Hokay, I understand. You just want to prevent dust from coming into the room that has the furnace and hot water heater in it. They seem to bring in fresh air for the gas-fired furnace and hot water heater. I'll bet a dollar to a donut that they are meeting code requirements.
Have you talked with your neighbors to see if they have a problem with dust and, if so, what they did to keep the dust at bay?
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