First of all you should find out if you have a closed system or an open system. If you have an open system you will have an expansion tank (in the roof probably) if you have a closed system you will have an expansion vessel located somewhere with a pressure gauge on it.
You should look for any leaks. If you cannot find any I would turn off the mains stopcock to your expansion tank, if you do not have one then tie up your ball valve so that fresh water cannot top up your tank if the water level drops over a period of a day or so you will know you have a hidden leak somewhere. You need to check the water levels when the system is cold so that readings are not changed by expansion of hot water.
Next you should check your pump speed is it running too fast. This may cause water to flow into your expansion tank causing the water to oxygenate.
The reason for all the above checks is because it sounds as if the heat exchanger has a build up of limescale which stops the heat being efficiently dispersed into the water circulating in the system causing it to overheat and cut out.
If you run your heating system at the lowest setting on the boiler you will probably not hear the noises that you mention this may help to confirm the above diagnosis.
To fix the problem you could use 1 can of Everad descaler and sludge dispersant in your system for 2 weeks or 2 cans for 24 hours and then flush the system and refill, also add a corrosion inhibitor.
You may find that in using the descaler that whilst you fix the boiler any weaknesses in the system may be exposed and you have to fix a few leaks. You should also check all your radiator valves to check the glands are not leaking.
I hope that this may help
Charles
March 2010
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