We have two cold air returns in our house. One is in the living room and is quite large(3 grates wide), the other is in the upstairs hallway, which is open to the livingroom. The one in the upstairs hallway blows a lot of cold air. When we took the grate off, we discovered that they used the bay as the vent instead of a metal vent, but now there is a hole in the main cold air return vent, where normally a metal vent would tie in. The main vent runs through the garage. Is it safe to cover the hole in the main vent, and block off the upstairs cold air return? The air and breeze is cold enough that it is cooling the upstairs.