Hi Robert, External air has been discussed a fair amount here, having its pros and cons but typically favouring the cons. The best place I found that covers it well is here:
the-outdoor-air-myth-exposed
In the RMH world, a chimney fan is only necessary (for some people) when you have poor draft at startup with a cold mass and want to give a little boost at the beginning of a burn, then turned off.
A bypass is a more low -tech solution that is hard to beat for its simplicity and reliability.