High efficiency furnaces can take combustion air from inside the house. Providing the room is large enough and the home is not hermetically sealed (or too tight). New construction home here in the Chicagoland area require blower door tests to ensure the home is air tight, but not too air tight. We have to have fresh air/oxygen so we don’t get sick. The opening on this intake does not have an elbow on it. If you open any furnace installation manual, you will find the acceptable installation method so you can take combustion air from the inside of the home.

The phot below shows the proper installation method. The elbow helps prevent inadvertent blockage such as resting clothes, towels, books, or boxes on the hole. Blocking this opening should stop the furnace from starting but should it start, the oxygen will be limited and high levels of carbon monoxide will be created.