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A higher kW-rated unit, 10.5kW against 8.5kW for example, has more headroom to maintain flow rate even as it works harder to raise the temperature, which is why higher-rated units tend to feel stronger and more consistent through the colder months. That headroom depends entirely on the property's electrical supply being able to support it: a higher kW shower draws more current, and the circuit and consumer unit need the capacity to match, which is not automatic on every property, particularly older ones.
A mixer shower works the opposite way: it blends hot and cold water drawn from the property's existing supply, whether that is a combination boiler, a vented cylinder, or an unvented cylinder, and its performance rides entirely on what that system can provide. On a property with an unvented cylinder or a combi boiler delivering strong, consistent mains pressure to both hot and cold, a mixer shower can perform very well, often better than an electric shower, because it is not limited by the kW-versus-flow trade-off that constrains instantaneous heating. On a property with a low-pressure vented cylinder fed from a loft tank, the same mixer shower may deliver disappointing pressure unless a pump is added to the system.
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The heating element is the part doing the actual work, raising the temperature of the water as it passes through the unit, and it is also the part most exposed to hard water. In areas with harder mains supply, elements pick up scale over time in exactly the way any heating element does, which reduces efficiency and eventually causes failure, showing up as a shower that runs cold, runs lukewarm, or trips out shortly after being switched on.
Plumb2U carries water softeners and the resin, salt and spare parts to keep them running, at trade prices with next-day despatch across the UK, whether the job is a first softener install or a service call on an existing one, for trade buyers from Epsom to anywhere else in the country.
A water heater and the cylinder it serves are meant to work as one sized system, and when that sizing does not match, the symptoms usually show up at the tap long before anyone thinks to question the cylinder itself. A heater that is undersized for the cylinder it is heating cannot bring the full stored volume up to temperature within a reasonable time, leaving a property with hot water for the first shower of the morning and disappointing lukewarm water for the second. An immersion element rated for a smaller cylinder, fitted into a larger one, faces exactly this problem: it is being asked to do more work than its rating supports.