Gas and Oil Home Heating Furnaces - Does my chimney need to be lined?

I am in the process of getting estimates to have my 40 year old gas furnace replaced. One contractor told me that if a furnace with an AFUE of 80% or better was installed, the chimney would need to be lined. He said that furnace manufacturers state that as long as a water heater is also hooked into the exhaust system it would not be a problem. But he says that none of them will put it in writing. Because of this he recommends a Lennox Whisperheat (78% AFUE) because its "chimney friendly". Can anyone please give me the real story.

line the chimney and get a higher efficiency furnace, it will pay for the extra cost. if the chimney has a terra cotta (sp?) liner, it is ok to vent without the internal new liner, if not, the flue gas will eat the mortar b/w the bricks. used to be that a water heater vented into the chimney would mean no liner, as the pilot light would cause a steady draft , not sure of the residential code now, may want to call the inspectors office in your county, they will be glad to help.