Flue liners are necessary to ensure that toxic gas exhaust from gas appliances (furnaces, hot water tanks and especially gas fireplace inserts) goes all the way up and out of the living spaces, because it can cause drowsiness or be fatal in large amounts. As well, if the chimney does not have a hooded pot, rainwater reacts with this gas to degrade the mortar — turning it to sand on the exterior portion of the chimney and in the attic. So it’s important to ensure that the flue has no holes in the mortar between bricks in any part of the chimney which is inside the house or attic. Furnaces and hot water tanks also give off water vapor. A liner seals that gas within the flue. At the very least the mortar in the attic should be inspected and the chimney should have a proper pot with a lid.