Just to add to the excellent advice already given...
You have to appreciate that a digital image is just a bunch of coloured squares, or pixels....that's it.
Us humans can 'see' recognizable patterns of pixels and interpret them as 'objects' that we see in the real world.
In order for that to be done by a 'computer' is extremely difficult....simply distinguishing between an image of a Cat and an image of a Dog is today still an incredibly difficult task to do and more often than not a 'computer' will fail and do no better than a 50:50 guess.
To us though cats and dogs are like chalk and cheese....no problem.
Putting that into context where PS is concerned it too just sees a whole bunch of coloured pixels....it's US that makes the decision on what the pattern of those pixels represent.....in your case a 'vent'.
There isn't, therefore, a way to isolate these 'objects' merely by calculation alone, which is what PS has to use....two identically coloured pixels could [to us] be completely different objects in the image.....like in your case...the 'vent' is very similar to its background, as [USER=69670]IamSam[/USER] has pointed out.
To us it's obvious, but when you go by the numbers there is no 'obvious' difference between any of the pixels at all....they are all the same with only their colour changing.
Therefore in order to differentiate between 'objects' PS needs something numerical to work with.....a difference in luminosity, contrast, colour, sharpness etc etc.....it cannot simply use 'what the image looks like' and it cannot do it by itself....we have to 'tell' PS what method to use.....yeah, PS is actually pretty dumb when you think about it....it's also colour blind too but that's a different story.
The suggestion of a much more contrasting background would have given us the contrast needed for PS to distinguish between a 'dark' pixel and a 'light' one making selecting those 'dark' ones much easier...as it stands the bits we want are almost identical to those we don't......its the "white cat in snow storm" situation.
I'm sure you were fully aware of all this but when people start talking about 'objects' and how to get PS to select them they often don't understand that PS just sees a whole bunch of coloured pixels...well, greyscale pixels actually but whatever.
Using a 'Levels' adjustment layer and a 'Threshold' adj layer I came up with a mask that with a little more 'painting' gives this result...
[ATTACH]57342[/ATTACH]
...but your image is pretty small so its not at its best.
Regards.
MrToM.
