Back boiler won't fire up

[norfolkinesox]

I have a vented heating system with a back boiler,I have replaced the main pump and isolation valves after draining system,I turned off electricity to replace pump.i have refitted pump and new isolation valves(other didn't seal ) and refilled system and bled radiators however when I select heating or water or even both back boiler will not fire up
I know the three way valve is operating ,am I right in thinking the micro switches in that is what kicks up the boiler,and pilot light on in boiler
Any help would be appreciated

 

[norfolkinesox]

My back boiler fired up then went off again when heating selected only,then for a time trying heating water then both it wouldn't ,now it's running on just heating but as I type this it's gone off again after running for only a minute...........the control
thermostats quite high

 

[Agile]

Are pump valves open?

Have you bled pump?

Tony

 

[ianmcd]

Sounds like you have an airlock , possibly in the pump try bleeding it

 

[norfolkinesox]

Thanks for replys tried all above am now thinking the radiators are sludged up as get heat to radiators but one gets Luke warm in areas and every time heating is on its intermittent which get any kind of warmth,the pipes are hot to radiator but radiators not getting hot,check valves all ok,bled them no air...........would you say remove and flush radiators?

 

[CBF]

you have made sure you fitted the pump the right way round