The industry standard advice is to replace your boiler every 10 years, however some people still have boilers working that are over 30 years old. On the other extreme I have known a customer who decided to have a new Vaillant boiler fitted as he was so feed up with his one year old Ferroli boiler continually breaking down! The boiler was under warranty so the repairs where not costing him any money, he was just tired with the Russian roulette as to wherever he could have a warm shower.
My advice is the 10 years for the boiler replacement is not far wrong, for the better makes (Vaillant and Worcester) you can maybe push that up a little. The very old boilers that keep going almost indefinitely do so because there is very little to go wrong in them. This might sound ideal, but they are terrible on gas bills. Also a modern boiler contains a lot more of the equipment you require to be fitted outside the old boiler. A full system conversion to a modern condensing combi boiler will remove the need to have an external central heating pump, zone valves, header tanks, hot water cylinder and twin channel programmer. When you ask a customer who has a 30 year old boiler that has had “no problems” if any of the other components on the central heating system have had to be replaced they almost always have. A 30-year-old boiler contains a heat exchanger and a burner, it’s not much of a step up from heating some water up on a real fire (which brake down even less than an old boiler). If you do have one of these boilers it would be an excellent long term finical decision to have it up graded. Provided the installation is done correctly it does not have to be disruptive, the new boiler will be reliable and preform much better. Most system conversions can be completed in 2 days.
We replace lots of ideal combi boilers at around 8 years onwards, Worcester bosch combi boilers at around 12 years. We recently replaced an 18-year-old Vaillant combi, it was still working perfectly however the customer wanted more efficiency and didn’t want to push his luck. If your boiler is getting on you can just wait until something major fails in it and have a new boiler fitted then. Obviously the problem with that is they generally decide to break down at the worst possible time, you will then have a wait while you arrange boiler quotes. Good boiler installers tend to have a lead in time, they specialise in boiler installs so are expert in what needs to be done. When you have no heating or hot water the temptation can be to go with the person who can fit the boiler first. This can lead to paying over the odds or having the boiler fitted incorrectly due to lack of experience as the person who often fits bathrooms, kitchens and only occasionally fits boilers will probably be able to fit you in sooner in the cold winter months.