I had a new boiler installed 3 years ago. I have had 3 failures of the heat exchanger, the first repair was carried out by the manufacturer under guarantee, the subsequent repairs have been carried out under the manufacturers repair service (on payment of a monthly fee). I contacted the installer this year to ask them to sort out whether it was a "installation" fault or a boiler "fault". The solution that has been reached between them and the boiler manaufacturer is to fit a pressure relieving valve to control the water pressure. I have disputed the bill for this installation as I feel that the water pressure was an existing factor when the boiler was installed and therefore the installation/boiler was not "fit for purpose". I am correct in this ?