Turning on kitchen hot tap (~8 metres from boiler) and others further along the hot supply involves drawing off/waiting for quite a lot of cold in the line before any hot arrives. Is, I'm sure a common problem for many households where hot draw-off is some distance from boiler (& especially as mine is 22mm pipe). I'd like to put a small electric heater in the line, under sink, say about 15 lt capacity, probably unvented, so hot water close to instant, coming out of tap. Would also benefit the shower, (more precisely the person in shower!), which is next in the line. My thinking is - heater's vessel size needs to be just adequate so hot from this heater just running out as hot supply from boiler arrives into newly installed heater. Boiler is a Combi thus hot water basically at mains pressure. There is hot draw off to bath about 2 metres from boiler. Length of supply piping before new heater position nearly 8M, thus adequate length for hot water expansion in pipe?
You could, but connect to the cold mains not the hot water, but it'll cost more to heat the water. I would change all 22mm to 15mm pipe and take the quickest/ most direct route. Have you had the boiler checked to ensure it's working correctly, you will get legs of cold water when opening a hot tap with any combi, but it shouldn't take too long for the hot water to come through.
Sometimes heat can be lost through the diverter to the heating flow, or the gas pressures may be out/gas valve sticking.
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