Overview
E109 on a Potterton Assure combi or system boiler is a pre-circulation or poor-circulation fault. In plain terms the boiler has detected that water is not moving through the primary/heating circuit as expected before it allows normal ignition or operation. The internal control looks for flow (via a flow switch, pump current sensing or temperature change) and if it doesn’t see the expected circulation it will flag E109 to protect the boiler from dry running or overheating. Common reasons for this fault are a jammed or electrically failed pump, air trapped in the system, a closed isolation or zone valve, blocked pipework or a magnetic filter full of debris, or a failed flow sensor/flow switch or its wiring. The fault is a safety measure: it prevents the boiler firing when circulation is inadequate, so the fault itself is important but the boiler deliberately makes itself safe rather than continuing to operate unsafely. Some basic checks are suitable for a competent homeowner (pressure level, bleeding radiators, checking valves are open, reset attempt). Anything involving opening the boiler, electrical work, pump replacement, flow switch wiring or gas system work should be done by a Gas Safe registered heating engineer. If simple checks don’t clear E109 the boiler should be isolated and an engineer called to diagnose and repair.
Possible Cause: Pre-circulation fault
Troubleshooting Steps
Safety first
1. If you smell gas leave the property immediately and call the gas emergency number. Do not try to investigate. 2. If the boiler is making unusual noises, leaking, or performing repeated lockouts, switch the boiler to OFF and isolate the electrical supply at the fused spur or consumer unit if you can do so safely. 3. Never remove the boiler casing or attempt internal electrical or gas repairs. Those tasks must be carried out by a Gas Safe engineer.
Initial homeowner checks (do these before calling an engineer)
1. Note the exact error (E109) and any other codes or lights. Take a photo and the time it happened. 2. Check the boiler pressure gauge; typical working pressure is about 1.0–1.5 bar when cold. If pressure is very low (below about 0.8 bar) repressurise to the manufacturer’s recommended level following the boiler manual. 3. Try a simple reset: use the boiler reset button as per the manual (usually hold for 3–5 seconds). If it clears and runs briefly but returns to E109, do not repeatedly reset—record that it relocks out. 4. Confirm the heating demand is actually on: room thermostat set above room temp, programmer/cylinder stat calling for heat, and any timers are on. 5. Check that any external flow/return isolation valves and the boiler’s service valves are open (these are normally lever-type valves adjacent to the boiler pipework). 6. If you have a magnetic filter on the return, check its isolation valves are open. Only if the filter is designed for simple user cleaning and you are comfortable, isolate, drain the filter as per its instructions and clean; otherwise leave it for the engineer.
Basic circulation diagnostics you can perform (no internal boiler access)
1. Call for heating while listening at the boiler: do you hear the pump running (a soft hum or vibration) shortly after the demand starts? 2. Feel the flow and return pipes (careful, they may be hot) for a difference in temperature a few minutes after the call for heat. If both remain very cold the pump may not be running or there is no flow. 3. Check radiators: if most radiators stay cold and bleeding one releases air, this suggests an airlock. Bleed the radiators starting from the lowest point of the system to remove trapped air, then re-check pressure and circulation. 4. If pressure was low and you’ve re-pressurised, try the boiler again—some circulation faults clear once the system is at correct pressure.
When to stop and call a professional
1. If the pump makes no sound but valves and pressure look correct, or if you detect the pump hum but no flow/heat change, do not try to dismantle or manually free the pump—call a Gas Safe engineer. 2. If there are signs of blockage (large temperature difference only on one side, or a magnetic filter that looks full and you are not confident to clean it), call an engineer. 3. If the E109 remains after the basic checks and resets, or if other codes appear alongside E109 (E110 overheat, E125 circulation fault, etc.), switch the boiler off and book a Gas Safe registered heating engineer.
What the engineer will check (so you can provide useful information)
1. They will inspect pump operation (electrical supply, pump rotation, impeller jam) and the flow switch or sensor and its wiring. 2. They will check for airlocks, blockages in the heat exchanger or pipework, correct system pressure and condition of filters/magnetic traps. 3. They may run electrical tests on the PCB, relays and pump supply, and replace faulty components (pump, flow switch, valves) as required.
Notes and practical tips
1. Record model and serial number, how long since last service, when the fault started and any recent work on the heating system—this helps the engineer. 2. If your boiler is under warranty or service contract, contact the service provider first. 3. Repeated lockouts or running the boiler when you suspect poor circulation risks damage; do not try to force operation—leave the appliance off and call a professional.
Summary: do the safe basic checks (pressure, reset once, valves open, bleed radiators) but do not open the boiler or attempt pump/flow-switch or gas repairs yourself. For anything beyond those simple steps, or if the fault persists, contact a Gas Safe registered heating engineer to diagnose and repair.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Potterton Assure Combi and system.