Don't want to buy or hire tools ? draw around the vent whether square or round, then drill holes all the way around the pencil line using an ordinary masonry drill, then knock out with hammer and chisel, any rough edge will be covered by the vent grille. Time consuming but effective.
just a word of warning as previously said you can fit the circular vent but htg engineer forgot to mention that if you want to use that core drill bit you'll need to buy or hire a core drill, a normal drill will not cope with it.
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