If this is a Linea 24 then prepare for failure - The same part has failed on mine four times. It is a rubber seal (301185) probably costing less than one penny that connects to the diverter valve motor (E06326) through to the three way valve. The seal fails, water seeps into the diverter motor and it fails. This leaves you without hot water or heating. I have spent over five hundred pounds having this fixed over the last few years. Call Vokera on 08443 910999 and ask for Lindsay Walker.
Anthony Foster, February 2008