I was referring to converting the central heating 'system water' (the stuff that carries heat around the house) to a sealed/unvented configuration. This can normally be done without changing the existing system components (boiler, hot water cylinder, etc.) It involves the relatively minor operation of removing the feed/expansion tank in the roof and installing the sealed expansion vessel that takes over its function somewhere convenient. The plumber will also need to provide a 'filling loop' and air-bleed point, and check that there is a suitable pressure relief valve in or near the boiler and install one if necessary.
In my experience, converting to a sealed system won't 'cause' new leaks but it may manifest pre-existing minor leaks. Although annoying, this is a good thing from the point of view of system life. This is because a continual trickle of fresh 'top up water' into an open system to compensate for a small leak somewhere will cause corrosion.
This is unrelated to another modernisation, i.e. converting the 'domestic hot water' system that supplies showers and taps. This is a bigger job that does normally require a new cylinder and often modification/replacement of outlets such as showers and taps.
It's difficult to give any more-specific advice without a site visit. I suggest you get a local heating engineer to review the possibilities and quote.
