I have a natural gas 40 gal. A.O. Smith water heater that was installed in October 2004. It vents out the chimney as did the old furnace that sat beside it. Last summer, 2007, I had a new 95% efficient furnace/ac installed and that had to be vented separately out of the side of the house. That left only the water heater vented up the chimney. One month after the furnace installation, the thermocoupler had to be replaced on the water heater. That thermocoupler was replaced again in January 2008. I have been told that since the chimney has no liner, the water heater can not vent properly. The only solution is to line the chimney or replace the natural gas water heater with an electric, which does not vent. Any ideas to solve my problem?