It could be a medical reason, so checking with the vet would be good.
There also could be other factors. I have a cat that will not poo in the litter box if it is not clean, so she goes right beside it. She is simply a princess. They have amazing smell and will go in the same place. So at the pet stores you can get special cleaner that will take the smell out.
I have read about cats that were bullied by other cats that would guard the litter, so it really could be anything. And if they find another place to go and it is not cleaned properly they will continue to go there. So the heating vent could simply be a place they thought they were supposed to go, possibly a past animal went there, so I would get it cleaned and if possible limit access to that room for a while.