Hi, I have a glowworm 24 ci combi boiler and since having the heat exchanger cleaned of scale about a year ago it has been working fine. This morning however there was no hot water and the boiler displayed a red light with F4. I switched off the powere supply then put it back on again and the boiler worked fine. Could someone explain what the F4 means ( I have mislaid the manual ) and is this likely to be the start of more problems or simply a hiccup. Thanks in advance, Jocojohn