This is a little repair I did on a Vaillant ecoTEC PLUS 624 system boiler in March 2014. This was an issue on these early first generation ecoTEC boilers manufactured from 2005.The burner door gasket that seals between the heat exchanger and burner assembly would fail causing heat to escape from the heat exchanger.
In most cases I have come across the escaping heat causes the air intake trumpet to melt which cuts the air supply off and the boiler ceases to function. This is the best case scenario, sometimes the damage is a bit worse. Failure of these burner door seals was a problem on the Vaillant ecoMAX models as well.
In this case I just upgraded the burner door seal and replaced the air intake piece and thats all it needed.
Glowworm and Saunier Duval also shared this design and suffered from the same problem. Most of these boilers have been fixed but I still come across the odd Glowworm 30cxi and Saunier Duval Themacondens with the old burner seal type, some of which are about to fail.
The seal was upgraded to a graphite seal to fix the problem. Vaillant recommends you replace these graphite seals every 5 years. I haven’t seen any G10 graphite seals fail, but the older ecoMAX has an extra rubber seal on the bottom of the heat exchanger which always fails at some point and starts leaking. These typically last between 3-5 years before failing.