D.77 Low
Your boiler is temporarily reducing its power while heating your hot water cylinder because it is reaching its temperature limit too quickly.
Technical description: Limitation of cylinder charging in output in kW
Typical repair cost: £0-120 (including labour)
DIY fix possible
Check that the valves on the pipes leading to your hot water cylinder are fully openEnsure there is no trapped air in the cylinder coil by bleeding nearby radiatorsCheck that your hot water temperature dial on the boiler is not set excessively high
Protect yourself from future repair bills
A boiler cover plan would cover faults like D.77 — no call-out fees, no surprises.
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Parts commonly replaced
- External heating pump
- Hot water cylinder NTC sensor
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