How to Replace a Cylinder Thermostat (UK)
Fit a new dual-purpose or single cylinder thermostat on indirect and direct UK hot water cylinders.
HoneywellDanfossHorstmann
Cylinder stat vs immersion stat
Cylinder stat = strap-on external, controls the motorised valve. Immersion stat = internal to the electric element. Both required on unvented cylinders (safety redundancy).
Fit
Isolate the motorised valve wiring. Loosen the strap around the cylinder — 100 mm below the top for indirect coil systems. Set to 60 °C to meet L8 Legionella guidance.
Frequently asked questions
- Why won't the boiler come on for hot water?
- Cylinder thermostat is calling, but motorised valve won't open — sync check. Or stat has failed open circuit — test with multimeter.
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