open vent condensing boiler would most likely the best. You don't say what fuel it runs off. Both systems heat the same water in the same place the only difference is the boiler also heats the pipe work to the hot coil so if very long run or using oil, solid fuel, or LPG it could become more expensive. But with natural gas it is so much cheaper than electricity that it is very unlikely that electricity used to heat whole cylinder could be cheaper than gas. If an instant heater similar to the shower units or the very small approx 7 litres boilers were used very local to where you shave and wash in morning then they may be slightly cheaper.
Sorry I forgot to mention that it is a gas fired open vent condensing boiler which was installed in October 2006 before I moved here. I have heard that combi boilers are a better choice but will make do with this one.
The pipework is not very long, it's about 3 feet from the boiler in the Kitchen to the hot water cylinder in the airing cupboard in the hall plus the piping which I guess is another 4 feet down the wall.
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