Is it necessary to install an isolation valve on the pre-heat side of the WWHRS system?

Isolation valve requirements

We do suggest the inclusion of a full-bore isolation valve on the preheat side as best practice, in case of replacement or upgrading of the WWHRS unit. However, depending on the type of WWHRS unit and siting of it, if you were required to replace the unit at a later date, then it may be a rather disruptive process anyway, and so the inclusion of an isolator on the preheat pipe might be fairly mute. The specifier for the project will be best placed to make this call, on a project-by-project basis.

Further installation information for the Pipe HEX can be found here