Error S.34

Safety precautions:

1. If you smell gas, evacuate the building immediately, do not operate electrical switches, call the gas emergency number and your gas supplier from a safe location.

2. If you are unsure or uncomfortable at any stage stop and call a registered gas-safe/competent heating engineer. Do not open the boiler casing or attempt gas or PCB repairs yourself.

3. Isolate mains power to the boiler only if instructed by a qualified engineer; simple resets below are safe but avoid internal access.

Initial checks a homeowner can do:

1. Check the boiler display and controls: confirm the boiler is powered, central heating (CH) mode is enabled and the room thermostat/programmer is calling for heat. Sometimes the boiler is in summer mode or heating is turned off externally.

2. Check the programmer/room thermostat and any wireless thermostat batteries. Ensure CH temperature setpoint is above the frost threshold (set CH to a normal temperature temporarily).

3. Check the boiler water pressure on the gauge or display. Normal pressure when cold is typically around 1.0–1.5 bar. If pressure is significantly below 0.8 bar the boiler may lock out or not run properly.

4. Listen at the boiler for pump noise or feel nearby heating pipes when the boiler is calling for heat: pipes should start to warm after the boiler fires. If the pump is silent and pipes remain cold this indicates circulation failure.

5. Note any other codes or flashing lights on the display and write them down for the engineer.

Simple actions you can try (safe, non-invasive):

1. Reset the boiler: follow the boiler display instructions for a reset or switch the boiler off at the front panel or isolator for 60 seconds and switch back on. If S.34 clears and the boiler runs normally, monitor for recurrence.

2. Temporarily increase the room thermostat setpoint or set the programmer to call for heating so the boiler is forced to run and see whether it fires and circulates heat.

3. If the system pressure is low and you are competent, you can top up via the filling loop until pressure reaches the recommended level (check the boiler handbook for the correct procedure). If you are not confident, leave this to an engineer.

4. Check external controls that could block heating: timers, smart controls, zone valves, TRVs and make sure none are set to isolate the heating.

Diagnostics and next steps to report or hand to a professional:

1. If the boiler does not fire when there is a valid heating demand (room stat turned up, CH mode on) and you have reasonable system pressure, this suggests a circulation or ignition/sensor/electronics fault. Do not attempt internal repairs.

2. If the pump is cold and does not run despite demand, or you hear the pump struggle, don’t force or dismantle it — call an engineer. A seized or failed pump is a common cause of frost-protection failures because the boiler cannot raise flow temperature.

3. If the boiler shows additional F-codes (sensor, fan, flame or electronics faults) alongside S.34, note them and pass them to the engineer; these indicate specific component failures (NTC sensors, PCB, fan, gas valve etc.).

4. If topping up pressure temporarily allows normal operation but the boiler soon returns to S.34, report possible leaks, expansion vessel or pressure-sensor issues to the engineer.

5. If frost protection repeatedly activates in mild weather, or if the boiler cannot maintain flow temperature, this is likely a component/sensor/PCB issue requiring diagnostic equipment and should be handled by a competent engineer.

When to call a professional:

1. If S.34 remains after reasonable basic checks and a reset, or if the pump doesn’t run when the boiler calls for heat.

2. If you see additional fault codes (F.xx) or the boiler goes into lockout frequently.

3. If you are uncomfortable topping up the system pressure or suspect water leaks or sealed-system faults.

4. Any suspected gas, ignition, or fan faults, or if the boiler needs internal access to test sensors, valves, pump or the PCB.

What the engineer will likely do:

A competent engineer will run full diagnostics, check flow and return temperatures and sensors, verify pump operation and wiring, test system pressure and filling loop, inspect for leaks, and if required replace failed sensors, the pump, or carry out electronics repairs. They will also ensure the frost-protection function and any external controls are configured correctly.

Final note: S.34 can be a protective action (boiler deliberately trying to prevent freezing) or a symptom of a failure preventing the frost-protection routine from working. Do basic checks and a reset; if the fault persists, arrange a gas-safe competent heating engineer to diagnose and repair. Do not attempt gas or internal electrical repairs yourself.