Combi hot water scalding hot then cold: flow and control faults
A combi modulates its burner against how much water you are drawing. When that measurement or the heat transfer path is compromised, the temperature hunts between scalding and cold.
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.
Published business details
- Based at 24 Bristol Road, Radstock, BA3 3EE
- Telephone enquiries: 07516 118198
- Opening hours: Monday to Friday, 8:30am to 5:00pm
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.
Related help pages
- Total loss checklistNo heating and no hot water: the checks to make before you call
- Vaillant fault codeVaillant F22: low water content and what it is really telling you
- Fan faultWorcester C6 fault: fan speed not reaching target
- Fault isolationBoiler faults on hot water only: isolating the DHW circuit
- Water qualityHard water and limescale build-up across Bath and Somerset systems
- Warranty repair rulesWarranty work versus a chargeable repair: how the line is drawn
Services that cover this
Where we deal with this
Book a visit for swinging hot water temperature
In practice: Measure the mains, check the scale, then talk about hardware. Bigger is not automatically better on combis.
For gas work, use a Gas Safe registered engineer; for oil work, use an OFTEC-registered technician.
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