You have a leak somewhere. Constantly filling a boiler with fresh water leads to early death. But, an old one may be a prime candidate for replacement when you can swing it. It will save you money in the long term. It probably occurs more in winter because your leaks are somewhere in the heating runs to the radiators and in the summer, no water flows there. It could be the pump is leaking when it runs as well. Well, there are other things as well, but you get the idea.
An internal coil in a boiler for domestic hot water is probably one of the most inefficient ways to make hot water...you need to keep the boiler running winter/summer constantly. An indirect is MUCH more efficient, as the boiler may only need to run once a day or so after a bunch of showers. They're generally so well insulated that they will keep water hot for at least days without the boiler firing.