Whoops - I just found the obligatory introduce yourself thread 
The long red wire is 5V, the long black wire is ground, from a USB 5VDC power supply by means of a BEC connector.
I’ve cut the AC/5V track near the logo to get a 5V switched signal on the H terminal.
Then it gets a bit messy. I only then dug up the specifications of my heater. I don’t need a hot water terminal. Instead of a 5V signal my heater requires an open/closed circuit. So I moved the short red wire that supplies both relays Common contacts with 5V (for testing purposes to only supply Hot Water relay NO contact with 5V) and added the short black wire between the W terminal track and the Common contact of both relays.
This modification allows for setting open/closed loop via H and W terminals.
The short red wire should probably be removed on my board.