Flex Pipe Floor Framing Problem
I don't think that your local building code would approve
of this type of situation. You're not suppose to use flexible dryer vent
pipes, for long runs, because of situations like this.
If this was a solid metal pipe, there's a good chance that it wouldn't be sagging. Most building codes require a 4 inch galvanized sheet metal pipe to run inside of your floor framing.
Now you're probably wondering, what's the big deal? Well, there's a very good chance that debris will build up over a long period of time in these flexible pipes, especially where the pipe is actually sagging.
A smooth pipe won't trap as much debris as a flexible one. If you need proof, remove your flexible dryer vent pipe and examine it to find out how much debris is actually stuck in every single groove.
Home Repair Tip: Remove the flexible dryer vent and and replace it with a 4 inch metal dryer vent. You can usually find your new 4 inch metal dryer vent pipes at most home improvement centers or lumberyards.
Try to make home repairs like these as soon as possible, otherwise you could find yourself with dryer problems in the future. If you notice that your clothes are taking longer to dry than normal, your dryer vent could be plugged up with lint and other debris.