Ok I don't know if you guys have solved this problem or not but I remember that celler quite well since I got stuck there as well on my first play-through (and on many other places). What you guys probably missed is that as soon as he opens the valve he closes it again (and since he's a shitty play-througher he skips over the text) you probably only missed it. Look again and you can see he doing it. The test will be something in the line off: "That probably shock the whole building" or something like that and apparently the shaking of the valve slamming down again makes the dust on the walls to fall on the floor and thus revealing the light-switch. I only solved this by miss-clicking with the mouse. I was going down the celler once again and accidentally right clicked on the valve and something new happened. Dumb luck really.