Overview
The L5 fault on Ideal boilers commonly indicates a problem with the return side of the heating system. On many Ideal models L5 is used for either a low return flow temperature/return thermistor fault (water returning to the boiler is much colder than expected) or a lockout caused by too many automatic restarts (5 restarts within 15 minutes). In practical terms the boiler has detected an abnormal temperature or behaviour on the return pipe and has gone into safety lockout to protect the appliance. Causes range from circulation problems (blocked pipes or radiators, a seized or slow pump, closed valves, airlocks, system sludge), a faulty or incorrectly fitted return thermistor, low system pressure, or an electrical/PCB fault causing spurious restarts. Severity is moderate to high: the boiler will usually stop providing central heating and may go into repeated lockouts. Some simple checks and a single reset are safe for a homeowner, but recurring L5 faults or any work involving gas, internal electrical components, replacing sensors, pumps, or PCB should be handled by a Gas Safe registered engineer.
Troubleshooting Steps
Safety precautions:
1. If you smell gas, do not touch the boiler—turn off gas supply at isolation if safe to do so, ventilate the area and call the gas emergency number immediately.
2. Always isolate electrical power to the boiler at the fuse spur before touching or opening any covers. If you are unsure, do not proceed.
3. Do not open the boiler casing, attempt repairs to the gas valve, burner, fan, or PCB, or replace sensors unless you are a trained Gas Safe engineer.
Initial checks a homeowner can do:
1. Reset the boiler: turn the boiler off at the isolator/fuse spur for 5–10 seconds, then turn back on. Do not attempt repeated resets if the fault returns immediately.
2. Check the system pressure on the boiler gauge. If pressure is below about 1.0–1.2 bar (cold), the system may need topping up using the filling loop to around 1.2–1.5 bar. Follow the boiler manual for your model when repressurising.
3. Bleed radiators to remove trapped air, then re-check pressure and top up if needed.
4. Ensure all radiator thermostatic valves, zone valves and the boiler’s isolation valves are open.
5. Listen for or feel the central heating pump when the boiler calls for heat: a working pump should hum or vibrate slightly. If the pump is silent and the boiler is calling, circulation may be absent.
Specific diagnostic and fix steps (what to try and what indicates a professional job):
1. Reset and observe: after a single full power-off reset, try calling for heating and watch whether the boiler fires and stays running or quickly locks out with L5. If it clears and the system works normally, monitor for recurrence. If it locks out again, stop further resets.
2. Confirm circulation: bleed radiators (top to bottom) until hot throughout. If upper radiators stay cold, you may have an airlock, closed valve, or a failing pump. If bleeding fixes it and the boiler runs, monitor pressure.
3. Check flow/return temperatures physically (if you can safely touch the pipes): the return pipe should be noticeably warmer than room temperature when the boiler is running. A very cold return compared with the flow suggests poor circulation or blocked radiators/pipework.
4. Check pump operation: if the pump appears stuck or extremely noisy, it may need replacement or freeing—this is a job for a qualified engineer. Do not dismantle the pump yourself unless you are competent and Gas Safe registered.
5. Check wiring and sensor clips: visually inspect (with power off) the thermistor wires and the clamp/clip on the return pipe if accessible. A loose or displaced return thermistor can give false low readings. Re-clipping a loose sensor is sometimes a simple fix, but replacement or electrical diagnosis should be done by an engineer.
6. System cleanliness: if your system has a history of sludge or black iron oxide, poor circulation and cold returns can be caused by blockages. Powerflushing or fitting a magnetic filter is a professional job and will likely require an engineer.
7. If the boiler indicates the fault after electrical resets (5 restarts in 15 minutes), leave it powered off for a longer period (turn off at fuse spur for a few minutes) and then call a Gas Safe engineer if the fault returns. Repeated resets can mask an underlying intermittent electrical or PCB fault.
When to call a professional / mandatory actions:
1. Call a Gas Safe registered engineer if the fault persists after the basic checks above, if the pump needs unblocking or replacement, if a thermistor needs replacing or re-wiring, if there are suspected PCB/electrical faults, or if you are uncomfortable with any step.
2. If the boiler is under warranty, contact Ideal customer support or your installer before arranging third-party work.
3. Do not attempt gas or internal electrical repairs yourself. Recurrent L5 faults, sensor replacements, pump replacement, powerflushing, PCB faults, and gas valve or flue issues require a qualified engineer to diagnose and repair safely.
Summary: try one full power-off reset, check pressure and bleed radiators, confirm pump operation and that valve positions are correct. If the problem is unresolved, call a Gas Safe registered heating engineer and quote the L5 fault so they can bring appropriate diagnostic tools and parts.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Ideal Ideal Boiler.