Ferroli boiler keeps cutting out on CH only

[nightjar]

I have an old Ferroli Boiler that works fine for heating water, but quite regularly it cuts out on the Central heating meaning the radiators never really get warm when it does this. This is an intermittent fault. The CH temperature is always turned up fully, but when playing up it constantly cuts out without reaching an adequate temperature. We don't have a room temperature switch so simply turn the CH dial down on the boiler when we are warm enough. Can anyone give me some pointers as to what might be failing and if there is anything I might be able to try to identify the fault. Many thanks Vince.

[htg engineer]

Could be many things, I would start by looking at the Central Heating Sensor/thermistor as the hot water is working as it should.

When you say it cuts out - can you see if the burner is lit or drops to a reduced gas rate?

[nightjar]

Thank you all help is appreciated - when it is acting up the boiler cuts right out rather than the flame reducing in size. If I remove the cover and watch when it happens I can see the relays on the control board activate and hear a click in the timer control unit as if it is being told by something to switch down before it should cut out. This is with heating control set to continuous and the temperature control on maximum. The pressure in the system is fine.

[htg engineer]

Does the fan stop running?

If it does I would go with:

A faulty sensor/thermistor for the CH temperature.
Or a blockage/restriction in the flow, this could be a slow running pump, blocked filter or choked heat exchanger.

Can you hear the pump, does it seem noisy or does the body seem to get too hot?

[nightjar]

Many thanks again for the help. The fan stops as soon as the boiler cuts out and not before. The pump has three settings and it is set to the highest. It does not get noisy or hot. When it is working fine all the radiators get nice and hot and are of an even temperature all over. I don't know where any filters might be located? A thermistor for the boiler costs about £9 so I will change that as it makes a lot of sense that, that could be the problem. Any thing I need to know about changing it - It looks straight forward enough? If this does not do it and a blockage is the next best answer should I get someone in to power flush or try and drain and refil myself? Thanks again for the great help!