The radiator in our hallway sometimes does not heat up but if i turn off all the other radiators downstairs the hall radiator heats up, so i spoke to the plumber about it and he told me that all the radiators need to be balanced and it could be a long process so you could do it yourself and save your self some money, so as i didn't want to appear stupid i told him ok, so how do i go about balancing the radiators, and why should this need doing?