heating sys. and into the walls. Look for holes in the walls too. Check your wiring for damge too.
If you have access to the ducts from below, buy a short length of vinyl pipe with a slightly larger diameter. Cutout a section big enough to cover the holes and use a two-part epoxy to affix it to the outside of the duct. If you can reach the hole fron the vent end, use the same diameter pipe and glue it on the inside of the duct. If your rodents are quite small and the holes are not very big,(less than an inch) you could try expanding spray foam.

Get rid of the rodents first though. They love to build nests out of expanding foam.

Try to put some pressure on the Terminix guys to expedite their work. Rodents tend to breed like... well, like Rodents!

Good luck finding Mickey,

Katoo

I had that problem in a add on duct run, heating guys who did it are too lazy to use the metal, Arughhh! I had it replaced with a metal run..
From the size of the droppings, I'd say its a rat! hate them things!
Find where they are getting in first! I see people's garage doors open all the time, and I think "well, I bet they got mice!" haha...

Thanks a lot Katoo and crableg!

I guess i might need the duct replaced too, but I don't know which section. How to find out which part has damaged, especially if it starts from the inside?

Do you know if replacing air ducts is "DIY" project? If you ever had it done, is it a high cost project? Thanks!

Guess it could be DIY, If my hubby was alive, he could have done it, I might have even done it, if I could crawl under the house, haha, them days are over!
All my ducts are metal, except that add on, and now it is also.
Are all your ducts flexable? I hope not, you have to find the source of invasion first, then repair damage...

Some sections of the air duct are vinyl, some are metal, according to my husband who went down the crawlspace. May be I should replace with metal one, but don't know how to get that thing under the crawlspace! We found quite a long gap where the duct went from furnace into under crawlspace and have fix that. My husband also fixed a gash in the duct. But we don't know if there's any other holes where rodents come in. I have tried to put pressure on Terminix and they'll send 2nd person to inspect. Hopefully he has better solution in finding the gaps/holes that need fixing...

I would replace ALL flexable with metal, if I were you, it comes in short enough pieces, (flat) to get it under there, it it asembled as you go to proper lenght, it is cut with tin snips and sealed with furnace tape. Just hire it done, worth everything to get rid of the rodents!

Help. Every time my furnace comes on I can smell urine. I'm pretty sure it's a mouse ... but where? Is it IN the ductwork, or ON the ductwork? Could it be in the bottom of the furnace itself? I've set snaptraps and caught one. I've also placed rodenticides, and I knonw I probably shouldn't have, because they can go back to their nests and die there. I'm at my wits end. Should I call a furnace guy, a duct cleaner, or an exterminator? I'm getting sick - literally.

I'm sorry to hear about your rodent problem! I dealt with that a while ago in winter and it was not fun. (we had to shut down the furnace.) Does the urine smell stronger in one room more than other? Where did you catch the rodent? Perhaps look into the duct close to where you caught the rodent? If you're able to, have you tried inspecting the air ducts in your home to see if there're any holes or insulation coming out? I noticed mine first from the insulation that came out of the duct. Rodents like to make nests from insulation materials. Try inspecting your crawlspace insulation as well - you might see droppings, urine. I'd try to talk to pest control company and see what they can do to help. In my case, what Terminix did was putting traps and baits but we didn't catch any rodents. It helped me feel a little better that there isn't any more rodents. I use a company that did crawlspace clean-up, insulation and duct works. The salesperson came and inspect the crawlspace and told us what needed to be done. You may want to try to find such company and talk to them as well. Good luck!

I've never done it but have heard of wrapping flex duct with hardware cloth to keep rats/mice out of ductwork.
One contractor told me he used felt paper.

If I had to choose I'd opt for the hardware cloth.
After replacing the damaged flex.

best of luck.

You mention noodles being eaten. In addition to blocking ALL holes with metal (have you thoroughly checked other places besides the ducting?) you should probably make sure that all food (including stuff in the garage like spare grass seed, bird seed, anything a rodent might like to eat) is put in something rodent tight. Metal cans, the fridge etc. If you have pets, make sure you feed them, they eat it, and you don't have pet food laying around. No food, they typically move on, at least as soon as winter is over.

Here's what happened to me: Last Sunday morning, it got really cold, so, for the first time this year, I turned my heat on. Not fifteen minutes had passed before I smelled this AWFUL smell, like burning hair, and my smoke alarm went off. :-(

Now, to explain a bit, I live in a fairly new trailer, which I got in June 2014, and, all last winter, I didn't have any trouble with rats or mice. But, for a few weeks prior to turning the heat on, I had been hearing sounds like mice or rats scratching around in the ductwork.

Still, I didn't really worry about it, since I had had mice in the ductwork of my old trailer, which I had from 1993-2014 every single winter, and none of them had ever caught fire. They would just run around like crazy, when it would first kick on, and then, I would assume, they simply got the hell out of Dodge, and waited for it to kick off again, before they would get back up in there. Whenever I would actually see them INSIDE the trailer, I would set traps for them, and dispose of them, of course. But, as for them being up in the ductwork, I just figured "Out of sight, out of mind", and didn't really worry about it.

I had heard quite a bit about mice and rats chewing on the electrical wires inside the walls of trailers, and causing them to catch fire in the winter, when the heat would kick on. So, needless to say, when that one caught fire, it scared the hell out of me, and I immediately turned the heat off, not wanting to run the risk of my trailer burning down, particularly with me in it. ;-(

So, yesterday, I put some poison traps down under the trailer, but, the whole time, I kept thinking: "What if they eat this and crawl off back into the ductwork and die?" "How will I ever get them out then?" :-/ I realize now that it was the wrong thing to do, and will take them out, and use snap traps in the vents instead.

But I guess I was just wondering whether anybody else had had the same experience, with one actually catching fire like that, and the smoke alarm going off? You'd think mice would have better sense than to just hang around and let themselves get singed like that, unless it was actually STUCK in there and COULDN'T get out? That's the possibility that worries the hell out of me. :-( Do they ever go up there and then get stuck?

A mouse wouldn't catch fire inside a duct. It does not get hot enough. Possibly if one had died laying on top of the heat exchanger inside the furnace but that is it. Snap traps are always better, so their rotting bodies can be disposed of instead of rotting in the walls or vents.