I recently put in a high efficiency furnace that used to be vented through my chimney. Some contractors wanted to put in a chimney vent because my hot water heater is still venting through the chimney. I have a clay liner in my chimney. Do I need to put in an additional liner?
I agree, the exhaust from the hot water heater will deteriorate the mortar and clay liner, and you should install a liner. For most water heaters a 3" or 4" is all you would need. The size is calculated by the BTU's of the heater and the height of the chimney, and there should be a cap installed if you do not have one already.