Hot water runs hot then cold — cold water sandwich
Temperature swinging between scalding and cold within a single shower is one of the most common combi complaints we get called to — and one of the most frequently misdiagnosed as "the boiler's had it".
What's actually happening
The boiler is losing and regaining flame during the draw. Either the flow sensor isn't registering demand steadily, the diverter valve is sticking part-way, or a scaled exchanger is causing the burner to overshoot then cut. Each has a distinct signature when you watch the flow rate and burner behaviour together.
Diagnostic sequence
- Observe burner modulation against a steady measured draw
- Test diverter travel and check for a partially seized cartridge
- Check flow sensor output and turbine free movement
- Assess exchanger scaling with inlet and outlet temperature readings
Frequently asked
- Is it worth repairing on an older combi?
- A diverter cartridge or flow sensor is a modest part cost and worth doing on most boilers under twelve years old.
- Does a thermostatic shower valve help?
- It masks small swings but not a real cold-water sandwich, and it can make diagnosis harder — we test at an unmixed tap.
- Why is it worse in winter?
- Colder incoming mains means the boiler works harder to hit target temperature, so any weakness in modulation shows up more.
One visit, clear answer
This is a fault that responds well to methodical testing. We'll name the component, quote the repair, and only mention replacement if the boiler genuinely warrants it.
Related solutions
Core services
Where we cover
New Worcester boiler with a 10 year guarantee*
Free no-obligation quotes for installation, servicing & repairs across BA & BS postcodes.