Overview
Two red flashes on a Baxi 816/825/830 Heat boiler indicate a high temperature protection trip (over-temperature). The boiler has detected that the primary flow or the heat exchanger has reached an unsafe temperature or the safety thermostat has opened to protect the boiler. The controller uses this alarm to prevent damage to the heat exchanger, pump, seals and other components and to avoid a hazardous condition. This fault most commonly happens because there is insufficient water flow through the heat exchanger (pump stopped, airlock, closed valves, low system pressure or blocked circulation), a safety (over‑temperature) thermostat has operated or failed, a wiring/connection to the thermostat or air pressure switch is open or intermittent, a PCB has registered an over‑temperature condition, or a flue/venting issue is causing recirculation or abnormal exhaust temperatures. Severity ranges from moderate to high: it’s a protective shutdown so the boiler is safe while tripped, but the underlying cause can rapidly damage components if ignored. Some basic checks and a single reset are reasonable DIY tasks, but if the fault persists, repeats, or you are unsure (or smell gas), you must call a Gas Safe registered engineer—do not attempt gas, sealed component or complex electrical repairs yourself.
Possible Cause: Over-temperature thermostat on heat exchanger activated. No water flow through heat exchanger. Over-temperature thermostat fault or bad connection (open circuit). PCB over-temperature condition. Flue fault resulting in recirculation or activation of air pressure switch (if fitted).
Troubleshooting Steps
Safety precautions:
1) If you smell gas, stop using gas appliances, leave the property and call your gas emergency number immediately. Do not try to fix the boiler.
2) Before touching anything on the boiler, switch off the electrical supply at the isolator or fuse and allow the appliance to cool for at least 20–30 minutes. Do not work on live wiring or gas components unless you are qualified.
3) Wear basic PPE (gloves, eye protection) and avoid opening sealed covers on the combustion/air/gas assembly. Any internal repairs should be carried out by a Gas Safe engineer.
Initial homeowner checks you can safely do:
1) Let the boiler cool and then try a single reset. If your unit has a reset button, press and hold per the manual; if it has a selector switch, set to R (Reset) and hold for 3–5 seconds. Wait to see if it restarts normally. Only attempt one reset; repeated resets without fixing the cause can cause more damage.
2) Check the system pressure on the gauge. For most domestic Baxi boilers the normal central heating pressure when cold is around 1.0–1.5 bar. If pressure is well below 1 bar, repressurise using the filling loop following the boiler instructions, then reset. If pressure is too high, isolate and call a professional.
3) Bleed radiators around the property to remove trapped air which can cause poor circulation and local overheating. After bleeding, recheck system pressure and repressurise if needed.
4) Check that radiator thermostatic valves (TRVs) and the central heating zone valves are open and the programmer/stat are calling for heat (so the pump should be running).
5) Inspect the flue terminal outside for obvious blockage (birds, leaves, debris). Do not place anything in the flue. Make sure vents and airways at the boiler intake are clear.
Specific diagnostic and basic fix steps (if you are comfortable with visual checks only):
1) With the boiler powered down and cooled, visually inspect visible wiring and connectors to the over‑temperature thermostat and any air pressure switch for loose or corroded connections. Do not attempt to measure/live test unless qualified. Reconnect any visibly loose plugs.
2) Restore power and observe the pump: when the heating call is made, you should hear the pump run or feel slight vibration at the pump body. If the pump does not run and pressure is correct, this is likely the cause and needs a professional pump inspection/replacement.
3) If the boiler trips immediately when calling for heat even after cooling and repressurising, note whether the trip happens with heating only, hot water only, or both. A trip on heating flow suggests circulation/primary circuit issue; a trip on hot water may indicate flow temperature sensor/valve issues.
4) Check condensate and flue paths if you can safely access them; a blocked condensate or flue can cause abnormal flue temperatures or recirculation that may lead to over‑temperature trips. If you suspect a flue or condensate blockage that you cannot clear safely, call an engineer.
5) If a reset gets the boiler running but the fault returns after a short run time or only during heating demand, do not keep resetting. Record when it happens (after how many minutes, whether when all radiators are open, etc.) to give useful information to the engineer.
When to call a professional and final notes:
1) Call a Gas Safe registered engineer if the boiler will not reset, the fault keeps recurring, you have low or fluctuating pressure after repressurising, the pump appears not to operate, you find wiring faults you cannot confidently isolate, or you suspect a failed over‑temperature thermostat, air pressure switch, PCB fault, heat exchanger blockage or pump failure. These parts involve sealed gas or electrical systems and must be handled by a qualified engineer.
2) Do not attempt to bypass safety thermostats, replace gas valves, or perform live electrical diagnostics unless you are trained and registered to do so. Repeated lockouts or ignoring an overheat condition can cause more serious damage.
3) When you contact an engineer, tell them the model, that the fault is 2 red flashes (over‑temperature), what you have checked (pressure, bleeding, pump sound, flue clear), and whether the fault follows a reset or occurs immediately—this speeds diagnosis and repair.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Baxi 816 Heat, 825 Heat, 830 Heat.