Glow-worm Space Saver 50 - Problem

I wonder if anybody has any ideas about the likely cause of a problem that appeared from one day to the next on this boiler. Boiler had been very reliable previously.

I have had a gas-safe plumber round to look at it, but after 3 hours he has not come up with a solution.

Symptoms are that, from cold, main burner comes on as per normal, and shuts down when boiler thermostat reaches trigger temperature. However after the temperature of the water in the boiler has dropped and the main burner lights again (again as per normal) at some time during the second burn the gas supply to the both the pilot and the main burner is cut off.

If you wait the prescribed 3 minutes and re-light the pilot, then on the first main burn it operates as normal, but on the second main burn again the pilot and the main burner get snuffed out prematurely before the boiler thermostat reaches trigger temperature.

Testing to see if pilot will stay alight continuously without main burner operating (by turning the boiler thermostat to OFF) confirmed that the pilot light by itself was stable and flame height looked normal.

Engineer checked Expansion and Supply tank in attic (OK), looked at flues for blockage (no blockage), checked pilot light and main burner flames (seemed normal), and then decided problem lay with the thermocouple and replaced this with a "Universal" thermocouple.

At several points the boiler seemed to go back to normal operation but when the outer metal jacket was replaced premature cut-out reappeared.

Finally, before leaving without solving the problem, engineer decided that cause of problem lay with the pilot light burner. He said that this could be partially blocked though it could be seen to deliver a very respectable flame when the outer metal jacket of the boiler was off.

First question is: Is a faulty thermocouple consistent with the symptoms?
Second question is: Could a fault develop with the thermocouple from one day to the next?
Third question: Is a "Universal" thermocouple likely to be suitable for this boiler?
Fourth question is: Could the pilot burner get suddenly blocked from one day to the next?

Any light you can cast on the problem, gratefully received.

Call someone who knows what they're doing. A simpler boiler is hard to find. Please read the forum rules.
There are many reasons for your boiler to cut out as described. A thorough service and inspection should find the cause.

Trawling through the forum over the weekend, I found a couple of posts that seem to describe a similar problem with a space saver 50 that I had experienced. In both cases it was suggested that there was a partial blockage of the pilot burner injector.

This proved to be the problem in the case of my boiler and when the engineer returned on the Monday a blast of compressed air administered to the pilot injector not only changed the appearance of the pilot flame when re-ignited but also seemed to fix the problem. The boiler is back to working as it should.

With hindsight, I could have immediately directed the engineer's attention to cleaning the injector. Informed with the appearance of the pilot flame after cleaning, it was obvious that the previous pilot burn was not right.

I didn't take long for the engineer to fix the problem on his second visit, and if he hadn't spent so long on the first visit looking at other things, I could have avoided a freezing weekend.

So if you experience your space saver 50 behaving like mine did, have a good look at the pilot flame through the observation window. If it seems to flicker, and the top of the flame is yellow and the flame is not nicely divided into two clear blue flames with one playing on the thermocouple and the other directed towards the main burner, then get the pilot injector unblocked as a first step.

By the way this boiler had been very reliable for some 36 years, and I now hope it will see me out.

It may not be the most efficient boiler but what modern boiler would be so trouble free for such a long period? And according to the figures on the inside of the removable tray at the bottom of the boiler (they show nominal input and output heat) this boiler is 74% efficient.