Cutting a round hole in a window

[johnf]

I want to install an extractor fan (100mm) in a utility room, and the best position for it is in a narrrow-ish top-opening window. Has anyone used one of those circular glass cutters? Any hints or tips on avoiding disaster?
Or would it be more sensible to take the complete window down to the local glass shop and pay them to cut it for me?

 

[Diyisfun]

Old glass is more brittle than new. Probably best to take it to glazier. If it breaks they may even fit the new pane.

 

[biffvernon]

Old glass is more brittle than new.

That's probably an urban myth. The other glass myth is that, being liquid, it slowly sinks, which is why old windows are thicker at the bottom than the top. Not true.

 

[big-all]

as long as it isn't double glazed or toughend as you can't cut these

 

[noseall]

agree with big-all, there should be a mark in the corner of the glazed unit indicating that it's toughened.