I seem to have yellow colored drip marks coming out of the base of my chimney and on the smoke pipe of my heating unit. What is this and why is it happening?
A This is becoming more common as heating appliances become more efficient. Usually it is due to improper sizing of the chimney flue in relation to the outlet size of the heating unit. Most heating system chimneys utilize an8″ x 8″ flue pipe which has an interior dimension of about 7″ x 7″. This was suitable for heating systems of the time when this standard size originated. Now, as oil and gas burners become more and more efficient, they use smaller outlets and put less heat up the chimney in the interest of keeping it in the home. The disadvantage to this is lower stack temperatures in the chimney. For every cubic foot of gas that is burned, 2 cubic feet of water vapor is produced. If the appliance is hooked up to a flue that is too large to heat to the proper operating temperature, the warm gases cool upon contacting the cold chimney walls and condensate forming droplets that drip down to the chimney base while picking up sulfur and forming sulfuric acid. This is very corrosive and
