I recently bought a older house (1928) house with steam heat. The boiler is in the basement. Next to the boiler is an insulated duct shown in the attached picture. At the top of the duct is an open vent to the outside. The bottom of the duct is open.
The tempurature in Michigan is in the teens right now and cold air is flowing it the basement make it (and the house) very cold.
What is the purpose of this vent? Is it OK to cover it?