Overview
The 1 flash green fault on Baxi 816/825/830 Heat models is a temperature-protection shutdown. It means the boiler detected an unsafe condition related to water temperature: the flow temperature has gone above the allowed maximum, the flow temperature rose too quickly, or there is an excessive difference between the flow and return temperatures. In short, the boiler has limited or stopped operation to protect the heat exchanger and other components from overheating or dry-firing. This fault can be caused by poor circulation (pump fault, closed or seized motorised valve, airlock, blocked radiator or heat exchanger), a faulty flow or return temperature sensor (NTC), wiring/connection faults, or less commonly a fan or flue-sensor issue that affects combustion control. Severity ranges from moderate to high: if circulation is the problem it may be a system fault you can partly diagnose, but if a sensor, fan, PCB or gas-related component is faulty the repair must be done by a qualified engineer. Because this code is a safety shutdown, do not ignore recurring occurrences or attempt internal repairs yourself — call a Gas Safe registered engineer if basic user checks do not clear the fault or if you smell gas or suspect carbon monoxide.
Possible Cause: Maximum flow temperature exceeded, Flow temperature rise rate too high, Excessive difference between flow and return temperature
Troubleshooting Steps
Safety precautions:
- If you smell gas, hear a gas leak, or suspect carbon monoxide (nausea, headache, soot, pilot lights behaving oddly), immediately turn off the gas supply, isolate the electrical supply to the boiler if safe to do so, ventilate the property, leave the building and call the gas emergency number and a Gas Safe engineer. Do not attempt internal repairs.
- Do not remove the boiler casing, touch electrical wiring, or attempt to service gas valves, the burner, fan or PCB yourself. Only carry out simple user-level checks.
Initial homeowner checks you can do:
1. Check the boiler display and note the error and whether it clears after a reset. Do not keep repeatedly resetting if the fault returns.
2. Check system pressure on the boiler gauge. Aim for about 1.0–1.5 bar when cold. If pressure is below 1.0 bar, topping up may help circulation-related faults.
3. Look and feel for obvious problems: visible water leaks around pipes/boiler, frozen condensate pipe (in cold weather), blocked external flue terminal (leaves, bird nest) or blocked vents.
4. Check central heating demand: ensure room thermostat or programmer is calling for heat, and any external switched live or smart controls are allowing the boiler to fire.
5. Listen for the pump. With the heating on, the pump should hum and pipes should warm. Carefully feel (without touching the boiler casing or hot components) the flow and return pipes: a healthy circulating system will have both pipes warm with a moderate temperature difference. A very hot flow and cool return suggests poor circulation.
6. Bleed radiators to remove airlocks that can impede circulation. After bleeding, check and top up system pressure if it falls below the recommended range.
7. If pressure is low, use the filling loop to top up: locate the flexible filling loop or valves under the boiler, open the valves slowly to let water in until gauge reads about 1–1.5 bar, then close both valves and remove/re-secure the loop if required. Monitor pressure for stability.
Specific diagnostic and simple fix steps:
1. Reset the boiler once after completing the checks above. Use the boiler’s reset button or selector (hold R or press reset for 5–10 seconds depending on your model). Observe whether the fault returns immediately or after a short time.
2. If circulation looks poor (pump silent, big flow/return difference, cold radiators while boiler fires), try: bleeding radiators, checking motorised valves on the system are in the correct position, and ensuring external thermostats/timers are calling for heat. If the pump appears seized you may hear a clicking or nothing; a seized pump requires a Gas Safe engineer.
3. If the fault follows after a short run with high temperatures, lower the boiler flow/CH temperature setting temporarily and see if the fault still occurs. This can indicate either settings too high for the system or poor heat transfer due to blockage.
4. If the condensate pipe is frozen (common in cold weather) thaw the pipe with warm water or a heat source and then reset the boiler. A frozen condensate or blocked flue can cause related faults.
5. If you have isolated or fixed any visible issues (topped up pressure, bled radiators, cleared flue blockage) and a single reset clears the fault and it does not return, monitor the system for repeat occurrences.
When to call a professional and what they will check:
- If the fault returns after basic checks and one reset, if the pump does not run, if you find wiring loose, or if sensors or internal components are suspected, call a Gas Safe registered engineer.
- A pro will check pump operation, motorised valves, heat exchanger condition and flow rates, NTC temperature sensors (flow, return and flue thermistor), wiring/connectors, fan and flue operation, and the PCB and gas components if needed. They will log the fault and carry out any necessary part replacements or repairs.
Additional homeowner guidance:
- Do not repeatedly reset the boiler; repeated resets can mask a developing safety issue and may cause further damage.
- Keep a record of when the fault happened, what you tried, and any symptoms (noisy pump, cold radiators, pressure drop) to give to the engineer.
- Any work involving gas, internal sensors, the burner, fan, heat exchanger or opening the casing must be done by a Gas Safe registered engineer. If the boiler locks out repeatedly, or the engineer identifies a sensor, fan, pump or heat-exchanger fault, arrange prompt professional repair to avoid further safety risk or damage.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Baxi 816 Heat, 825 Heat, 830 Heat.