I av a Biasi 24s and when the dhw is turned on the boiler fires on and off not letting the water get up to full temp, but when I was avin a check round I found when the sealed unit front cover was off it worked perfect
my boiler is a biasi 24s about 11 years old, I have changed dhw diaphram but when I run the hot water tap the boiler fires up for a few seconds then shuts down and carries on doing this therefore the water goes hot and cold not reaching its max temp