I just bought this house built in 1959. It has a fairly new HVAC system installed in the attic. It also has a separate register in the hallway about 4 feet from the furnace return air vent for a seperately controlled outside air fan. When the furnace is on, it sucks air from the nearby outside air vent and makes the hallway ice cold. It also makes the rest of the house drafty. My question is this: can I block off that outside air register? Will blocking it off negatively affect the operation of my furnace?