i have just had a new gas valve put onto my ideal classic rs40 boiler as the old one had packed in. since it has been on i have noticed the water is red hot and sounds like it is boiling. its sounds like a giant kettle. This also causes banging in the pump. i have turned the boiler down to one (it goes from 0-6)on the dial, and the thermostat on the tank is set to 50C which is the lowest it can go and still it is happening after about an hour of being on. When the boiler is on it is constantly fired up but my friend thinks this shouldnt be the case. Also, the mechanical 3 port valve wont switch over to hot water only.Any ideas what the problem could be? i would really appreciate any advice or ideas. Thanks