BoilerFaultCodesYour boiler is temporarily reducing its power while heating your hot water cylinder because it is reaching its temperature limit too quickly.
This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.
Technical description: Limitation of cylinder charging in output in kW
What causes this fault?
This occurs when the boiler is generating heat faster than your hot water cylinder can absorb it, causing the water inside the boiler to get too hot too quickly. To prevent overheating, the boiler automatically throttles its power back to a lower level to allow the heat transfer to catch up. The most common reasons are a buildup of limescale or sludge inside the heating coil, or a pump that isn't moving water fast enough through the system.
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
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Protect yourself from future repair bills
A boiler cover plan would cover faults like D.77 — no call-out fees, no surprises.
Affiliate links — we may earn a commission at no extra cost to you.