Saunier Duval F.81 Fault Code: Causes, Fixes & Repair Costs
What does the Saunier Duval F.81 fault code mean?
The F.81 code indicates that the boiler has detected an abnormal overheating event on the domestic hot water (DHW) circuit. As a safety response, the boiler locks out and stops producing hot water, although central heating may continue to operate normally. You may notice the pump running and making noise, yet the stored water stays cold (around 23–24 °C) while the heat exchanger outlet climbs to 70–80 °C — a classic sign that heat is being generated but not transferred into the water supply. The boiler display will flash two alternating numbers to indicate the fault. Once the underlying cause is resolved, pressing the reset button should clear the lockout.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Limescale build-up in the plate heat exchanger Common
In hard water areas, limescale deposits gradually coat the internal surfaces of the DHW plate heat exchanger, narrowing the water passages and dramatically reducing flow. Heat can no longer transfer efficiently into the domestic water supply, so the outlet temperature climbs unchecked and triggers the F.81 safety lockout. This is the single most common cause of this fault in the UK.
- Faulty or seized DHW circulation pump Common
If the pump has seized or is not circulating water effectively, the coolant stagnates in the DHW circuit and overheats rapidly. Importantly, the pump can still vibrate and sound like it is running even when it is not actually moving water — so an apparently active pump does not rule this out as a cause.
- Mispositioned or defective DHW temperature sensor Sometimes
A sensor that has worked loose from the heat exchanger outlet, or one that has developed an electrical fault, can send inaccurate readings to the control board. If the boiler 'thinks' temperatures are higher than they actually are, it will trigger the F.81 lockout prematurely. Corroded sensor connectors can produce the same effect.
- Overheating safety thermostat (overheat stat) failure Sometimes
The overheat thermostat is a secondary safety device that trips if the DHW circuit reaches dangerously high temperatures. Once a thermostat has thermally tripped or burned out, it can hold the F.81 fault signal permanently, even after temperatures have returned to normal. A failed overheat stat cannot be manually reset and must be replaced.
- Three-way valve or non-return valve malfunction Sometimes
The three-way valve directs flow between the heating and hot water circuits. If it sticks or fails to switch correctly, coolant may not reach the DHW circuit at the right time or in sufficient volume, creating localised overheating conditions that trigger the fault.
- Air locks in the system Sometimes
Trapped air prevents proper coolant circulation, which can cause hot spots in the DHW circuit and fool the boiler's safety systems into triggering a lockout. Low system pressure (below 1 bar on the pressure gauge) is often associated with air ingress.
- Faulty PCB (control board) Rare
If the main printed circuit board is misinterpreting temperature signals — despite all sensors and components being in good working order — the board itself may be at fault. This is typically only diagnosed after all other components have been checked and found to be serviceable.
How to fix it
- Check your system pressure and top up if necessary DIY safe
Look at the pressure gauge on the boiler front. It should read between 1.0 and 1.5 bar when cold. If it is below 1 bar, use the filling loop to top up slowly to around 1.2 bar. Low pressure restricts water circulation and can contribute to overheating faults.
- Attempt a boiler reset DIY safe
Press and hold the reset button on your Saunier Duval boiler for 5–10 seconds, then allow the boiler to complete its full restart sequence. Open a hot water tap and observe whether the F.81 code returns. If it clears and does not come back, monitor the boiler over the next 24 hours. Do not reset the boiler more than two or three times — repeated lockouts mean something needs professional attention.
- Listen to the pump and check for obvious circulation problems DIY safe
With the boiler running, listen at the pump location for a steady hum. If you hear only a faint vibration but hot water is not being produced, the pump may be running but not circulating — note this down for the engineer. You should not attempt to remove or work on the pump yourself.
- Engineer inspection: check and test the DHW temperature sensor Gas Safe engineer
A Gas Safe engineer will measure the electrical resistance of the DHW sensor and inspect the wiring harness and connector for corrosion or poor contact. A faulty or mispositioned sensor is replaced with a manufacturer-approved part.
- Engineer inspection: descale or replace the plate heat exchanger Gas Safe engineer
If limescale is suspected — particularly in hard water areas or if the boiler has not been serviced recently — the engineer will either chemically descale the heat exchanger using an appropriate acid solution followed by a thorough flush, or replace it entirely if the plates are damaged or severely fouled.
- Engineer inspection: assess the DHW pump Gas Safe engineer
The engineer will confirm whether the pump rotor is free and whether water is actually being circulated. A seized or underperforming pump is cleaned if possible, or replaced if it has failed.
- Engineer inspection: check the overheat thermostat Gas Safe engineer
The overheat stat is tested to confirm it is tripping at the correct temperature threshold (typically around 90–95 °C). If it has failed open or is triggering prematurely, it is replaced. A burned-out stat cannot be reset manually.
- Engineer inspection: test the three-way valve Gas Safe engineer
The engineer will verify that the three-way valve switches correctly between heating and hot water modes and that flow is being directed as expected. A sticking or faulty valve is replaced.
- Engineer inspection: PCB diagnosis and replacement if necessary Gas Safe engineer
If all components test correctly but the F.81 fault persists, the control board may be misinterpreting temperature data. PCB replacement is a last resort, carried out only after all other causes have been ruled out.
- Call a Gas Safe registered engineer if the fault returns or persists Gas Safe engineer
If the F.81 code reappears after a reset, or if you notice water leaks, unusual noises, or other fault codes appearing alongside it, contact a Gas Safe registered engineer. Do not continue attempting resets or operating the boiler if it keeps locking out — repeated overheating can cause further damage.
Parts you may need
- DHW temperature sensor · from £35
- Overheat safety thermostat · from £25
- DHW circulation pump · from £90
- Plate heat exchanger (DHW) · from £180
- Three-way valve assembly · from £65
- PCB (main control board) · from £220
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £120–£380, depending on the underlying cause.
Frequently asked questions
Can I reset an F.81 fault myself?
Yes — a single reset attempt is reasonable. Press and hold the reset button for around 5–10 seconds, let the boiler restart fully, then run a hot tap to see if the fault clears. If the F.81 comes back after one or two resets, the boiler needs professional diagnosis. Repeatedly resetting a boiler that keeps locking out on an overheating fault can cause additional damage and should be avoided.
Why is my pump running but I still have no hot water and an F.81 code?
This is a known characteristic of the F.81 fault. The pump can vibrate and sound active while not actually circulating water — either because the rotor is partially seized, because the pump impeller has failed, or because an airlock is preventing flow. The heat exchanger then overheats rapidly despite the pump appearing to work. An engineer will need to verify actual water flow rather than just checking whether the pump is making noise.
Could limescale cause the F.81 fault even if my boiler has been serviced recently?
Yes, particularly in hard water areas. Limescale accumulates inside the plate heat exchanger over time, and a standard annual service does not always include a descale of the DHW heat exchanger unless specifically requested. If you live in a hard water region and the boiler is more than a few years old, a descale is worth discussing with your engineer when investigating this fault.
Is it worth repairing an F.81 fault or should I replace the boiler?
For most common causes — a sensor, pump, or overheat stat — repair costs typically fall in the £120–£380 range and are usually worth doing if the boiler is under 10 years old and otherwise in good condition. However, if the plate heat exchanger needs replacing (which can cost £350–£600 including parts and labour) or the PCB is at fault (typically £250–£450 all-in), and your boiler is over 10 years old, replacement may be more economical in the long run. An annual service costing around £80–£150 is the best way to catch limescale and pump issues before they escalate to an F.81 lockout.