The standard way to deal with that is simple to replace your current boiler. But of course, this can involve a significant cost, and it’s always painful to replace something that still has many years’ life in it.
Well, there is now a very simple an inexpensive solution to this. It allows you to increase the capacity of the system by simply adding a new, small, electric flow boiler. These are now available from manufacturers such as Amptec (Heatrae Sadia) and Trianco in a range of capacities.
For example, if you are adding a conservatory which will have a couple of radiators, adding one of these in to the heating circuit (they can be added in anywhere) will boost the temperature of the water, reducing the load on the main boiler. And of course for something like a conservatory, which may be in use all of the time, you can turn this off when you do not need it.
This is a viable option regardless of whether your current central heating system is gas, oil or electric.
Usually, when sizing an electric flow boiler for new radiators, the rule of thumb is that an average sized radiator requires 1.5kW (so a 6kW flow boiler such as the Trianco TRI 6 could happily cope with 4 radiators). When sizing for additional radiators in an existing system, however, you just need to take account of the level to which the current boiler will be
So, often this will mean a small model such as the Trianco TRI 2 or the Amptec C400 will do the job fine, and unless your central heating pump is also overloaded, then you will not need to add another.
The net effect of this is that rather than having to replace your boiler at a cost of several thousands of pounds, you will probably be able to just augment the power of your existing boiler with an electric flow boiler for a few hundred pounds.