Ravenheat F84 Fault Code: Causes, Fixes & Repair Costs
What does the Ravenheat F84 fault code mean?
The F84 code appears when the Ravenheat boiler detects a persistent, abnormal difference between the temperatures recorded by the flow NTC thermistor (the sensor monitoring water leaving the boiler) and the return NTC thermistor (monitoring cooler water coming back from the heating circuit). A temperature gap between flow and return is perfectly normal during operation — heat is transferred to your radiators and the returning water is cooler. The fault triggers when that difference becomes excessive or stays fixed in a way the boiler's control logic cannot account for, suggesting that one or both sensors are giving inaccurate readings, there is a wiring problem between the sensors and the PCB, or something physical such as limescale or a heat exchanger fault is causing genuine abnormal heating conditions. The boiler shuts down as a safety measure to prevent overheating.
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 failed NTC thermistor Common
Ravenheat boilers use 25kΩ NTC thermistors at room temperature, dropping to around 4kΩ at 80°C. If either the flow or return thermistor drifts out of this characteristic range — due to age, heat stress, or moisture ingress — it will report an implausible temperature to the PCB, triggering F84. This is the single most common cause of this code.
- Damaged, loose, or corroded wiring connections Common
The wiring loom carrying signals from both thermistors to the PCB can develop faults over time. Loose connectors, corrosion at terminals, or damaged insulation can create intermittent or out-of-range signals. Boiler vibration from the fan and pump gradually works connections loose, which can cause the F84 fault to appear and disappear seemingly at random before eventually becoming permanent.
- Limescale build-up in the heat exchanger Sometimes
In hard-water areas, limescale accumulates inside the primary heat exchanger and creates localised hot spots. This causes the water immediately around the flow thermistor to reach a genuinely higher temperature than the rest of the system, producing an abnormal flow-to-return differential. Over time, severe scale can also cause micro-cracks in the heat exchanger, compounding the problem.
- Heat exchanger leak causing sensor or PCB contamination Sometimes
A cracked or leaking heat exchanger can allow water to seep onto nearby electrical components, including the thermistor connectors or the PCB itself. Moisture on these components corrupts the resistance readings sent to the control board, producing false temperature differentials and triggering F84.
- Faulty PCB Rare
The PCB reads the resistance values from both thermistors and calculates the temperatures. A failing PCB can misinterpret correct thermistor signals as a mismatch, or its analogue input circuits can develop faults that mimic a sensor error. PCB failure is relatively uncommon as a root cause and is typically diagnosed only after the sensors and wiring have been ruled out.
How to fix it
- Reset the boiler once or twice DIY safe
Press and hold the reset button on your Ravenheat boiler as described in your user manual. Wait for the boiler to attempt a restart. If the F84 code clears and the boiler runs normally, monitor it over the next few hours. If the fault returns immediately or after a short time, a component fault is likely and an engineer is needed. Do not reset more than two or three times in quick succession.
- Check your gas supply is active DIY safe
Confirm that other gas appliances in your home (such as a gas hob) are working normally. If there is no gas supply to the property, contact your gas supplier rather than an engineer. If gas is available elsewhere but the boiler continues to fault, the issue is internal to the boiler.
- Inspect the thermistors and wiring loom — engineer required Gas Safe engineer
A Gas Safe engineer will isolate and remove both the flow and return NTC thermistors and test their resistance with a multimeter at a known water temperature. A correctly functioning Ravenheat thermistor reads approximately 25kΩ at room temperature and around 4kΩ at 80°C. Any sensor reading outside its expected curve needs replacing. The engineer will also trace the full wiring loom from each sensor back to the PCB, looking for damaged insulation, corroded pins, or loose connectors.
- Assess the heat exchanger for scale and leaks — engineer required Gas Safe engineer
If the thermistors and wiring test as healthy, an engineer should inspect the primary heat exchanger for limescale build-up and signs of leakage. A power flush or descale may be recommended if scale is present. Cracking or leaking will require heat exchanger replacement, which is one of the more costly repairs on this boiler.
- Test or replace the PCB if all other components check out — engineer required Gas Safe engineer
If sensors, wiring, and the heat exchanger are all found to be in order, the PCB is likely misreading the thermistor signals. An engineer may be able to source a replacement PCB; however, on older Ravenheat boilers the cost of a PCB replacement can make a full boiler replacement a more economical decision.
- Call a Gas Safe registered engineer Gas Safe engineer
Beyond a reset and a basic gas supply check, all F84 diagnostics and repairs require a qualified Gas Safe engineer. Attempting to work on gas components, the PCB, or internal wiring yourself is unsafe and illegal. Contact a registered engineer to carry out a full diagnosis and advise on whether repair or replacement is the most cost-effective route.
Parts you may need
- NTC thermistor — flow or return (Ravenheat compatible) · from £18
- Wiring loom / sensor harness · from £35
- Primary heat exchanger (Ravenheat RSF/CSI) · from £180
- PCB (Ravenheat replacement control board) · from £150
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–£350, depending on the underlying cause.
Frequently asked questions
Can I fix the Ravenheat F84 fault myself?
The only safe DIY actions are resetting the boiler and confirming your gas supply is on. Everything else — testing thermistors, checking wiring, inspecting the heat exchanger — must be carried out by a Gas Safe registered engineer. Working on internal boiler components without the correct registration is both dangerous and illegal in the UK.
How much does it cost to fix a Ravenheat F84 fault code?
For the most common causes — a thermistor replacement or a wiring repair — most homeowners pay between £100 and £350 including parts and labour. If the heat exchanger needs replacing, costs typically rise to £400–£600, and a PCB replacement usually falls in the £300–£500 range. On older Ravenheat boilers, repairs costing more than around £500 often make a full boiler replacement the better financial choice.
Why does my F84 fault keep coming back after a reset?
An F84 that clears on reset but returns within minutes or hours almost always points to a genuine component fault rather than a one-off glitch. The most likely culprits are a thermistor that is drifting in and out of range as it heats up, or an intermittent wiring connection that loses contact under vibration. An engineer needs to test the sensors cold and at operating temperature to catch this kind of intermittent fault.
Is it worth repairing a Ravenheat boiler showing F84, or should I replace it?
If your Ravenheat boiler is under ten years old and the fault is limited to a thermistor or wiring repair (typically £100–£250), repair is usually cost-effective. If the boiler is more than ten years old and the fault turns out to be the heat exchanger or PCB, the repair bill can easily exceed £400–£500 with no guarantee other aging components will not fail soon after. In that situation, getting quotes for a new boiler installation alongside the repair quote makes good sense.