normally put it in BEFORE the system is filled depends on system non pressurized tip it in the feed and expansion in the loft
a pressurized system in through the large type bleed valves via a tun dish or flexi pipe with no pressure in the system
if you have the very tiny bled valves in your system may have to partly drain down disconnect a rad upstairs remove the valve and put the inhibitor down the pipe then reconnect pipes and refill system
the experts use a pressure pump device to save the drain down hence the cost
wirecutterman, March 2010