What's up with my Boiler (Glowworm Spacesaver 50)

[Puzzled2016]

I have an ancient boiler, Glowworm Space saver 50, with separate hot water tank. The boiler has been very reliable over the last 40 years,
However, lately, I have had to relight the pilot light several times. You are probably thinking that I need a new thermocouple.

But if the boiler is not on (boiler thermostat knob set off), the pilot light is stable. And viewed through the pilot light observation window I can see a healthy looking blue flame playing on the thermocouple

When there is a call for heat (eg when the hot water tank thermostat signals the water tank temperature has sufficiently dropped), the boiler lights up and and until the boiler thermostat signals gas off, the burn is normal.

The problem arises then. At that point either the pilot gets extinguished or it stays on. If it stays on the boiler continues to cycle on and off until the water tank thermostat says its water is hot enough.

What could be going on? I have a vague suspicion that it's all related to the weather.
But my only evidence for that is that if the boiler has been shut down over night (electricity off, boiler thermostat off, but pilot light left on), then it seems to be almost certain that the boiler will go through multiple burns to get the hot water up to temperature, and the pilot light will stay on.

 

[kidgreen61]

Gas valve fault, you need a gas safe bod to look at it.
Those boilers are the most simple, basic things ever built, I hope you can obtain spares, there's nothing else to go wrong with them,

 

[gas112]

pilot being drawn away from t/couple get it serviced and pilot injector split and cleaned thoroughly very common fault on these

 

[Puzzled2016]

Thank you for your suggestions K and G. However whatever is making the boiler behave this way has to account for:-

1. the pilot light is stable if the boiler is off (ie the boiler thermostat turned to OFF and electricity switched off.)

2. I get multiple burns of the boiler to bring water tank to temperature AND pilot stays on afterwards, provided that the boiler has been off (see 1, for meaning) sufficiently long for the hot water tank to cool to the point that its thermostat would call for heat.

Perhaps I am being thick but I don't understand how your suggestions explain this.

Fortunately, I am not being seriously inconvenienced, in this Summer's heat, by just switching the boiler back on when I really need hot water. I only use hot water for washing dishes by hand and having a shower- and even tepid water is often OK for these jobs. The hot water tank is fabulously well insulated and its water stays hot for long time. Also it quickly comes up to temperature when boiler restarted.

I may eventually have to call in someone to have a look at the boiler, but it would be useful to have in advance some idea as to why the boiler is behaving as described.

 

[gas112]

I may eventually have to call in someone to have a look at the boiler, but it would be useful to have in advance some idea as to why the boiler is behaving as described.

you have got an idea its a pilot problem