Inhibiter into central heating system - How much to drain ?

[Bankrupt]

I need to put some Inhibiter into my central heating system. If I bung the Sentinel X100 in the expansion tank and leave the water make-up on, how much water would I need to drain down to get it all (or most of it) into the circulating part of the system ?

The expansion tank is in the attic ( two storey house ) and I don't want to shut off the water make up (unless absolutely necessary) as I'm afraid I might drain down to the point where air is introduced to the system and I end up having to bleed all ahe radiators.

The system is currently full of clean water - My plumber drained down when extening to two new radiators but didn't put any inhibiter when he refilled the system....

 

[bengasman]

Draining one standard bucket full should do.

 

[Bankrupt]

Draining one standard bucket full should do.

Thanks for the advice ! - .

Is that a metric bucket or an imperial bucket ? . - ( Only joking )

 

[bengasman]

Missus says its orange

 

[Bankrupt]

Missus says its orange

Drat ! - I've only got a red one... I'll have to see if I can buy an orange one on the way home tonight....