Why does my water heater produce lukewarm water?
The short answer
On a combi boiler, lukewarm water usually points to a failing diverter valve or a blocked plate heat exchanger. On a cylinder system, it's more often a thermostat set too low, a failed immersion heater, or a programmer that isn't running the hot water for long enough. If it's lukewarm at just one tap, suspect that tap's mixer or a thermostatic valve rather than the boiler.
Also asked: Why is my hot water not hot enough?
Work out the scope first, because it narrows things immediately. Every hot tap in the house lukewarm points at the heat source. One tap lukewarm while the others are fine points at that tap — a failing mixer cartridge, or a thermostatic mixing valve that has drifted or stuck.
On a combi, the diverter valve is the part that decides whether heat goes to your radiators or your hot water. When it starts to stick, the classic symptom is exactly this: heating fine, hot water disappointing. The other usual suspect is a plate heat exchanger that's blocked up and no longer transferring heat properly.
It's worth being precise about what blocks it, because most plumbing articles you'll read are written for the South East and get this wrong for Macclesfield. In a hard-water area a plate heat exchanger furs up with limescale. Macclesfield's water is soft — soft enough that scale is not really a thing here — so when one of ours blocks, it's typically sludge and corrosion debris circulating from the heating side, not scale. Same symptom, different cause, and a different fix: a system cleanse and a magnetic filter, rather than a descale.
On a stored hot water cylinder, start with the boring things. Is the cylinder thermostat set high enough? Is the programmer actually calling for hot water, and for long enough to heat a full cylinder? If you're on an immersion heater and it's dead, you get a tank of tepid water and no obvious error message.
One important caveat: don't simply crank everything up to fix it. Hot water storage temperature is a safety matter in both directions — too low risks bacteria, too hot risks scalding — which is worth understanding before you start turning dials.
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