This is how I cleaned the heat exchanger on MY Combi boiler – it sounded like it was just about to leave the wall, so I took a look.
PLEASE MAKE SURE YOU TURN OFF THE ELECTRIC AND WATER & TAKE CARE NOT TO DAMAGE THE PCB BOARD. I took advice from my plumber mate & found out the heat exchanger was blocked, so i turned off the water supply drained the system and removed the exchanger (2 Screws) flushed it out and put it back. Bingo works great!! The magnaclean was also cleaned and YES the Top one is heating & the Bottom is hot water. Thanks for watching, I'm a computer engineer.