Grandee boiler locking out

[James evans]

Hi guys,
New here. I’ve got a old grandee boiler that started to display a fault about 8 months ago. The boiler would periodically lock out after a few hours of running and would sometimes take a couple of resets to start. Recently the fuel tank ran very very low and I manually poured in 100 litres of oil that I had stored to keep it running until my delivery arrived. Ever since I poured in the 100 litres the boiler has not failed once for roughly 2 weeks where as before it would fail every day.

fast forward to today the boiler has locked out again but this time it would not restart. I could here the priming pump run, electrodes spark and then ignition. The boiler sounds normal but would run for roughly 15 seconds and cut out. I read up about the photocell inspecting for lack of flame before ignition and for a flame afterwards.

I removed the photocell which was clean and tested the resistance by covering it with my hand and it showed open loop when dark and resistance when it saw light.so the photo cell worked.

however when I went to reinstall it I noticed that the photocell had two positions that it could locate into. One at 90° from the position it was previously set at. I measure the resistance at both positions and the resistance was a lot lower at the new position. Not only that but the photocell was reconnected the boiler runs where as once returned to the original position it cuts out again after 10 seconds.

my question is, how could it have run in the previous position if it appears not to have been facing the flame correctly? Why is there two different positions for the photocell and is there any reason it is unsafe to run it in the other position?

I have removed the photocell while it is connected to the boiler and ran it through the sequence of no light, ignition, light with my torch and my hand and the photocell is doing it’s job perfectly


Thanks for your help

 

[oilhead]

How long is it since the boiler/burner was serviced? any contamination on the flame diffuser can affect the flame sensor. It is not uncommon for the p/cell to appear to face away from the flame, but as most are side and end sensing, then they still see the flame. Turning the p/cell to full face can result in the control box receiving a false flame signal from the electrodes prior to ignition causing lockout or continuous purging.

 

[James evans]

Awesome thank you so much for your response. It is due a service.is the diffuser another work for the spray nozzle?

I think you are right in saying that the p/cell is side and end sensing.
so previously, you think it could have been side facing to the flame and was picking up enough light from the ignition to make the switch? Causing it to lock out prematurely.

now I moved the p/cell 90° and is only end sensing it is only picking light up once there is a flame?


Thanks again so much it’s much appreciated the boilers been doing my head in

 

[oilhead]

The diffuser is the plate that is around the nozzle. It evens out the airflow from the fan to mix with the nozzle spray and create a fine mist for clean combustion. The part about false flame from the electrodes is that if the p/cell sees the spark, the control box assumes the burner is lit so does not open the solenoid valve, eventually locking out.

 

[James evans]

Thanks so much, the boiler will now only run if it’s getting direct light from a torch aimed at the photocell. If I keep the photocell happy with torchlight, I can see all the stages of ignition and the boiler flame that seems to be running well. So I’m guessing either the nozzle or diffuser is soiled and the flame isn’t running effective enough to make the switch from the photo cell. Or the photocell it’s self is on the way out. The solenoid coil from the pump is working well so that’s ruled out.

 

[oilhead]

If, as I suspect, your burner is an EOGB Sterling, if replacing the MZ photocell, it needs to be the 80mm version rather than the shorter standard version.