Ravenheat F85 Fault Code: Causes, Fixes & Repair Costs
What does the Ravenheat F85 fault code mean?
The F85 fault code on a Ravenheat boiler indicates that one or both of the heating circuit's temperature sensors — the flow thermistor (measuring water leaving the boiler) and the return thermistor (measuring cooler water coming back) — are sending readings that the PCB considers unreliable or out of range. These sensors are NTC thermistors, small resistive components whose electrical resistance changes predictably with temperature. When the PCB detects erratic or implausible signals from either sensor, it shuts the boiler down to prevent the system from overheating. Typical symptoms include radiators that only warm partially even when the thermostat is turned up high, fluctuating hot water temperatures, or the boiler cycling on and off without reaching the set temperature. Note: Ravenheat Manufacturing Limited was dissolved in 2025, meaning factory support has ended and spare parts are becoming increasingly scarce — this is an important factor when deciding whether to repair or replace the boiler.
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
- Faulty or degraded NTC thermistor Common
After years of heating cycles, the thermistor's resistance characteristics can drift away from specification — particularly in hard-water areas where limescale builds up around the sensor pocket and insulates it from the water temperature it is meant to measure. This causes the PCB to receive readings that no longer match actual system temperatures, triggering F85.
- Damaged or chafed sensor wiring Common
The wiring loom running from the thermistors to the PCB can fray where it passes close to the heat exchanger or around sharp metal edges inside the casing. A partial short or open circuit in this wiring produces erratic voltage signals that the PCB interprets as a faulty sensor, even if the sensor itself is fine.
- Poor or corroded sensor connector Sometimes
The plug-in connectors joining the thermistor leads to the main wiring harness can corrode or work loose over time, especially in boilers installed in damp utility rooms or unheated spaces. Intermittent contact at the connector mimics a failing sensor and can cause the fault to appear and disappear.
- PCB fault misreading sensor signals Rare
In some cases the thermistors and their wiring test within specification, but the input circuitry on the PCB itself has deteriorated and is misinterpreting the signals. Given Ravenheat's now-ceased manufacturing status, sourcing a replacement PCB is increasingly difficult and expensive.
How to fix it
- Do not continue resetting the boiler repeatedly DIY safe
A single reset (holding the reset button as described in your user manual) is reasonable to attempt once. If F85 returns straight away or within a short time, further resets will not resolve an underlying sensor or wiring fault and may mask a developing problem. Limit resets to two or three attempts before moving on.
- Check that the boiler has adequate system pressure DIY safe
Look at the pressure gauge on the boiler front — it should read between 1.0 and 1.5 bar when the system is cold. If it is below 0.8 bar, a low-water condition can cause unusual sensor behaviour. Top up via the filling loop (two small valves, usually beneath the boiler) until pressure reaches around 1.2 bar, then close both valves. If pressure is already correct, this is not the cause.
- Visually inspect the wiring around the sensors — engineer task Gas Safe engineer
A Gas Safe engineer will power down the boiler and trace the sensor harnesses from each thermistor back towards the PCB, looking for insulation damage, fraying, or burn marks — particularly where cables pass near the heat exchanger. Damaged sections can sometimes be repaired with solder and heat-shrink sleeving, restoring correct signal continuity.
- Test thermistor resistance with a multimeter — engineer task Gas Safe engineer
With the boiler powered off and cooled, the engineer disconnects each thermistor and measures its resistance. A healthy NTC thermistor typically reads close to 10,000 ohms (10 kΩ) at 25 °C. A reading that is open-circuit, short-circuit, or significantly outside this value confirms the sensor has failed and needs replacing.
- Replace the faulty thermistor(s) — engineer task Gas Safe engineer
If a sensor has failed, the engineer fits a compatible NTC thermistor into the sensor pocket, ensuring good thermal contact. Parts currently cost around £15–£25 each, though availability from Ravenheat-specialist suppliers is declining now the manufacturer has ceased trading. The engineer will run a full heat cycle after replacement to confirm stable temperature readings on the boiler display.
- Inspect and test the PCB if sensor and wiring checks pass — engineer task Gas Safe engineer
Should the thermistors and their wiring all test correctly, the fault lies in the PCB's sensor input circuitry. Given Ravenheat's dissolution in 2025, new PCBs are no longer manufactured; the engineer may be able to source a used or reconditioned unit, but availability cannot be guaranteed. At this point a cost-versus-replacement assessment is strongly advisable.
- Call a Gas Safe registered engineer if you have not already done so DIY safe
All diagnostic and repair work beyond pressure top-up and a single reset must be carried out by a Gas Safe registered engineer. You can find registered engineers at GasSafeRegister.co.uk. Given the parts-availability situation with Ravenheat, ask the engineer to confirm parts can be sourced before committing to repair costs — and request an honest comparison with the cost of a new boiler installation if yours is more than ten years old.
Parts you may need
- NTC thermistor (flow or return sensor) · from £20
- Sensor wiring harness · from £35
- Sensor connector/plug set · from £12
- PCB (reconditioned/used — increasingly scarce) · from £180
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 £100–£280, depending on the underlying cause.
Frequently asked questions
Can I fix the Ravenheat F85 fault myself?
Not safely. While you can check system pressure and attempt a single reset, diagnosing and replacing NTC thermistors or repairing the wiring harness requires the boiler to be powered down and components handled near gas pipework and electrical circuits. This work must be carried out by a Gas Safe registered engineer. Attempting it yourself could void any remaining warranty and create safety risks.
Ravenheat has gone out of business — can I still get parts for an F85 repair?
Ravenheat Manufacturing Limited was dissolved in 2025. Some independent heating parts suppliers still hold old stock of Ravenheat NTC thermistors and wiring components, but availability is shrinking. It is worth asking your engineer to confirm the part is obtainable before agreeing to a repair. If the fault has reached the PCB, sourcing a replacement is significantly harder and may not be cost-effective — at that point a full boiler replacement is usually the more practical option.
How much does it cost to fix an F85 fault on a Ravenheat boiler?
If the problem is simply a failed thermistor, the part itself costs around £15–£25 and total repair bills (including Gas Safe engineer call-out and labour) typically fall in the £100–£280 range. If the wiring harness also needs attention, costs sit towards the higher end of that range. Should the PCB need replacing, you are looking at significantly more — and given parts scarcity, it may be more economical to replace the boiler entirely, particularly if it is over ten years old.
Why are my radiators only half-warm when F85 appears?
The boiler's PCB uses the flow and return thermistor readings to regulate burner output and protect the system from overheating. When one or both sensors send erratic data, the PCB either reduces firing rate or locks out entirely as a precaution. This means the boiler cannot deliver full heat output, leaving radiators lukewarm even at maximum thermostat settings. Once the faulty sensor or wiring is replaced, normal temperature control should be restored.