Worcester greenstar utility 18/25 oil constantly locking out.

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I put a spare photocell in and it ran for a couple of hours great and then started playing up. I am not sure the HT leads are in the way are they.
I took a pic with the torch shining down where the photocell is to see if the path was OK. You can just see if you look carefully? If I pull the photocell out and watch for the spark it isn't that easy to see but I don't know how bright it should be or what else to try.
So I started the boiler with the photocell in my hand and then showed it light once fired so i could see the flame and it looks clear. Sometimes the fan spins just for a few seconds to start with so I will replace the start capacitor anyway but pretty sure this isn't the main problem.
Are you able to remove the motor and test it on the bench, Robbie? If you can the motor should start readily each time but if you can stop it spinning with your fingers its goosed.
John :)
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What state is your blast tube in? Clean it up so it's like new, drill a hole through the fins in line with the photocell also if you want to, nothing huge, 2.5mm is adequate
A good test for the photocell is more or less what you have done, if I'm reading you correctly when it is removed from the burner.
As the burner starts up, keep the photocell completely dark.
Immediately the burner lights, show the photocell as much light as you can and the burner should keep running.
There is a certain matching between the photocell and the control box....how accurate this is I can't comment unfortunately but do check you have the correct one.
John :)
The MZ photocell is available in two lengths. The longer one is used as standard on Wallstars with a long blast tube, and larger Sterling burners, except for Worcester down firers such as this boiler. I have often replaced the standard cell with the 80mm cell and it meets with some success. The flame pattern on these downfiring burners is very restricted, particularly if someone has adjusted the head setting. Paying particular attention to cleanliness of the diffuser and the blast tube inside is also essential. Any 'drip' carbon on the diffuser can distort the flame sufficiently to diminish the sight for the photocell.
Ok here’s a suggestion.

I had a problem with the same burner with the Satronic control box. Basically the burner would occasionally fire up ok and on other occasions it went to lock-out.

With the Satronic control box the normal firing sequence is 12 seconds pre-purge, flame establishes, 5 seconds flame proving and if cell sees no flame the. Burner will lock-out.

The fault on my example was that the oil pump was letting by. In other words oil was passing through the nozzle during the pre-purge and as the ignition starts immediately on these burners the oil would light so the cell saw false light. The difference in timing for this fault is that you will immediately hear the burner fire and then the control box will lock-out after around 20 seconds instead of 12 seconds.
Thanks for replies. I will start with the easiest! A longer photocell and good clean of blast tube area. I will report back.
Thanks to all the great suggestions for helping with the boiler. I have had help fixing my boiler for over 10 years with some of the same guys helping every time - that's quite something if you ask me.
I am three days in after oilhead suggestion of a longer photocell and not one lockout! The family are thrilled to have reliable heat again.
Thanks to all who helped. Touch wood I can move on to fix other problems now!
Hello sorry to jump in on this, I am having similar problems I’ve had two people out to look at the boiler and new control box, and a lot of other stuff. It does fire up but only if I repeatedly click lockout maybe 10-20 times, then it will eventually fire up? Could the longer photocell as previously suggested solve this? I have tried looking for an 80mm photocell but wondering if anyone could link me please, thanks in advance!

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