A non-sealed-flue[1] gas boiler means there needs to ventilation in the room,
which is provided by air brick(s) (on the opposite wall to the gas voiler).
This tends to produce a howling gale of cold air in winter, especially when
the wind is blowing against the wall. Is there any kind of heat exchanger
that would attempt to recover some of this heat without impeding air flow
too much?

I'm guessing it wouldn't work without forced airflow (which is possible, but
complicates matters)? Or is there some kind of passive (eg convection)
solution?

What sort of airflow is required for safe combustion in the average gas
boiler?

Thanks
Theo

[1] I don't know the exact term, but it's the type of gas appliance which
operates like an open fire, in that the oxygen comes from the room, rather
than outside, but the combustion gases go up the flue.