ducts

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it depends, say you have a long run to a kitchen and you open a vent on that duct. You will reduce the pressure of that line and reduce the amount of heat the kitchen gets. On the other hand if you open a vent right at the furnace outlet you will reduce the pressure to the whole house evenly and not affect your balance, same with a totally unused line if you have one. But if the furnace has the capacity (isn;t running all the time as it is) bite the bullet and open what you need and then rebalance.