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Rick Ridley
Rick Ridley, UK Heating Engineer
Category: UK Plumbing
Satisfied Customers: 2708
Experience:  30 Years on the tools, now retired, Ex CORGI
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I have a Worcester Heat Slave 20/25 boiler. For some time

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I have a Worcester Heat Slave 20/25 boiler.
For some time now when switched to heat water only, the radiators sometimes also heat. Not always.
Now, about five minutes after switching on (water heat) the lock out lamp lights. I wait 2 minutes and press the reset button and it fires up and runs for about 3 minutes until it locks out.
Repeating this the lock out lights after shorter and shorter times.
Also noticed that the pressure gauge reading increases as water heats.
Can you diagnose please?
Hi I am Rick, UK Plumber, and I specialize on Boiler problems,OK several faults here, for the rads to heat on hot water only the three port motor valve is passing, this could be because the rubber ball is damaged in which case a new valve will be required, or as it is intermittent it could just be the spindle is stiff, so you could try removing the head and check the spindle it should be free to turn with fingers (only through 90 deg)if stiff apply wd40 and try again, if free then it could be the gear box in the head, again replacement is the only cure. Re the lock out, I think this is most likely the flame photo sensor is in need of a clean, remove the control box on the top of the burner and you should see it in between the electrode connections. It is normal for the pressure to rise as the boiler gets hot cold you should see around 1.5 bar all hot around 1.8 to 2 bar much higher than this and I would need more detailed info, Rick
Customer: replied 5 years ago.

Hi Rick,

I have removed the control box, the box with the reset button, and the wiring base but cannot see photo sensor or electrodes. Next behind control box is Danfoss EBI ignition transformer. Where do look from here please?

Regards Colin

Customer: replied 5 years ago.
Hi Rick,
Have found the photo sensor removed and cleaned (Looked pretty clean). It is to the left of the control box. Just going to replace everything and try it.
Regards Colin
Customer: replied 5 years ago.

Replaced everything and tried again. No change, locks out after 5 minutes. Any more suggestions?
OK could be you have a blocked nozzle, can you tell if it actually fires for a time or is it just trying to ignite, if it fires without smoke and not pulsating then just stops it could be the pressure is failing due to a restriction in the oil line or possibly control box failure, Rick
Customer: replied 5 years ago.
It is firing because the water pipes in the boiler heat up.
Regards Colin.
OK if it is firing for at least a few minutes then shuts down it has to be a lack of oil, or pump/motor problem, you need to check that a good continuous flow of oil is available at the pump, if OK then it is likely that the fan/pump motor is failing as it gets hot this could be a dry bearing, it could also be a faulty solenoid valve coil that fails as it heats up, Rick
Customer: replied 5 years ago.
Hi Rick,
In your email the "reply to expert " link does not work.
I measured the resistance of the solenoid at lock out and it showed OL on the 40 K range. As the temperature dropped the resistance slowly dropped to about 2.5 K. I have ordered a replacement solenoid and am awaiting delivery. My boiler is not yet working so do not yet know if your answer was helpful. Will accept your answer ASAP.
Regards Colin.
Certainly looks like the solenoid failing as it warms up, wait to here, Rick
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