Main lines are usually located on the furnace. There can be from one to four lines on one furnace. Main lines are the large metal square or rectangular lines that connect the furnace to the vents and returns. When main lines are cleaned...

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Vents or register vents blow air into the rooms and return vents are sucking air in. Register vents are usually smaller than return vents. If you are still not sure what is a register vent and what is a return, you can make a simple test. ...

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