We’re planning an extension to our master bath, and would appreciate input on adding an electric tankless WH as supplement to our existing plumbing within that expansion. Current configuration has a single MB sink at the outside (framed) wall that we intend to remove for the extension. Existing plumbing comes up out of the slab into that wall, so we understand some of that slab will have to be demoed/replaced to connect to that location for the hot/cold water source to the expansion. Planned addition includes a double shower, two sinks and possibly a tub. Our intent is to have the tankless heat the hot water line (cold water slug?) between the natural gas tanked WH, (about 40 ft away in the garage) and the master bath.

Questions:

1. As we haven’t opened that wall as of yet (thus suspect ½” supply at that existing sink) , is it reasonable to reroute that existing hot water supply line (under the future slab extension) up thru the master bath electric tankless to the new shower heads and sinks?
2. Are we correct in assuming that since the tankless will only be a supplement at the remote location (master bath), that a smaller (less wattage) unit will work well?
3. Is there a need for a water filter at the input to an electric tankless WH?
4. As we eventually intend to convert the garage gas WH to tankless, are there any issues we need to be aware of?

Thank You, in advance for your expertise,
Jerry