Hot water temperature fluctuating: why taps run hot then cold

Detail that affects your quote for fluctuating hot water temperature

• Combi fluctuation during use often means flow near the boiler's minimum
• A worn diverter valve can let heating water blend into hot water flow
• Cylinder systems: a failing thermostat is a common intermittent cause
• A drifted blending valve produces a similar symptom on stored systems
• Note whether other taps or appliances were in use when it happened

What is actually going on with fluctuating hot water temperature

On a combi boiler, fluctuation during a single tap run often means the flow rate is close to the appliance's minimum, so any change in demand elsewhere in the house, such as a dishwasher filling or a second tap opening, momentarily drops flow below the point where the boiler can maintain steady temperature. A worn diverter valve can also let a little heating water blend with hot water flow intermittently. On a stored hot water system with a cylinder, fluctuation more often points at a failing cylinder thermostat cycling incorrectly, or a blending valve that has drifted out of calibration. Mains pressure changes at busy times of day can also produce a similar effect on combis in areas with variable incoming pressure.

Where we deal with this

• Melksham (SN12) · 12 mi
• Weston (Bath) (BA1) · 9 mi
• Oldfield Park (BA2) · 8 mi

Questions we get asked

Does this mean I need a bigger boiler?

Rarely. It is more often a control component behaving intermittently than the appliance being genuinely undersized for the property.

Could this be a shower valve rather than the boiler?

Yes, thermostatic shower valves can wear and cause exactly this pattern on their own, independent of the boiler or cylinder. It is worth mentioning which outlets are affected.

Is fluctuating hot water linked to a fault code appearing?

Sometimes. If a fault code also appears when the temperature swings, that code takes priority and should be reported alongside the symptom.