Error F.77

Safety precautions:

1. Before doing anything, put the boiler into standby or switch it off at the programmer/room thermostat and wait a few minutes for components to cool. 2. Do not open gas covers or attempt to repair gas or sealed combustion components — only a Gas Safe engineer should work on gas or internal boiler parts. 3. Wear gloves and eye protection for outdoor work and when handling condensate (it is slightly acidic). 4. Do not pour boiling water on plastic condensate pipes or components; use warm water only. 5. If you smell gas or suspect a gas leak, leave the property immediately and call the gas emergency number — do not attempt any checks.

Initial checks a homeowner can do:

1. Note the displayed error (F.77) and take a photo of the screen. 2. Try a normal reset: press and hold the boiler reset button for about 3 seconds, then release. If the fault clears and does not immediately return, monitor the boiler for normal operation. 3. Locate the condensate discharge pipe (typically a white plastic 21mm pipe that runs to an outside drain or gulley). Follow its route and look for visible kinks, blockages, or frost/ice. 4. Inspect the external flue terminal (the circular/rectangular outlet on the outside wall) for visible debris, bird nests or leaves that could block flow. Only clear obvious external debris with gloves — do not insert objects into the flue while the boiler might run.

Specific diagnostic and simple fixes you can try:

1. Thawing a frozen condensate pipe: if the external condensate run is frozen, pour warm (not boiling) water slowly along the frozen section, working from the outside toward the boiler. Repeat until the ice clears. After thawing, insulate the exposed run with foam pipe insulation or wrap to reduce re-freezing. 2. Clearing external blockages: remove visible debris from the flue terminal and ensure the condensate discharge end is not blocked or under standing water. 3. Check for kinks or trapped sections: straighten any sharp bends in the condensate pipe and ensure it has a continuous downwards fall to the drain. 4. If your system has a separate condensate pump unit (a small plastic pump bottle often located near the boiler or under a sink), check visually that it has power (indicator light) and is upright. With the boiler powered off, you can check whether the pump float moves freely; stuck floats or debris in the pump sump can prevent operation. If you feel confident, you can carefully clear grime from the pump sump or float, but do not dismantle sealed parts or electrical connections. 5. After performing the checks/fixes above, reset the boiler and observe: if it runs normally and the fault does not return, monitor over the next 24–48 hours.

When to call a professional (Gas Safe engineer):

1. The fault returns after simple DIY checks or after resetting multiple times. 2. The condensate pump does not run even though it has power, makes unusual noises, or the float is stuck and will not free. 3. You need to access or remove boiler internal panels, replace the condensate pump, replace or repair the flue or non-return flap, or there are electrical faults indicated. 4. The flue appears to be blocked internally, there is visible water ingress inside the boiler, repeated freezing or recurring blockages, or you are unsure about any step.

Additional advice and prevention:

1. Do not cap or permanently block the condensate drain. 2. Insulate any exposed condensate pipework on external runs, or have a Gas Safe engineer fit frost protection/trace heating where pipes are exposed to cold. 3. If the boiler is installed in an unheated location (garage, outhouse), consider relocating the termination or adding protection as recommended by a professional. 4. Keep records of what you tried and share them with the engineer if you call one. 5. Always use a Gas Safe registered engineer for repairs beyond basic clearing/insulating and for any work that involves the boiler internals, flue work or replacement of the condensate pump.