David presented the following explanation :
Ideally I need to cut a 6" hole in the side of my house and put some
sort of vent thing in. The only way I can think of doing this is to
draw the circle on the wall and then drill lots of little holes round
the circumferance.

Is there a better way of doing this? I looked at a large bore diamond
cutter from HSS, but they wanted about £70 a day to hire it!

You gave no clues as to the material from which the wall is built, nor
its thickness. Assuming a normal cavity brick outer wall, you might
consider simply taking bricks out as best you can (hammer and chisel),
inserting some pipe, then filling in with bricks and morter to the new
pipe.

Drilling the morter out first will aid in getting the bricks free and
reduce the hammering effort.

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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.org