BoilerFaultCodesWhat the manual says
Manufacturer manualMaximum set value for target cylinder
What causes this fault?
This occurs when your boiler reaches its temperature limit while heating your hot water cylinder, causing it to pause temporarily to prevent overheating. It often happens because the boiler is producing heat faster than the cylinder can absorb it, which is typical once the water in the tank is nearly at its target temperature. The boiler is simply regulating itself rather than failing.
DIY fix possible
Check the hot water temperature setting on the boiler control panelReduce the target cylinder temperature if it is set unnecessarily highWait for the system to cool down naturally before the boiler resumes
Avoid the next call-out bill
A cover plan would handle faults like D.20: no call-out fees, no surprise bills.
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Related fault codes
All 135 fault codes on the ecoTEC pro 24
Seeing a different code? Every code this model can display, in one place.