Can I block large open vent in basement
I believe the vent is for combustion air. If there is no fresh air to the furnace and water heater this could create serious problems both with carbon monoxide and inefficient operation. If you block the large vent air may be pulled in from the vents on the water heater and furnace causing a buildup of carbon monoxide and because of the reduced fresh air the gas may not be completely burn. Plugging the fresh air vent could be quite hazardous and cost in inefficiency.