if you have a small header cistern in the loft, turn the cold water off at the mains drain some water out of the system (enough to empty the tank) pour the inhibter in to the cister, turn on the mains, bleed any rad as requied.
If a combi drain the system to the boiler reads 0 bar.
Using a constrate inhibter put in to the system by the filling loop or rad, top up as normal
erj, March 2010