Having drained the central heating system to replace a radiator, after following correct procedure,no water is going back into the system. Expansion cistern fills but nothing else.
Open a vent on a radiator - connect a sentinel dosing bottle and pressurise - or I have done this and it has worked - anti bacterial wipe for rad - place mouth over and blow, normally clears it if it's just air.
If it's a blockage - get a length of hose - in tank and push down the pipe feeding the heating system see if it'll clear.
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