Ferroli Combi 77 FF Cutting Out

Hi,
I have a Ferroli Combi 77 FF.
When running Hot Water ths boiler works perfectly and is never a minutes trouble.
However, when running Heating, it will frequently cut out - all the burners AND the pilot light go out. I have discovered that the Gas Valve control knob pops out slightly when this happens (in the same way that it pops out if you turn it to want to turn off the gas to the boiler).
I must stress that this only happens when running Heating. During the summer when heating is not required, hot water will work fine for months. Now that we are into colder weather I am having to re-light the boiler 2-3 times a day when using the heating.
Has anyone got any ideas of the fault or problem? Is it a faulty gas valve? Why doesn't it happen on Hot Water? The boiler has recently had a full service and is in excellent condition (according the the gas man).
I would appreciate replies by email to snipped-for-privacy@takeout.cimarron.co.uk (remove the takeout. bit).
Many thanks for your help,
Phil
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Phil Brown wrote:

There is perhaps not enough flow of water around the radiators when the CH is on.
IIRC this model does not have a diverter valve the pump only works for CH so it might be as simple as a stuck pump.
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Ed Sirett - Property maintainer and registered gas fitter.
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