Navien combi

Hello,

I have a customer that has a Navien combi boiler. It is currently hooked up to city water (Port Washington, NY). The homeowner is installing a whole house water filtration system including a water softener. I need to re-pipe most of the water piping in the house. Should I keep the boiler on "city water" or connect to the new filtered/softened water? I thought I remember reading that "soft" water in no good for these boilers (and most high efficiency ones). Any input would be greatly apricated.

Thank you.