BoilerFaultCodes.com
D.78 Low Warning

Your boiler is limiting the temperature of the water being sent to your hot water cylinder, which typically means the water won't get as hot as usual.

Monitor it

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 temperature in celsius

What causes this fault?

This fault occurs when the boiler detects that it cannot safely heat your hot water cylinder to the requested temperature, often due to a conflict between the boiler's settings and the cylinder's actual heat demand. It is typically caused by a faulty temperature sensor or a thermostat that is sending inaccurate data to the boiler's control board.

Typical repair cost £0-120 (including labour)

DIY fix possible

Locate the hot water temperature dial or digital setting on the boiler control panelAdjust the hot water target temperature slightly to see if the status clearsCheck if the hot water cylinder thermostat is set higher than the boiler's output limit

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A boiler cover plan would cover faults like D.78 — no call-out fees, no surprises.

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