back boilers fired up but radiators cold?

[norfolkinesox]

we have an old back boiler ,its fired up when panel has water /heating selected,but the radiators are cold ,but ive noticed the main pump was a little noisey on start up,it still is a little but nowere as bad.the room thermostat is turned up and checked everything thats simple ie three way valve is in mid position when the control panel has both water and heating selected,any idea guys please
will the main pump be knackered?
just done a bit more investigation,i turned of hot water then just selected heating ,the radiators are getting hot but sound like a lot of air or bubbling in pipes near to main pump through airjec???
soory guys just to update,everything seems to have settled now got heating and water on,pump little quiter and bubbling calmed down,so could it have been pump starvation so the noise was cavitation? if so what can i do as i remember this happening once before a few months ago

 

[kevplumb]

check the header tank has water in it

 

[norfolkinesox]

hello kev,the header tank has water in it,since this morning it seemed to bleed its self and radiators were hot and no noise from pump,but this afternoon it kicked in and was making a noise again,so i knocked pump off tried to bleed radiators and there wasnt much water flow through them so ran system and it was bubbling through airjec and then radiators were full and hot,so i knocked pump off bled radiators and they were full,so it seems to be draining,have you any ideas why it may be doing this.
i am mechanically minded but not too sure on the system.
any ideas would be great

 

[norfolkinesox]

can anyone suggest why radiators might drain,really appreciate any answers
thanks