Low
D.78 on Vaillant Atmomax

Your boiler is limiting the maximum temperature it uses to heat your hot water cylinder, which may mean your water takes longer to heat up or doesn't feel as hot as usual.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

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.

What the manual says

Manufacturer manual

Limitation of cylinder charging temperature in c

Shared across the range
This code is shared by the boilers in this range, which use the same control board. It may not be printed in this exact variant’s manual.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

DIY fix possible

Check the hot water temperature dial or digital setting on the boiler control panelEnsure the cylinder thermostat (usually found on the side of your water tank) is not set too lowObserve if the fault clears after the water cylinder has reached its target temperature

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