Error D.78

Safety precautions:

- Do not open the boiler casing or touch internal components. Internal wiring, gas and electrical parts are hazardous and must be handled by a qualified technician.

- If you smell gas or suspect a gas leak, evacuate, call the gas emergency number and do not operate electrical switches or the boiler.

- Before doing any external checks, switch the boiler off at the room thermostat and/or the isolator and allow the system to cool.

Initial homeowner checks:

1. Check what the user controls show: on the front display and in the VSmart/app note the DHW user setpoint and any displayed diagnostic messages. If you have set DHW to 55°C and the app shows 55°C for water, that matches D.78 set to 55°C.

2. Measure the hot water temperature at a tap with a thermometer after a run of hot water to confirm actual output temperature.

3. Restart the boiler: switch off at the mains or the boiler isolator, wait 30 seconds, then switch back on. See if the D.78 display or message clears and if hot water behaviour returns to normal.

4. Look for additional fault codes: if any F-codes (for example F.10, F.13, F.71, F.91 etc.) or safety limit messages appear alongside D.78, record them and do not continue to operate the boiler. Those indicate sensor or safety faults and need an engineer.

5. Check the cylinder/stat controls and the hot-water programmer: make sure there is an active demand for hot water when checking temperatures (e.g. DHW mode on or timer calling for hot water).

Diagnostic and possible fixes (homeowner-level and guidance for engineer):

1. Confirm the difference between parameters: D.20 is typically the DHW target you set (the temperature you want the cylinder to reach), while D.78 is the maximum flow temperature allowed when charging the cylinder. If D.20 (target) is higher than D.78 (max flow), the boiler cannot exceed the D.78 limit and will not reach the higher target. If you need higher DHW temperature, either reduce the user target to be ≤ D.78 or have an installer raise D.78.

2. If D.78 is purely informational and there are no accompanying fault codes and hot water performance is acceptable, no repair is needed—this is a configuration/limit setting. If you want the limit changed, contact the installer or a Gas Safe engineer to change installer-level parameters; do not attempt to change installer/service settings yourself.

3. If hot water is too cool or fluctuating and the D.78 message is shown together with sensor or cylinder-related faults (e.g. F.13, F.84, F.91 or any temperature-sensor related F-codes), this often indicates a defective NTC sensor, loose sensor plug, wiring fault or actoSTOR cylinder electronics problem. Do not try to replace sensors yourself. Turn the boiler off and call a Gas Safe registered engineer to diagnose and replace the NTC, wiring harness, or cylinder electronics as required.

4. If the boiler has safety shut-offs (overheat, STB trips or temperature limiter events) or shows signs of blockage, pump failure, or persistent lockouts, isolate the boiler and call a Gas Safe engineer immediately. These conditions can create risk and usually require specialist testing (continuity/resistance checks, inspection of pump, flow/return sensors) and may involve part replacement.

When to call a professional:

- Call a Gas Safe registered engineer whenever you see any F-code fault, temperature limiter trips, or if adjusting user settings does not restore correct hot-water temperature.

- Call a professional to change D.78 (installer/service parameter), to check or replace NTC sensors, wiring harnesses, the actoSTOR cylinder electronics, or to investigate pump/blockage issues.

What to tell the engineer:

- Note the exact messages and codes displayed, the user-level DHW setting, the D.20 and D.78 values if visible, and whether the boiler shows any F-codes. Provide times, symptoms (e.g. not reaching set temperature, lockouts), and a photo of the display if possible. This information speeds diagnosis and repair.

Do not attempt internal electrical or gas repairs yourself. Only a Gas Safe qualified engineer should perform sensor replacements, wiring repairs or changes to installer settings.