If you have a musty smell chances that you are not circulating the air are big and that means you don’t have the number of return vents you need in your Home.If you hear a whistle around your furnace or a return vent that is because you don’t have enough returns and the air handler is forcing the air to come through holes in the systems.
Some Old Homes have one big return installed near the furnace or in a main area. If this is your case you should update your system, there is nothing wrong with it but is not the most efficient.
What is recommend and more efficient is to have a return in each room of your House. This allows the system to recirculate the air in your entire House. This also allows your system to last clean longer because you are pulling the contaminants from the entire House.
Having return vents in each room allows for lower humidity in your home which is important in controlling bad odor and Mold Growing.Return Vents are recommended to be install in wall because cold air stays in the floor and you want the hot air to come out from the top of the walls.