Hi again. I have some more information on the boiler. when the boiler is at 70 degrees. I can turn the dhw tap on and it will start as normal,but the water is only just warm. I tested it by turning it off and on three times and it turned off then on as it should do. as soon as it reaches 90 degrees the burner will not cut in and the dhw is coming out cold.please can you help me in this matter.