Boiler
109
Error code 109 on a Potterton boiler indicates that the boiler has detected an overheat condition — meaning the water temperature inside the heat exchanger has risen beyond its safe operating limit and the boiler has shut itself down as a safety measure. This is a serious fault that should not be ignored, but it doesn’t necessarily mean your boiler is beyond repair. In this article, you’ll learn exactly what’s causing the 109 fault, what you can safely check yourself, and when to call a Gas Safe registered engineer.
What does this error mean?
The 109 error code on Potterton boilers (including popular models in the Assure, Titanium, and Profile ranges) signals a high-limit thermostat lockout — sometimes referred to as an ‘overheat fault’. Your boiler contains a safety device called the overheat thermostat (or high-limit stat), which is designed to trip and cut the boiler off if the water temperature inside the primary heat exchanger climbs to an unsafe level, typically above 95–100°C. When this happens, the boiler locks out and displays the 109 code to tell you it has protected itself from potential damage. Think of it like a thermal fuse in a plug — it’s done its job, but now you need to find out why it was needed in the first place. The boiler will not fire again until the fault is investigated and the lockout is reset.
Est. repair cost
£80–£300 for common repairs (pump, thermostat, or re-pressurisation); £300–£600 for a full system power flush; up to £800+ if the heat exchanger requires replacement
🔍 Likely causes
🛠️ Step-by-step fix
Step 1 — Do not immediately reset: Before you touch anything, allow the boiler at least 15–20 minutes to cool down. Resetting a hot boiler in lockout repeatedly without investigating the cause can damage the heat exchanger or other internal components. Safety first.
Step 2 — Check your boiler pressure gauge: Look at the pressure gauge on the front of your Potterton boiler. It should read between 1.0 and 1.5 bar when cold. If it reads below 0.5 bar, your system may be low on water, which can cause overheating. Repressurising the system via the filling loop (check your boiler manual for your specific model’s instructions) may resolve this.
Step 3 — Check all radiator and isolation valves are open: Walk around your home and make sure every radiator valve (both the thermostatic head and the lockshield valve at the other end) is open. Also check that the isolation valves on the pipework directly connected to the boiler (usually with a flat-head screw slot) are in the open position — the slot should run in line with the pipe, not across it.
Step 4 — Bleed your radiators to remove air locks: Using a radiator bleed key, open each radiator’s bleed valve (usually at the top corner) until water flows steadily with no spluttering air. Start with the radiator furthest from the boiler and work back towards it. Re-check the boiler pressure after bleeding and repressurise if it has dropped below 1.0 bar.
Step 5 — Listen for the pump: Once the boiler has cooled, attempt a reset by pressing and holding the reset button (refer to your Potterton model’s manual — on most models it is a button or rotary dial marked with a flame and cross symbol). Listen carefully near the boiler — you should hear a low hum from the central heating pump within a few seconds of firing. If you hear nothing, the pump may have failed and you must stop here and call an engineer.
Step 6 — Attempt a single reset: If the pressure is correct, valves are open, and you can hear the pump running, press the reset button once and observe the boiler. If it fires, runs for a few minutes, and then locks out again with the 109 code, do not keep resetting it. Repeated lockouts indicate an underlying fault that will not self-resolve and could cause damage.
Step 7 — Stop here if the fault repeats: If the 109 code returns after one reset attempt, this is beyond safe DIY territory. Boilers involve gas, high-pressure hot water, and electrical components. Do not attempt to open the boiler casing, bypass any safety devices, or tamper with the overheat thermostat. Call a Gas Safe registered engineer and describe exactly what you’ve found.
📞 When to call a professional
You should call a Gas Safe registered engineer if: the boiler locks out again after a single reset attempt; you cannot hear the pump running; the pressure is correct but the fault keeps returning; your boiler is making unusual banging or kettling noises (a sign of sludge or scale); or if you are in any doubt whatsoever. Because this fault involves a gas-fired appliance and a safety lockout, professional diagnosis is strongly recommended even after a successful reset — the 109 code is your boiler telling you something is wrong. When you call the engineer, tell them: the exact error code (109), that it is an overheat/high-limit lockout, how many times it has tripped, whether the pump sounds like it’s running, and what you have already checked (pressure, valves, bleeding). This saves time and money on the call-out. Always verify your engineer is Gas Safe registered at gassaferegister.co.uk before booking.
⚖️ Repair or replace?
Whether to repair or replace your Potterton boiler when facing a 109 fault depends largely on the age of the appliance and the underlying cause. If the fix is straightforward — a power flush to clear sludge (£300–£600), a pump replacement (£150–£300 including labour), or a new overheat thermostat (£80–£180 including labour) — then repair is almost always the right call if your boiler is under 10 years old. Potterton is a well-established UK brand (part of the Baxi group) and spare parts are generally well-stocked and reasonably priced. However, if your boiler is over 12–15 years old, is a back boiler, or if the engineer finds the heat exchanger is cracked or severely scaled — repair costs can quickly exceed £500–£800, at which point a new boiler makes more financial sense. A new A-rated Potterton or equivalent combi boiler installed by a Gas Safe engineer typically costs £1,800–£3,000 fully fitted in the UK, but will be far more energy-efficient, potentially saving £200–£350 per year on gas bills compared to an older G or F-rated appliance. As a general rule: if the repair costs more than 50% of a new boiler’s installed price, replace it.