Our boiler (worcester greenslave oil-fired) has developed a fault. Often when we come back inside after having been away for a few hours, the boiler will no longer be circulating hot water through the radiators and the house is cold. There's no sound of the pump running either, although the usual pilot lights are on as if nothing is wrong. We've discovered by accident that if we switch the boiler off, wait one minute and turn it on again, the problem goes away and the pump fires up again. But then some hours later, the problem repeats itself.
I suspect there could be a number of causes but, based on what you?ve posted- especially that cycling the power seems to reset things- maybe the boiler is overheating due to a lack of flow in the system. This could be due to air in the system or a problem with the pump. I?m assuming it isn?t a sealed/pressurised system- if it is, have you checked it is up to pressure? If it isn?t, top it up.
You will need to trace the call for heat through each stage, and see where its going wrong.
Do you know what kind of system it is (i.e. what kind of valves are there, programmer, thermostat etc)?
Start with some good diagrams of the typical systems:
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On a typical Y plan system (one three port valve sharing the boiler flow between rads and hot water cylinder), you would start by looking at the call for heat signal out of the programmer (using a multimeter to look for a voltage on the wire).
If it is present and getting to the thermostat, then check the output of the stat changes as you wind the temp up and down. If its live, look for a demand to the into the three port valve's call for heat. Check you can hear the valve move if you wind the stat up and down. Then check the output from the microswitch on the valve. If that is there then Check the call for heat at the pump and the input to the boiler (assuming the pump and boiler are wired in parallel - some boilers will control the pump themselves).
At each stage, not getting the expected output can signify a component failure. So no output from the programmer, could be a programmer fault, none from the stat a stat fault, none from the microswitch on the valve, a valve fault (say sticking, not moving full travel), or a switch fault. Pump getting power and not running, a stuck pump. Boiler getting a call for heat but not firing a boiler fault or its in a lockout state.
That model of boiler could have different options of burner and the particular ignition sequence control varies too. If you could post a few photos to a hosting site with a link here it would be immensely helpful. You mention "pilot lights" which are actually indicators. If you have the installation and service booklet there should be a means of identifying what they are. Most sequence controls require manual resetting by pressing a button on the control box but there are exceptions which reset on power down. It's common for the cable to the burner to comprise live and neutral, sometimes a lockout indicator feed and occasionally other cores which control fan run on timer to purge the combustion chamber.
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