Combi Boiler Preheat Not Working
A noticeable delay before hot water arrives, where there didn't used to be one..
A noticeable delay before hot water arrives, where there didn't used to be one.
What is actually going on
A noticeable delay before hot water arrives, where there didn't used to be one.
The realistic causes, in order
Preheat or comfort mode switched off, sometimes during an efficiency adjustment A failed preheat thermistor, so the boiler no longer maintains the exchanger temperature A long pipe run to the outlet, which is a plumbing layout issue rather than a boiler fault
What we test on site
Preheat setting in the boiler's user or installer menu Thermistor readings and preheat cycle behaviour Draw-off distance and dead-leg volume to the affected tap
Get combi preheat problems diagnosed properly
Where the delay is pipe length rather than the boiler, we say so — a boiler change wouldn't fix it.
Questions we get asked
What causes combi preheat problems on a gas boiler?
Preheat or comfort mode switched off, sometimes during an efficiency adjustment
What normally puts combi preheat problems right?
Where the delay is pipe length rather than the boiler, we say so — a boiler change wouldn't fix it.
Is it safe to keep using the system with combi preheat problems?
Preheat improves convenience but uses gas keeping the exchanger warm. The right setting depends on how you use hot water, and we'll explain the trade-off rather than just switching it on.