Overview
H.01 – .21 on a Baxi 200/400 combi indicates a temporary fault where the boiler detects the flow temperature is rising too quickly when producing domestic hot water (DHW). The boiler monitors flow and return temperatures and if the flow sensor reading jumps fast or the system isn’t circulating water correctly the board will suspend normal operation to protect the heat exchanger and safety controls. Typical causes are poor or no circulation (stuck/failed pump, closed valves, airlock, blocked heat exchanger or scale), a faulty flow temperature sensor (NTC) or a loose/poor connection to the sensor or PCB. Severity: this code is classed as a temporary fault and often resets, but repeated occurrences are a sign of a real fault that can cause overheating, trips of safety thermostats, reduced hot water performance or long-term damage to the boiler heat exchanger. If the fault appears once and clears after a reset, monitor the boiler closely. If it repeats, or if you have no hot water, visible leaks, unusual noises, or a gas smell, treat it as urgent and stop using the appliance until inspected. DIY vs professional: homeowners can carry out basic safety checks and simple resets, check boiler pressure and obvious visible issues, but diagnosing and repairing pumps, sensors, wiring or internal components requires a qualified, Gas Safe registered engineer. Any work involving gas, mains electricity or opening the boiler casing must be done by a professional.
Possible Cause: Temporary error – flow temperature increasing too fast in domestic hot water (DHW) mode, insufficient circulation or sensor error.
Troubleshooting Steps
Safety precautions:
- If you smell gas, stop, turn the gas supply off at the meter/isolator, ventilate the area and call the gas emergency number immediately. Do not attempt repairs.
- If the boiler is showing overheat symptoms, heavy leaks, or persistent lockouts, switch the boiler off and isolate the electrical supply and gas until a Gas Safe engineer attends.
- Do not open the boiler casing or attempt internal electrical or gas work unless you are a qualified engineer.
Initial checks a homeowner can safely do:
1. Note the exact code (H.01 – .21) and when it appears (only when you run a hot tap, or also during central heating?). This information is useful to the engineer.
2. Try a simple reset: with the boiler powered, press and hold the reset button for about 3–5 seconds. Wait and see if the code clears and if normal hot water returns.
3. Check the boiler pressure gauge. Normal is usually around 1–1.5 bar when cold for a combi. If the pressure is below 0.5 bar the boiler may not operate correctly; repressurise to about 1–1.5 bar following the manufacturer’s instructions using the filling loop.
4. Run a hot tap and observe behaviour: is hot water available briefly then lost, is temperature fluctuating, or is there no flow? Listen at the boiler for pump or valve sounds when DHW demand is present.
5. Check that external isolation valves to the boiler or the DHW service valves (if accessible) are open. Ensure any external mixing/thermostatic valves on outlets are not closed.
6. In very cold weather check that external condensate/drain pipes are not frozen or blocked.
Specific diagnostic and fix steps (safe, non-invasive actions):
1. If the fault cleared after reset and does not return, monitor for recurrence. If it returns, record frequency and conditions (e.g., only on hot taps, after long inactivity, or during high demand).
2. If pressure was low and you repressurised the system, reset and test DHW again. Low pressure can cause poor circulation which may trigger flow/temperature faults.
3. Bleed a radiator to remove airlocks in the central heating circuit (if you have CH enabled). Airlocks can sometimes affect circulation; only do this if you are familiar with bleeding radiators and re-check pressure afterwards.
4. Check for obvious leaks around the boiler and pipework. Leaks reduce system volume and can lead to poor circulation.
5. If you have access to the boiler manual, follow the manufacturer reset and fault-clear instructions and note any guidance specific to H.01 codes.
When to call a professional (must-call conditions):
- The H.01 – .21 fault recurs after reset, or the boiler locks out repeatedly.
- There is no hot water or central heating despite normal pressure and reset.
- You hear the pump trying to run but there is no circulation, or you suspect a stuck diverter valve or seized pump.
- You see leaks, smell gas, or notice scorch/overheat signs on the boiler.
What a Gas Safe engineer will typically check and do:
- Inspect wiring and connectors to the flow temperature (DHW) sensor and PCB for loose or corroded connections.
- Test the NTC sensors (flow and return) for correct resistance/operation and replace any faulty sensor.
- Check the primary circulation pump operation, impeller and electrical supply; replace or strip/repair pump if seized or failing.
- Inspect and test the diverter valve operation (if fitted) to ensure DHW demand correctly routes water and that there is no partial blockage.
- Check for blocked or scaled heat exchanger or restrictors that reduce circulation; perform a chemical flush or repairs if required.
- Test the PCB for intermittent faults or communication issues between gas valve and control board if indicated.
Final notes:
- H.01 – .21 is a protective temporary code but should not be ignored if it repeats. Keep a log of occurrences and what you did when it happened to help the engineer diagnose the cause.
- Do not attempt gas or electrical repairs yourself. Arrange a visit from a Gas Safe registered engineer if the fault persists or you are not confident performing the safe checks above.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Baxi 200 / 400 Combi Gas Boiler.