I am working in a very old home (circa 1850)and am putting in a new gas water heater. A chimney looks like it was added when oil burning heat was added years ago. My question is, can I run the new w/h flue into the existing 8 " chimney flue. There is a line of tar or creosote or something that comes out of the 8" pipe opening on wall about 4" long and rock hard. The w/h will be the only unit using that vent. Thanks in advance for your opinions in this matter. Dennis

Should be fine but check with local codes o venting... may want to clean chimney first