thorn apollo supressor question

[falloverpete]

Hello, this is a question i have of an Apollo thorn EMI boiler. Previously the boiler would fire and cut off, fire and cut off to heat the water, the firing process being approx 30 seconds. After a while this stopped and it just fired untill the tank water reached the temperature and then the boiler would fire/ cut off to maintain that temp.
Why did it do this and what part of the boiler regulates the timing of firing the the gas valve?

Thanks in advance
FP

 

[Gasguru]

The boilers own high or low thermostat (dependent on the switch setting) regulates the gas valve.

 

[falloverpete]

Would you diagnose a faulty high/low switch or a faulty thermostat? where is the switch thermostat located on a thorn EMI apollo? in the water tank?