My Central Heating system has no header tank. The HW cylinder is directly above the boiler and there is a large header tank for the HW cylinder but no separate header for the CH. The central heating seems to get its water topped up through a 1-way valve in the hot tank or boiler. I can drain the CH and get the dirty liquid out then it fills itself up again from the HW tank but how would I get flushing fluid or anti corrosion fluid into the system?