Boiler Flame failure

[ALANKING]

My Boiler is a THORN MARATHON 56/76C converted to run on Propane.
The pilot light fails when the Boiler thermostat turns the Burner off.
A Heating Engineer fitted a new Thermocouple to the pilot but this does not cure the problem.
The Pilot will burn continuously if the burner thermostat is turned off and the burner will light O.K. when the Boiler is thermostat (knob) is turned. and it will relight when the knob is turned on and of manually, but when the burner is turned of automatically with the thermostat, the pilot goes out with it.

 

[chrishutt]

It doesn't make sense that the pilot remains alight when you turn the stat off manually but not when it switches off automatically. Are you sure about this?

Is there an overheat stat or something linked into the thermocouple circuit? If such an overheat stat were operating, it would shut down the pilot.

 

[DP]

Chris, the pilot is going out because the burners coming on causes gas starvation at the pilot. Changing the thermocouple will be 'taking a shot in the dark' to effect repairs

Boiler service might cure all your ills.

Call an engineer who has LPG ticket (check his CORGI card at the back), and is able to diagnose the fault and not be a 'bottle changer'. Bottle changer term comes from valve TV days. Engineer who called to repair these, carried a suitcase full of valves, which were changed one by one until the TV was fixed.

 

[ollski]

Chris, the pilot is going out because the burners coming on causes gas starvation at the pilot. Changing the thermocouple will be 'taking a shot in the dark' to effect repairs

Boiler service might cure all your ills.

Call an engineer who has LPG ticket (check his CORGI card at the back), and is able to diagnose the fault and not be a 'bottle changer'. Bottle changer term comes from valve TV days. Engineer who called to repair these, carried a suitcase full of valves, which were changed one by one until the TV was fixed.

Doesn't explain the difference between manually turning the stat and the stat reaching temperature on its own.

 

[CH4]

ALANKING,

The problem may be due to your boiler overheating.

If your boiler has an overheat thermostat connected in series with the thermocouple, then this may be just doing its job - interrupting the current to a coil, within the primary gas valve, whenever the boiler overheats. The coil acts as an electromagnet (energised by the thermocouple) and holds the gas valve open. Thereby, keeping the pilot burner lit.

A heavily scaled heat exchanger or a slow running pump can cause overheating. Also, some boilers have a pump-overrun thermostat that keeps the pump running for a short while after the main burner extinguishes. This removes residual heat from heat exchanger, to prevent momentary overheating. If your boiler does have a pump-overrun thermostat then it may be stuck in the switched live position.