I am trying to get some background information on whether an expansion tank is needed when a gas tankless water heater unit is being installed.
Currently I have a 40 gallon tank gas hw heater and investigating its replacement with a Navien 210A gas unit (in a different placement in my basement). The tank unit has an expansion tank on the cold inlet (as would I expect) but what about tankless units? Do they need them, and should I have one installed with a tankless replacement.
Yes, you will need an expansion tank if this is a closed system. tank or tankless, it does not matter when it comes to an expansion tank.
Maybe you would like to research Noritz or Rinnai, are far better units then the one you are going to choose.