back boiler heating circulation issues

[Nas]

I hope someone can help as its driving me round the bend.
Its and open vented system with a baxi 552 back boiler. the water work perfectly but the heating flow pipes to the radiators get hot but there is no circulation . The flow pipe is hot to all the radiators but the return pipe is cold

Things ive checked:-
1) the pump is fine (its new)
2) the 2 port valve controlled by the room stat work is ok.
3) all the return pipes have water in them indicating no blockage.


the sytem has 10mm pipe running off the 22mm flow and return.

Am I Missing something obvious? Any suggestion appreciated

Thanks in advance

 

[wattsie1987]

Sounds like blockage the return will have water in regardless as it feeds through boiler and coil. Manifold?

 

[Last Plumber]

As above. Restricted circulation.

Did pump valves open properly after pump was changed ?

 

[Last Plumber]

As above. Restricted circulation.

Did pump valves open properly after pump was changed ?

 

[Gas74]

Is there a by pass? What's the flow and return temps at bb?

 

[Gas74]

Is there a by pass? What's the flow and return temps at bb?

 

[Vee]

Check for blockage where cold feed meets the heating system return. Also check for failed anti gravity check valve fitted in return pipework from heating often enclosed in fireplace adjacent boxing if still on gravity hot water or not removed when converted to fully pumped.

 

[Elgordo]

Close off the feed to the hot water cylinder.

 

[Gary80gas]

Has system been recently drained down? Air lock?