What does the Vaillant D.78 fault code mean?

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.

How much does it cost to fix Vaillant error code D.78?

Typical repair cost for the Vaillant D.78 fault is £0-120 including labour.

Vaillant D.78 Fault Code: What It Means & How to Fix It

What the manual says

Manufacturer manual

Limitation of cylinder charging temperature in celsius

From this model's manual

Verified against the Vaillant/Vaillant Ecotec Plus Pro 839592 05 2006 installation & service manual.

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.

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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Related fault codes

D.22View →D.25View →

All 135 fault codes on the ecoTEC plus 612 R1 VU GB 126/3-5

Seeing a different code? Every code this model can display, in one place.

35 96 con Connection D,11 D.0 D.1 D.2 D.3 D.4 D.5 D.7 D.8 D.9 D.10 D.12 D.13 D.17 D.18 D.20 D.22 D.23 D.25 D.27 D.28 D.30 D.33 D.34 D.35 D.40 D.41 D.44 D.47 D.50 D.51 D.60 D.61 D.64 D.65 D.67 D.68 D.69 D.70 D.71 D.72 D.75 D.76 D.77 D.80 D.81 D.82 D.83 D.84 D.90 D.91 D.93 D.96 D.97 F.0 F.1 F.10 F.11 F.13 F.20 F.22 F.23 F.24 F.25 F.27 F.28 F.29 F.32 F.49 F.61 F.62 F.63 F.64 F.65 F.67 F.70 F.71 F.72 F.73 F.74 F.75 F.76 F.77 F.78 F70 P.0 P.1 P.2 P.3 P.4 P.5 P.6 S. 2 S. 5 S.0 S.1 S.3 S.4 S.6 S.7 S.8 S.10 S.11 S.13 S.14 S.15 S.16 S.17 S.20 S.21 S.22 S.23 S.24 S.25 S.26 S.27 S.28 S.30 S.31 S.32 S.34 S.36 S.41 S.42 S.52 S.53 S.54 S.96 S.97 S.98

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.

What the manual says

What causes this fault?

DIY fix possible

Avoid the next call-out bill

Related fault codes

All 135 fault codes on the ecoTEC plus 612 R1 VU GB 126/3-5

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