JMA46

Hi all Logic tells me that an external air in take will definitely have benefits such as stopping draughts and would asume the air intake could be controlled via a chimney valve / plate? My question is I am wanting to install a boiler stove 21kw to run rads and H/W. My chimney is central to the house, so no external wall and concrete floors. Could I use the chimney cavity to run an external air feed for the boiler along side the boiler flue? Maybe not have it terminating at he height of the burner flue, so it does draw any fire gases or even terminate in loft? Thanks in advance Joe