Eigenvector wrote:
I have a couple of ideas as to what these are there for, but I have vents in
my ceilings that go to nowhere. They are in all the rooms, all of them go
into the attic and are all connected to each other by regular ducting. But
they don't exhaust anywhere.

I'm not exactly sure why they exist.

I'm thinking of two possibilities
1) They are from an aborted attempt at installing AC
....

I'd venture w/ Suzzanne they were installed either when the house was
built or during a major remodel w/ the intent of adding central
heat/air which hasn't been done. If they're uninsulated, they were
undoubtedly the returns.

What got me wondering about it was whether or not I should seal the vents
during the winter to conserve heat.

Wouldn't hurt a thing, might help a tiny amount, but if they're not
open, doubt if would see any real difference. If they were open, that
would be another story, as then you could set up an nice flow path
driven by thermal and pressure differences, but w/o the exit, there's
no flow.