return vent size change
 

I am re flooring my family room and thought it an opportunity to replace the original 15.5" x 10" vent with a smaller nice looking vent cover. Is there a reason not to do this?

The house was built in the early 1950s with gravity heat system, It is now a high efficiency forced air system.

Is this the only return?

An undersized return can damage equipment. It is usually a bad idea to decrease the return size.

Yep.
Got to watch decreasing return size.
Could harm equipment. Make lots of noise through the grille. And not be able to heat the house as well(as in not reach set temp of stat).

return vent size change
 

It is the only one in a room 13.5 x 17.5. I guess the consensus is, it's a bad idea.

Even considering the fresh air intake added to the outside?

You can't put air into a room if you don't have a way for air to get out of a room.

So the fresh air intake, isn't providing a path for air to leave that room.

Also. With a fresh air intake. If you remove, or reduce return air from rooms. Due to the house leaking air out of it. You will increase the amount of fresh air that intake brings in. And increase your heating bill.

Again, Is this the only return?

Yes,if anything, without a pros advice make it larger ,keep it the same,don't make it smaller.