As of last fall, you were not required to use a liner. RG&E said one was
not necessary when I put my HighE furnace in because it was already tiled
and ran up through the center of the house.
Anyhow, you miss the reason for a liner. In the summer the humidity from
the water heater just goes up the chimney. In the winter it can chill and
condense before it get out. Being acidic, it can attack the chimney.
And if I recall correctly, the liner was supposed to be SS. One company
said they alway use liners, one said they never use liners, and the other 2
did inspections and measurements and said I didn't need one.
I was happy with my installation, if you want a recommendation; though I
still wonder if I should have used a liner.