When adding UFH to an existing Y plan central heating system, ideally the system should be converted to an S plan configuration, if this is impossible then an additional 2 port zone valve should be installed on the inlet pipe immediately before the 3 port (Y plan) valve. The UFH primary pipework should be taken off the primaries before this 2 port valve. This configuration will allow independent operation of the UFH.
See the schematic for wiring. – Y Plan with EWC-4 V2