My Glowworm micron 60FF runs fine from cold but once the system is warmed up (but not fully up to temperature) it keeps going around the cycle: turn on, fan starts, gas ignites, runs about 10 seconds, turns off, waits 10-30 seconds, repeats. This is all with the central heating thermostat still calling for heat.
So, is this indicative of a failing themistor? In other words is it indicating that the boiler water temperature is hot enough and hence not allowing the boiler to run? The boiler stat is turned full on.
Also, when it is running it does a good impression of a steam engine .... chuff .... chuff .... chuff.