3 amp fuse keeps blowing disabling system

[john weir]

i have oil cent heating not combi boiler .system wd work o k for two or three hours then 3amp connected to pump and honeywell controller wd blow. on putting new fuse in right away it wd blow immediately.if i wait a couple hours and replace fuse it wd run for a short time then blow.if i wait overnight it wd last 2 or 3 hrs.iv changed pump , pump valves,controller and thermostat. the system has one motorised valve.sometimes pump runs but boiler does,nt fire up

[stoneyboy]

john weir,
Most likely to be the burner motor.
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