some radiators not working

Hi - a few apparently random rads within a single zone have just stopped working. They have been checked for air (they're full of water). The pipe feeding one of them is hot at the on/off valve, but two others are cold at the valves. Our pressure dropped just before this happened, I adjusted it back to normal, and switched the heating back on and all seemed to be well, but then I discovered that these rads had stopped taking hot water in.

What could cause this - a build-up of sludge maybe? Will I have to remove a rad or two to check it (I have installed/moved rads before, so I can handle this!)? Thanks in advance for any help.

Purge

Try purging out the system with fresh water first. Get as much flow through the complete system available. If you can bypass the boiler and increase the pressure above 30 psi while you are purging, it may help to flush out the crap. If not, set your pressure to 20 and let it purge for 1/2 the day. Then if all else fails, remove your radiator and take it to an automotive radiator repair facility for an acid bath. You will end up with a like-new (and unpainted) radiator.