Cylinder Coils and Thermostats
The coil inside an indirect cylinder transfers heat from the boiler's primary circuit into the stored water, while the cylinder thermostat tells the boiler when to stop heating it.
The coil inside an indirect cylinder transfers heat from the boiler's primary circuit into the stored water, while the cylinder thermostat tells the boiler when to stop heating it. Slow reheats or overly hot water often trace back to one of these two rather than the boiler itself.
What it does
Boiler-heated water flows through the coil, transferring heat by conduction into the surrounding stored water without the two ever mixing; the thermostat clamped to the cylinder wall senses when target temperature is reached.
How it fails
The coil scales up internally in hard water areas and transfers heat more slowly, or the thermostat's sensor drifts and either undershoots or overshoots the set temperature.
Repair or replace
A scaled coil can sometimes be chemically cleaned; a badly scaled or leaking coil means cylinder replacement, since coils aren't usually replaceable in isolation. Thermostats are straightforward to replace on their own.
What points to a coil or thermostat issue
- Hot water taking noticeably longer to reheat than it used to
- Water not reaching a comfortable temperature at all
- Water scalding hot despite no changes to settings
- Boiler firing for hot water far longer than expected
Who does the work
- Gas Safe registered engineers — registration 554328
- Worcester Bosch accredited installer
- John The Plumb Ltd, company registration 10026483
Investigate slow or scalding hot water
Tell us how long hot water takes to reheat after a bath and we'll check whether the coil, the thermostat, or the boiler is behind it.
Questions we get asked
Is coil replacement possible without replacing the cylinder?
On some older vented cylinders yes, but on most modern unvented cylinders the coil is integral and not separately replaceable.
Does cylinder work fall under Gas Safe rules?
The cylinder itself is a plumbing item, though where the fault turns out to be at the boiler end of the primary circuit, that part is Gas Safe registered work.
Can a water softener protect the coil?
It reduces scale build-up considerably in hard water areas, extending the coil's effective working life.