AO Smith has a "15 year" but it is 12 years on the heat exchanger and and 5 years on warranty, but I don't know what the restrictions are. They give no warranty if you install it yourself.
http://www.navienamerica.com/WarrantyInformation/ http://www.navienamerica.com/WarrantyInformation/NPE/ seems to exclude damage due to build up, but I don't see a specific hardness cuttoff. One exclusion readsThey specifically say no warranty for any unit bought via the internet.Any damage caused by local adverse conditions including but not limited to hard water deposits, lime or mineral build-up, operating in corrosive atmospheric elements.
For A O Smith the hardness limit number is 7 grains.So don't assume you get a warranty just because they advertize a warranty. Don't assume that I found all of the weasel words for you.Water hardness levels must not exceed 7 grains per gallon (120 ppm) for
single family domestic applications or more than 4 grains per gallon (70 ppm) for all other types of applications.