read this see if this would work for you
if you have modern rads with the large vent nuts loose some water out of the lowest rad vent or drain point a couples of litres will do,(needs to be more than the additive being put in ) refit the plug, go upstairs and remove the upstairs rad whole vent nut again , now pour in all the additive via a funnel and small bore pipe, (PROTECTTHE CARPET in case of SPILLAGE) refit the large nut, repressurise the water system.start boiler to circulate additive
some fitters will inject additive in via a pressure pump
wirecutterman, October 2010