Parts and labour are rather expensive when it comes to heating. It's ok to have a few parts replaced, however if it begins to form a trend, there's a limit to what you want to spend on repairs because the cost tends to go up with each new repair bill.
Medium quality boiler manufacturers give 5 – 7 years warranty on their products. Makers like Worcester-Bosch or Vaillant may offer up to 10 years. It's unlikely for any boiler to last longer than 12 – 15 years. Most parts are subject to high temperatures and extreme working conditions.
If your family has grown since you last installed a new boiler, or if you've added a new shower or a new tap and it looks like the old boiler is no longer coping with the load, it might be a good time to shop around for a new one. With parts affected by wear and tear, your heating system may take longer to heat up. If you regularly have to wait for hot water or for your radiators to heat up, it means the old appliance is no longer coping.
All modern boilers come with an A energy rating meaning that their efficiency is around (or slightly over) 90%. This is pretty cool as they only waste 10% of the energy going into them. At the same time, there are still millions of low-efficiency boilers in active use in the UK. Their efficiency may hover around 70% or even lower.
This is adding unnecessary cost to your energy bills and is also quite bad for the environment. Replacing your boiler with an A-rated appliance can save you a significant amount on energy bills, however, the buy-back time on boilers is quite long. If we assume you manage to save around £200 per year, it will take more than 10 years to recoup the cost of a new boiler. So, unless you're a Greenpeace Warrior and want to make a tangible difference to the environment you're unlikely to be persuaded to buy a new boiler just based on the saving you might make.
Nevertheless, this is a very convenient additional excuse for wanting a combi boiler replacement, and if any other signs from this list match your situation, you know that it's time to go for it.
A regular humming noise is ok and boiler noise is not something you usually pay attention to, until something goes wrong. Louder than usual buzzing or screeching noise is a telltale sign that it's time to replace your boiler.
The most usual faults related to strange sounds are broken fans or water pumps. Neither of these parts are cheap to replace, especially if you have an old model. And as they say, it never rains but it pours. Once the fan is gone, it's almost certain another expensive part will fail within the next few months.