How do you drain down a vented system. I have only ever drained a combi
boiler. I seem to have two large header tanks in loft.

It's slightly easier than a sealed system, as there is already a way for air
to get in at the top. Just tie up the ballvalve/turn off the water supply
and open up the drain cock at the bottom (preferably with a hose to a nearby
drain). Any 'U's (such as drops to downstairs radiators with concrete
floors) should have separate drain cocks.

Christian.