Valiant Turbomax of some sort. Unfortunately I can't tell what's connected to what up there.
Just Googled this and it does look like the most promising candidate. Thanks!
It's responsible for circulating air around the vents in my apartment, though since it's above the boiler I'm unsure whether it is connected to the gas/heating/hot water as well.
The main issue is that it's extremely noisy and I want to shut it off. If it only serves the vents, then that...