My roof exhaust is too powerful. When the electrician set it up he asked whether
I want it to full power or half. It was set to full power.

The exhaust fan takes two 230V lines. What if one 230V line is disconnected will
it be half power or will I damage to fan? I am sure to get a fully qualified
answer one would have to inspect the connections and such but generally speaking
would disconnection the other 230V line be the usual solution? I am thinking of
just shutting the breakers to the other line.