Combi hot water scalding hot then cold: flow and control faults

Where swinging hot water temperature usually comes from

Common causes are a scaled plate heat exchanger reducing transfer, a partially blocked flow restrictor, a failing flow turbine sending the wrong demand figure, or simply low incoming mains flow rate at peak times. Where mains flow is genuinely poor, no combi will deliver a steady shower and a stored system may be the honest answer.

Practical notes on swinging hot water temperature

• Measure incoming mains flow before blaming the appliance
• Scale in hard-water areas is the leading cause here
• Flow restrictors silt up and are cheap to clean or replace
• Thermostatic shower valves mask, but do not fix, the swing
• Simultaneous draw-offs will always destabilise a combi

Questions we get asked

Is it the shower or the boiler?

Test the temperature at a bath tap too. If the swing follows both, it is the appliance or the mains supply.

Would a bigger combi fix it?

Only if mains flow can support it. Output means nothing without water to heat.

Where we deal with this

• Bruton (BA10) · 13 mi
• Peasedown St John (BA2) · 3 mi
• Chew Magna (BS40) · 8 mi