big noise and downstairs rads not working

[starmog]

help.all of a sudden my heating system has thrown a wobbly,i get a big noise when boiler fires up sounds like a kinda explosion wakes up house etc and also my downstairs radiators dont seem to be working as well. ive looked at the pump and it seems to be working but how do u tell?does anybody have any ideas?p

 

[breezer]

see this

 

[soubriquet]

Pump Pump Pump..... If the pump is not running the boiler will be very noisy, may heat upstairs by thermo syphon. should eventually trip the overheat stat. check the pump is running. feeling hot is not proof, vibration is not proof. Switch system off, undo screw or plastic knob at centre of pump, insert screwdriver into slot. Turn. Does the pump rotate freely? take screwdriver out, shine torch into end of pump ehere the screw was.... a few dribbles of water are normal... get a helper to switch the heating on. Did you see the rotor start to whiz round? if not, check electrics to pump, if yes, put screw back in. and relax a bit.
If the pump fails to start, the capacitor may have failed, the electris may have a poor connection, or there may be muck in the bearings. The pump bearins run in the water of your heating system. If the water is nasty and black, the bearings are suffering. Flush the system, add an inhibitor, fernox or sentinel are good. And a new pump is about £40.