Heating fine but hot water only lukewarm: what causes the split

Things worth knowing about lukewarm hot water despite normal heating

• Combi: a partly scaled plate heat exchanger reduces heat transfer
• Cylinder: a low thermostat setting or scaled coil limits stored temperature
• A partly open zone valve restricts hot water flow without stopping it entirely
• Thermostatic mixing valves can be the true cause on showers specifically
• Descaling a combi plate is far cheaper than replacing it outright

How we read lukewarm hot water despite normal heating on site

On a combi, a partially scaled plate heat exchanger is the classic cause, transferring less heat into the passing mains water than it should even though the boiler itself fires correctly. On a stored hot water system, a cylinder thermostat set too low, a partly failed zone valve restricting flow to the coil, or the cylinder's own coil scaling internally are the equivalent culprits. Mixing valves set incorrectly at the tap or shower can also mimic this fault entirely independently of the boiler.

Where we deal with this

• Winford (BS40) · 10 mi
• Staverton (BA14) · 11 mi
• Melksham (SN12) · 12 mi
• Bathford (BA1) · 11 mi
• Englishcombe (BA2) · 7 mi

Questions we get asked

Why is the water warm rather than fully hot or fully cold?

A partial restriction or partial scale build-up reduces heat transfer proportionally rather than stopping it, which is why the result sits in between.

Could the boiler's hot water temperature dial just be set low?

Yes — it is always worth checking the boiler's own hot water setting before assuming a component fault, since it is sometimes turned down unintentionally.