Timer/programmer
Your timer switch, which you’ll recognise if it has a built-in clock and electronic display, allows you to set your heating and hot water separately, so you can programme it to come on at certain times of the day. These often have a shelf life of 5-10 years, and when they fail, will often mean you won’t get any heating or hot water as it control the functions of the boiler. But to replace it can usually be a simple task, particularly if its a straight swap.