By far the majority of central heating problems are discovered in winter when it is already too late. There are two principle reasons for this. Firstly, a home's central heating system tends to be used more in colder weather so it comes under greater pressure from repeated usage. The other major factor that has an effect is that the pipework which connects the radiators to the system's boiler can suffer from metal fatigue as they get colder. In very cold conditions, especially when the system has not been used for a few weeks, the system's components can crack which, in the worst cases, means a new boiler installation will be required.
In winter professional central heating engineers and boiler installation companies tend to be at their busiest. If you choose to call someone to replace your ageing boiler at this time, then it is commonly on a weekend or as an emergency because that is the only time you can be fitted in. With colder weather, you simply cannot always wait for an appointment in a few weeks. On the other hand, having your old boiler replaced for a more efficient one in the summer frequently means not having to wait. The equivalent phrase for another aspect of looking after your property would be, 'repairing the roof while the sun is shining' and the same concept can be applied to your boiler.
Don't wait for your system to break down before you have your boiler replaced. Instead, book a new boiler installation in the summer when it is more convenient to deal with. Not only will the service be more prompt, but if you have to do without hot water for a few hours while it is fitted, then at least you won't be cold in your home.