I recently had a problem with an air lock in my central heating with upstairs rads staying cool and domestic h/w (thermo syphon) not heating following a radiator change. I refilled the indirect system through a downstairs radiator and all was well, rads up and down AND hot water. Today the h/w is not heating. The expansion tank is full so there is a head of water and I re-bled the upstairs rads, but the last one was rather reluctant to let out any air or water. Is it likely I've got another air lock in the domestic h/w or a blockage somewhere between the header tank and the boiler. If so how on earth do I find it or clear it? Thanks!