Hi there, sorry for the delayed response. I live in a relatively clean water area (stoke on trent) and I say this looking my kettle element which is virtually scale-free. I did go up in the loft and had a look in the expansion tank and the water was clean; there was very little limescale on the side of the tank and overall everything looked in good nick.
I did some research and experiments this weekend with the various controls in the system and what I have noticed is explained below. I have my thermal-store hot water cylinder normally set at 62 degrees; as hot water is being used to prolonged periods of time, the temperature inside the cylinder drops. As you'd expect the boiler kicks in and starts re-heating the water inside the hw cyclinder and I suspect that the difference between the flow and return gets bigger than 20 degrees C. This causes the boiler to work at its max temperature (80 degrees C) until there is a difference of less than 20 degress C between flow and return. It is only during this time that the "boiling water" noise can be heard as the pump circulates the hot water through the hw cyclinder; once the hw cyclinder reaches a temperature that is withint 10 degrees of what it shoudl be. the "boiliong water" noise from the pump stops and the pump becomes smooth very hardly any noise at all. Let me give you an example....every morning me and the lady have a shower pretty much at the same time, me in the en-suite and she in the main bathroom. Both our showers are Mira mixer showers with the hot water feed coming from the thermal-store cylinder. When we finish the showers, I make a point of checking the temperature on the HW cyclinder and it is ususally down to 48-50 degrees C (from 62C) and my circulating pump is pumping away hot water and the annoying boiling water noise can be heard from the bedroom. We then have some b;fast, coffee, morning news etc etc a procession which usually lasts for about 30-40 minutes and again once this is over and done with, i go back upstairs to have a look at the temperature on the HW cyclinder...usually by this time it gets to about 58-60 degrees and the pump is still working away but this time no more "boiling water" noise. So there you have it in simple terms. What i have just said could be a lot of non-sense but this is what i make of it so far.
Bengasman, the reason I went for an open vent system was because the plumber said that because the previous system was also open-vent, if we go to a sealed system the radiators pipes might leak because there were never designed to take that sort of pressure. I know hindsight is a wonderful thing, but I have to live with what I got. Can you give me just a few exmpales of what could b wrong? I do agree with your suggestion of somebody competent having a good look at it and give it a thorough inspection...this is what I intend to do.
Thanks.