Error H.01 – .18

Safety precautions:

- If you smell gas, evacuate the property immediately and call the gas emergency number from a safe location. Do not attempt any checks on the boiler.

- If the boiler is making unusual noises, leaking, or showing signs of overheating (very hot casing or radiators unexpected), switch the boiler off at the isolator and the mains consumer unit and call an engineer.

- Do not remove the boiler casing or attempt to access internal components, wiring, gas valve or the heat exchanger — this must be done only by a qualified Gas Safe engineer.

Initial checks a homeowner can do:

1. Note the exact error code and any preceding events (power cut, recent work, low water pressure). Take a photo of the display for the engineer.

2. Check the system pressure gauge on the boiler. Normal cold system pressure is typically around 1.0–1.5 bar; many Baxi manuals recommend between 1 and 2 bar. If the pressure is below about 0.5 bar the boiler may lock out. If low, repressurise using the filling loop until pressure is in the correct range (follow your boiler handbook for the correct method). If you are unsure how to use the filling loop, stop and call a Gas Safe engineer.

3. Try a controlled reset: press and hold the boiler reset button for 3–5 seconds (or follow the model-specific reset instructions). If the fault clears but returns, further investigation is required.

4. With the heating demand on, listen at the boiler for the circulation pump running (you may hear or feel vibration). Feel the flow and return pipework (careful — pipes can be hot). If both pipes stay cold or there is a large temperature difference where flow is hot but return is cold or vice versa, circulation is likely impaired.

5. Check that any external radiator valves, thermostatic radiator valves (TRVs) and motorized zone valves are open, and that the central heating programmer/room thermostat is calling for heat.

6. Bleed radiators to remove trapped air that can cause an airlock. After bleeding, re-check and top up the system pressure if it has dropped.

Specific diagnostic and basic fix steps:

1. Repressurise the system if pressure is low, then reset the boiler and observe. Low pressure is a common and easily fixable cause of circulation faults.

2. If pressure is normal and the pump appears not to run: ensure pump isolation valves (if fitted) are open. Some pumps have an isolator switch or a mechanical paddle switch—do not attempt to remove the pump or open wiring. If you can safely reach the pump casing, feel for movement or light vibration when the boiler is calling for heat.

3. If pump noise indicates it’s running but radiators are cold, partially closed lockshield valves, blocked pipework or heavy sludge/magnetic filter blockage may be restricting flow. Bleed radiators and ensure magnetic filter isolates are open. Do not dismantle the filter unless you know what you are doing.

4. If the fault mentions rapid flow temperature rise or excessive flow/return differential, it can indicate a stuck closed valve, seized pump, or a failed flow/return sensor. A sensor fault often shows alongside other sensor-related codes; sensors and wiring checks should be left to an engineer.

5. If you recently had work on the heating system or electrical interruptions, recheck electrical isolation switches and boiler connectors are secure externally. Avoid opening the boiler casing.

6. If you see water leaks, pressure continuously dropping, or cannot restore circulation by the simple checks above, switch the boiler off and call a Gas Safe engineer.

When to call a professional:

- If the H.01 error persists after checking pressure, bleeding radiators, and resetting.

- If you suspect a seized or noisy pump, blocked heat exchanger, faulty flow/return/flow sensor, defective zone valve, or any gas/combustion issue.

- If there are leaks, repeated lockouts, or the boiler displays additional error codes.

A Gas Safe registered engineer will perform safe electrical and gas checks, test and replace sensors, inspect and, if needed, replace or service the pump, check PCB communication with the gas valve and perform any required repairs. Provide the engineer with the exact error code and what troubleshooting steps you have already done to help speed diagnosis.