Hi. We have a potterton he plus 28 combi boiler fitted 2 years ago in a house that previously had no boiler or heating system. When we turn on the hot water it asks for heat, but when the temperature gets to about 80 degrees it starts a knocking sound and pressure gauge wobbles between 1-2 bar. This stops either as boiler cuts out or we turn off the tap. We had a spirotech valve fitted 6months ago after the filters (?) were cleaned. This helped until about 2 months ago. Water seems to be clear when we empty the spirotech valve. I have no experience and may well call an engineer but just want to know what to ask them. Any help appreciated. Thanks