Can someone help me figure out what these vents are in my home !!??

I recently moved to a really old home and it has a a very old heating ducting system. I have heat vents that are on the side of the walls in every room but on the floors one in every corner of the house so that would be four I have these wooden grates over vents in the floor that are wood lined and do appear as if their a wood duct type system but nooo clue for what as the heat vents on the walls appear the same age as the floor vents. . I am totally clueless as to what these vents are for as no air blows through them and I feel no air being sucked into them..... does ANYONE have a clue what these are for?

 

Go under the house and see where they go.

 

A really old house probably had a convection furnace at one time. The rising warm air would come out of the wall vents and the sinking colder air would drift down to the furnace to be re heated. Some of these may have been blocked off if you now have forced air. As Joe suggested, do some eyeball detective work.

 

I have them too! They used to be for heating the house with coal. The fire would be in the basement and the heat would naturally rise through those vents! They can be almost impossible to cover without the exact measurements. I have 2 rectangle and 1 square with a circle in the middle about the size of a manhole cover!I thought of painting them and putting a piece of tin underneath because they are in these days for decoration instead of heating.