Essentially a boiler designed for the home should (if of a reasonable quality or better) last for between 15 and 25 years, dependent upon the standard of the design/build, type of boiler and frequency and quality of maintenance.
My parents have a 25kW Ideal Mexico (floor-standing heat only boiler [pump separate]) which has ran very smoothly since it was installed back in 1996, only needing an annual service (a simple unit that the utility firms are familiar with) and a new programmer (separate to the boiler). Given a fair wind, it may even beat its BG predecessor, which lasted 35 years (from when the house was built). The efficiency isn't that high (I can't remember off the top of my head, but its probably in the mid-70s%), but as others have said, the savings of a 90% efficient 'A' rated new boiler take a LONG time to recoup if the purchase and installation costs are factored in (probably about £2000+).
I would have to spend about £3500 to replace mine, just because the idiots designing the homes where I live thought it would be good to stick the boilers (non-condensing - so no drain for a new condensing boiler [it will have to be pumped to the drain in the k******/WC]) in a cupboard in the middle of the flat, so that it has long concealed flue and gas pipe runs, which will require lots of expensive work (see my earlier post about flues - it also sort of applies to gas pipes) to install a modern boiler in its place (nowhere else to put a new one). As such, my unusual (and not very reliable) 'Powermax' boiler will have to last nearer to 20 years before its worth me changing (hopefully I will have moved by then - its 13yo now).
My rule is that unless the boiler is unsafe, very unreliable (and thus the expense of having new parts being continually fitted being, say, the equivalent of an extra service per year, as well as the inconvenience of no heating/hot water [99% of time its in winter, when you need it the most]) or less than 60% efficient/reliant on a fuel that is (compared to the available alternatives) getting very expensive, then keep the existing one until the aforementioned criteria are satisfied.
The payback on new boilers is often at least 10 years, if not longer (especially if your existing one is of reasonable efficiency anyway [70%+] and reliable/easy and cheap to service). Its ironically a very similar argument for replacing a car, though perhaps without the 'aesthetics' element.
As all boilers go, simple, proven designs always last longest (as has been said above) - I've surveyed sites where some from the early 1970s are still going strong, with the odd replacement parts here and there (wear and tear), yet others which were poorly designed/made/maintained only lasted 10 years or so. In my line of work (and in my view/experience), Ideal aren't exactly renouned for their boilers' reliability, though the Mexico seems to fit my 'simple and reliable' mantra.
Makes like Worcester-Bosch and Vaillant always come top (and with Which? too), most of the other major players are average as far as reliability is concerned, and is reflected in the purchase price. One tip to always make sure of is that your nominated plumber is trained to service/repair the boiler you're looking to buy - some unusual ones (like mine) require special courses for the plumbers to attend, which they may feel is an unwanted expense that they probably won't recoup and may set themselves up for being sued if they don't get a repair/service right. The same goes for the availability of consumables/parts if it goes wrong - I made the mistake as a junior engineer of not checking this (hindsight is a wonderful thing!) as I liked my flat far better than all the others (with other boilers), and have 'paid the price' as non-generic parts are about 50-100% more expensive than more common makes/boilers. Lucky for me that my plumber does have the knowledge to do the repairs when they are needed.
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