Error D.77

Safety first

1) If you smell gas, evacuate the property immediately and call the emergency gas number—do not operate electrical switches or the boiler. 2) If the boiler is showing additional alarms that indicate overheating or safety cut-out, switch the boiler off at the main isolate and call a Gas Safe engineer. 3) Before doing any visual checks or touching wiring, switch the boiler off at the isolator and, if you will handle wiring, switch off the household mains power and only proceed if you are competent and comfortable with basic electrical safety.

Initial homeowner checks (safe, simple checks you can do)

1) Note any other fault codes or symbols shown on the display and take a photo or write them down. 2) Try a soft reset: press the fault-clear/reset key (or switch the boiler off, wait 30s, and switch on). If the code disappears but returns, continue troubleshooting. 3) Check the hot water demand and programmer/room thermostat settings—ensure the boiler is actually being asked to charge the cylinder. 4) Check cylinder thermostat (if accessible) and any external cylinder controller settings—ensure they are not set to limit charging intentionally or set to an eco/low-power mode. 5) Check boiler water pressure gauge—if pressure is very low (<1.0 bar) top up the system using the filling loop following the boiler manual; low pressure can cause circulation problems that affect charging.

Specific diagnostic checks and possible fixes

1) Look for accompanying sensor faults (e.g., F13) which indicate a cylinder temperature sensor problem. If a sensor fault is present, do not continue to run the boiler long-term—call an engineer. 2) If you have a visible cylinder sensor and connector at the boiler or accessory module, look for loose, corroded, or disconnected wiring. Only perform visual checks; do not disconnect or attempt electrical repairs unless competent. 3) If an external cylinder charging module or additional accessory is fitted (third-party controller, zone module), check its status LEDs and settings. Some modules can request a reduced kW charging rate; consult the accessory manual. 4) Check circulation: ensure any cylinder charging pump or motorised valve (if accessible) is operating when hot water demand is present. You may hear the pump run or feel pipework warming. If pump/valve does not operate and you are competent, a visual check for power to the pump can be done, but replacement/repair must be handled by an engineer. 5) Check for blocked or partially blocked cylinder coil/pipework (if your system has such a coil) or any isolation valves closed between boiler and cylinder.

If the issue appears to be a settings/configuration limit

1) The boiler menu can include limits for maximum flow temperature and output. If you are comfortable following the manual, check the maximum heating/hot water temperatures and any option menus that relate to cylinder charging output. Do not change advanced parameters unless you understand them—incorrect changes can cause unsafe operation. 2) If you suspect the boiler is set to deliberately limit output (e.g., for combi/cylinder configuration mismatch), note the current settings and contact a qualified engineer to review and, if appropriate, reconfigure.

When to call a professional

1) Call a Gas Safe registered boiler engineer if: the code returns after reset; you see sensor faults (F13 or similar); pumps/valves do not operate; wiring looks damaged; the boiler displays overheating or safety cut-out codes; or you are unsure of the accessory/module configuration. 2) Ask the engineer to check cylinder sensor continuity and positioning, accessory module configuration, pump performance, flow/return temperatures, and boiler parameter settings related to cylinder charge limits. 3) If the engineer replaces parts or reconfigures the boiler, request an explanation of the cause so you can monitor for recurrence.

Final notes

1) D.77 often results in restricted or slow hot water charging rather than an immediate dangerous failure, but it points to a control, sensor, accessory, or circulation issue that should be diagnosed properly. 2) Temporary resets may restore function briefly, but persistent or recurring D.77 should be investigated by a qualified engineer. 3) Do not attempt gas, combustion, or sealed component repairs yourself—always use a Gas Safe registered engineer for these tasks.