the coil is a magnetically operated device which sends a charge down a valve on the fuel pump. This valve subsequently opens sending oil to the nozzle. the magnetic field with in this coil is measured in resistance (this is nothing to do with the water side of the heating ) this valve is set on...
check tank stat to check water is not satisfied also check valve is the right way round
not all valves are mid position and some will prioritize hot water (valve could be wrong way round)
straight forward job turn of the pump valves remove the pump head using allen keys and replace with the new pumphead do not have to remove whole pump or touch any pipe work this will eliminate any possibility of disturbing any pipe work
probably down to combustion setting but have come across problem before will sticking coil use a multimeter to check resistance across coil when cold warm up in hand and check again need 90+(ohms) for a successful start up
may be due to running with lack of water as pump is new and system not been on check pump valves fully open and bleed pump by loosening bleed screw on front of pump (damage may already be done though)
:(