heating system additives

what is the best way to add additives to my heaitng system, ive read a few different ways on here ( i have open vented sys, non combi boiler, hotwater tank header tank in loft, thermostat) would it be drain some water out of my system and add it to the header tank, or to isolate a rad and drain some out of it and inject the solution into said radiator?
just wondering really, think i might put some into my system as i dont know if or when it was last done

If you have a feed and expansion tank in the loft it is much much much easier to tip it there.

To get it mixed quicker: Tie up the ball-valve in the F&E and bale out most of the water from the F&E first. If it contains sediment, clean this out with a sponge. Tip in the inhibitor and mix it well with fresh clean water (otherwise it may settle to the bottom).

Then drain off a bucket or so from the drain-off cock on one of your radiators (if you haven't got any, this is a good time to add them) or elsewhere.

Then untie the float and the flow of water will wash most of the inhibitor down and into the system. Over a longer period, the slight movement of water from expansion and contraction of the water as it heats and cools will gradully mix the contents of the F&E down, but this takes a long time (especially if it is not well mixed).

BTW it is a good idea to give the system a good chemical clean and rinse first.

nice one, cheers for that mate