Its in my utility room with the furnace, gas water heater, washer/dryer, etc. It goes up into the ceiling and across my home and leads to an exterior vent on the opposite side of the house. Its has a constant flow of exterior air coming from it and I'm fairly certain its the cause of my chilly basement. I have a radon mitigation system in place so that probably helps to draw air through this pipe as well. I'd really like to close it off but a friend has mentioned it probably is to provide fresh air for furnace combustion however my furnace and home are only 11 years old and I thought one of the two PVC pipes off the furnace was for the fresh air.