Chimney Liners Replacement/ Correct Size

[Christopher Collins]

Does anyone has experience with Chimney Liners? 

I need to add a chimney liner to a three story chimney. It will only be used to vent the gas from four traditional hot water heaters. The chimney is narrow so two contractors suggested two 4-5", each liner would be connected to two HWHs. The other contractor suggested we only need one 5" liner.

Is there a ratio of exhaust to diameter that anyone can help me out with? Installing one liner is significantly cheaper if possible, but I want to make sure it won't cause backdraft for the units.

Thanks!