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When the burner switches off on my oil fired central heating about 5 mins later hot water comes out the overflow from the heating expansion tank. I tried cleaning the system and bled the whole system but this had no effect. I also tried the following : with the heating off I let the expansion tank fill, I then shut the mains water off to the expansion tank. When the heat was put on the expansion tank drained, I ran the heating for a couple of hours then when I switched the heating off about 5 mins later the tank refilled with hot water from the bottom outlet pipe on the tank. The reason I am getting the overflow is that when the heating is on the expansion tank loses water into the heating system allowing the tank to fill from the water inlet then when the heating is switched off hot water comes back up the pipe into the tank then the tank has to overflow. I have also noticed if I switch the heating on before the radiators have cooled down there is a lot of noise from the first radiator in the system turning this radiator off puts the noise into another radiator. I am wondering if the boiler or burner could be causing the problem as most of the time when the heating is switched off (power removed) the burner will not relight as the reset button has popped on the thermostat and I have to reset this although the thermostat seems to be working ok as it switches the heat on and off at the correct temp.

barry1,
Bleed all the radiators, is your central heating pump on its maximum setting if yes turn it down.
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I have bled all radiators and tried adjusting pump at different settings. This system has been in 6 years and only started giving this problem in december.

barry1,
With the pump on its lowest setting did things improve?
Water is not compressable so there must be air (ie no water), somewhere in the system. You just have to find where.
Is there a vertical pipe with manual vent hidden in a cupboard or in the loft?
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Hi Barry, firstly the problem with the thermostat tripping when the boiler shuts down, could be the result of no pump over-run, meaning the heat in the boiler,at shut down, is not being dissapated into the system resulting in an overheat. On shut down only the burner should be controlled. It would be worth while having a look at the electrcal control of the pump and determine if an over-run is fitted or possibly not working.
The problem with the header tank would indicate incorrect pipework in that the the location of the pump in relationship to the cold feed/expansion pipework should be investigated.