Worcester Heatslave 20/25 not firing after new rad fitted

Hello there. Oil boiler has been running fine. Yesterday I drained the system down to fit a new radiator. Pressured the system back up and bled the air out, turned the HW and CH temp dials to lowest setting and turned CH on so the circulation pump would run without firing the boiler to flush the last of the air through and bled the radiators. Ran for about 30 mins. All good, no gurgling and no leaks.

Turned the temp dials up. Fan noises came on and ran but burner didn’t fire. Went to lockout. Reset 5-6 times, same result... similar to if I have ran out of fuel (I haven’t).

Any ideas why draining the system down means that the boiler now won’t fire and what the problem might be. Thank you.

What burner do you have fitted? If it is running through the prepurge, then it is not a limit stat issue. On the face of it, there is no reason why draining down should lead to burner lockout.
Loosen the burner and try to start it. As soon as it locks out, remove the burner and look for fuel on the blast tube. If there is none, then work back through the solenoid circuit. If there is fuel, then suspect the ignition transformer, but that may be integral with the control box. Also a photocell fault can cause your symptoms

Thanks! It’s a B11. I will do as you suggest.

Just turning down the temp dials might have left the burner firing for short periods, and if so possibly overloading the transformer. Check it to see if there is any visible sign of a bump or scorching. You can also test for a spark by withdrawing the burner, REMOVING THE SOLENOID LEAD FROM THE PUMP COIL, then trying to start the burner. Keep the blast tube in shade to prevent stray light to the photocell. as soon as the burner starts, you should get power to the transformer( a voltage sensor is useful here) and you should get a spark. If no spark but a feed, then the EBI unit is goosed. Should you need to run the pump again without the burner, then just disconnect the burner plug.
If you have no power to the EBI then the control box is suspect.

Great suggestion about pulling the burner plug, thank you. I shall definitely do that in the future. I just learnt that a friend of mine is a qualified boiler engineer... so rather than blow myself up, he’s going to take a look tomorrow a.m. Will report back.

Just for completeness... the problem was in the oil pump. Something to do with a sticking valve. It meant that the there was actually a puff of flame right at the very start of the purge cycle, then nothing, then it seemed to flame again right as the lockout cut in. Not something the boiler guy had seen before. Anyway... new pump fitted and all great. Thanks for you suggestions.

New one on me today. Heatslave 20/25 locking out. Changed nozzle, checked oil pressure and it ran ok. Left to run for a few mins and then disconnected my oil pressure gauge.
Boiler then wouldn't fire and went to lockout after 12 seconds but with no obvious sound of ignition. Decided to check for spark, so removed burner, disconnected solenoid coil lead and turned burner on. Spark, followed by immediate flame a fraction of a second later, across this chaps kitchen! Made me jump.
Decided oil stem was stuck open, causing immediate flame that would result in lockout caused by false light. New oil pump and all running correctly. [I don't carry spare oil pump stems as spares.]