Installing drain off valve?

Hi,

I have just bought a 3 bed bungalow and am intending to install a new CH / DHW system via a combi. I have read forums regarding draining combi systems via a drain off valve. Where should I put the drain off valve? all rads are on same level hence bungalow. If possible I would like to install a drain off outside. How would I drain the system should I need to?

Thanks

M - Norfolk
September 2009