Water temperature fluctuating in shower — combi boiler

Temperature fluctuation in a combi boiler shower happens because the boiler is trying to maintain a set hot water temperature, but when another outlet draws cold water (flushing the toilet, turning on a basin tap), the cold supply pressure drops briefly, which changes the hot/cold balance in a non-thermostatic shower.

A thermostatic shower valve or thermostatic bar valve eliminates this problem — it compensates for pressure changes and maintains your set temperature regardless. If you have a non-thermostatic mixer, upgrade to a thermostatic valve (£80–£200 for the valve, plus fitting).

Also check the combi boiler's minimum flow rate setting: some combis have an adjustable minimum flow rate for hot water activation. If set too high, the boiler may cut out when flow drops slightly (e.g., when a shower mixer valve is adjusted) and then refire — causing a cold blast followed by scalding hot water. A Gas Safe engineer can adjust this in the boiler settings.

If the boiler's hot water supply temperature is set very high (above 65°C), even a small change in cold supply causes a large perceived temperature change at the shower. Reducing the DHW temperature to 55°C and using the shower on a higher flow rate produces a more stable result.

On a gravity-fed system (not combi), temperature fluctuation when the toilet flushes suggests the cold tank in the loft is low, reducing pressure when the cistern refills. Check the header tank level and ball valve operation.

For combi systems with persistent temperature fluctuation, a specialist engineer can check the flow sensor sensitivity, heat exchanger performance, and whether a plate heat exchanger descale is needed — scale build-up causes uneven heating and erratic temperature control.