I’m a trainee gas engineer doing portfolio.

[Cbrown99]

For electrical thermostats to test you would establish main burner then turn on low, then burner should completely go out. Why is this?

 

[Brambles]

You need to be a bit more specific, are you referring to an electric thermostat ( a dial with numbers indicated) on a boiler? If so you heat up the appliance and when the set temperature is reached, the thermostat should close the main gas valve via the solenoid.

An electronic thermostat ( thermistor) once at or close to the set temperature will modulate the main gas burner down and / or possibly switch the boiler off.

I suspect that you are refering to a Vapour Pressure FSD ( on cooker ovens). Light burner, should light on low flame, after 30 secs rises to a high flame.
Turn gas supply off, wait 90 seconds and relight - should relight on low flame

 

[Cbrown99]

You need to be a bit more specific, are you referring to an electric thermostat ( a dial with numbers indicated) on a boiler? If so you heat up the appliance and when the set temperature is reached, the thermostat should close the main gas valve via the solenoid.

An electronic thermostat ( thermistor) once at or close to the set temperature will modulate the main gas burner down and / or possibly switch the boiler off.

I suspect that you are refering to a Vapour Pressure FSD ( on cooker ovens). Light burner, should light on low flame, after 30 secs rises to a high flame.
Turn gas supply off, wait 90 seconds and relight - should relight on low flame

Sorry I was referring to the central heating stat, on my testing sheet it states to turn boiler stat to low once main burner has establish and should go out.