Protherm F.20 Fault Code: Causes, Fixes & Repair Costs
What does the Protherm F.20 fault code mean?
The F.20 code appears when Protherm's built-in safe temperature limiter detects that the primary circuit flow temperature has climbed above 90°C and has cut the boiler out as a precaution. In plain terms, the boiler has overheated and shut itself down to prevent damage or a dangerous situation. Once the water temperature drops back to a safe level the boiler may attempt to restart automatically, but unless the underlying cause is identified and fixed, the fault will keep returning. F.20 sits in a range of codes that Protherm considers serious enough to affect both performance and safe operation, so it should not be ignored or repeatedly reset without investigation.
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
- Circulation pump failure or airlock Common
The pump is responsible for moving water around the primary circuit continuously. If it has seized, developed a fault, or become airlocked, heat builds up rapidly in the heat exchanger with nowhere to go, quickly pushing the flow temperature above 90°C and triggering F.20. A blocked or partially closed isolation valve on the flow or return pipework has the same effect.
- Low system pressure reducing circulation Common
When system pressure falls noticeably below 1 bar, there is less water in the circuit for the pump to move, leading to uneven heat distribution and hotspots that cause the overheat limiter to activate. Check the pressure gauge on the front of the boiler — it should sit between 1.0 and 1.5 bar when the system is cold.
- Limescale build-up on the heat exchanger Common
In hard water areas, limescale accumulates on the internal surfaces of the heat exchanger over time. This acts as an insulating layer, creating localised hotspots where water can approach boiling point, sometimes accompanied by a kettling noise. The F.20 limiter trips because the boiler cannot transfer heat efficiently into the circulating water.
- Air pockets in the system Sometimes
Trapped air in radiators, the heat exchanger, or pipework disrupts smooth water flow, creating gaps in circulation where heat concentrates. Bleeding radiators and automatic air vents can help, but persistent air ingress may point to a leak or a failing auto-vent.
- Faulty NTC flow temperature sensor Sometimes
The NTC thermistor monitors the flow temperature and reports it to the boiler's control board. If it develops a fault or drifts out of calibration, it can report falsely high temperatures, triggering an F.20 lockout even when the water is not actually overheating. Conversely, a failed sensor that reads low could allow genuine overheating before the board reacts.
- Faulty or blocked pressure relief valve (PRV) Rare
The PRV is designed to release excess pressure safely. If it becomes blocked or fails in a closed position, pressure inside the circuit can rise beyond normal operating limits, contributing to overheating conditions and an F.20 trip.
- Fan failure Rare
Although the fan primarily manages combustion air, a failing fan can affect the overall thermal balance of the appliance. If the boiler's ventilation is compromised, heat may not be dissipated correctly, potentially contributing to an overheating condition.
How to fix it
- Switch the boiler off and let it cool completely DIY safe
Turn the boiler off at the programmer or room thermostat and leave it for at least 20–30 minutes. Do not attempt to reset or restart an overheating boiler immediately — the limiter needs the water temperature to drop before any restart is possible, and forcing repeated resets on an overheating appliance can cause further damage.
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the boiler's front panel. It should read between 1.0 and 1.5 bar when the system is cold. If it is below 1 bar, top up via the filling loop following the instructions in your Protherm user manual. Do not overfill — stop at around 1.2–1.3 bar. If the pressure drops again quickly after topping up, there may be a leak that a Gas Safe engineer will need to trace.
- Check that radiator and isolation valves are open DIY safe
Make sure the thermostatic or manual valves on your radiators are open on at least most of the radiators in the house. A fully closed system gives the pump nowhere to push water, causing rapid overheating. Also check that the isolation valves on the boiler's flow and return pipes (usually located beneath the boiler) are fully open.
- Bleed your radiators to remove trapped air DIY safe
Using a radiator bleed key, open the bleed valve on each radiator a fraction until water dribbles out steadily (no more air hissing), then close it. Work upstairs first, then downstairs. After bleeding, recheck the system pressure and top up again if it has dropped slightly.
- Listen for the circulation pump DIY safe
When the boiler is back up to temperature and calling for heat, place your hand carefully on the pump body (usually a cylindrical motor attached to pipework near the boiler) or listen closely. You should feel a gentle vibration and hear a quiet hum. Silence or a loud grinding noise suggests the pump may have seized or airlocked — this needs an engineer.
- Attempt a single boiler reset DIY safe
Press and hold the reset button on the boiler's control panel for 3–5 seconds as described in your Protherm user manual. Allow the boiler to complete its ignition sequence and run for several minutes. If F.20 returns promptly or the boiler fails to fire, do not keep resetting — move on to the next step.
- Call a Gas Safe registered engineer for further diagnosis Gas Safe engineer
If the fault returns after your checks, a qualified engineer is needed. They will inspect the circulation pump for correct speed and output, check the NTC flow thermistor with a multimeter, power-flush or chemically clean a scaled heat exchanger, test the PRV and fan, and add inhibitor fluid and a magnetic filter if not already fitted. Never open the boiler casing or attempt to adjust internal gas or electrical components yourself.
Parts you may need
- NTC flow temperature thermistor · from £25
- Circulation pump (compatible replacement) · from £80
- Overheat safety thermostat / limiter · from £20
- Pressure relief valve (PRV) · from £18
- Magnetic system filter (e.g. Fernox TF1 or equivalent) · from £65
- Central heating inhibitor (e.g. Fernox F1, 1 litre) · from £15
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 just keep resetting the Protherm F.20 and carry on using the boiler?
No — repeatedly resetting an overheating boiler is not safe and risks damaging internal components including the heat exchanger and pump seals. F.20 means the boiler reached a temperature above 90°C and shut down for a reason. One reset after the boiler has cooled is reasonable to see whether the fault clears, but if it comes back you need a Gas Safe engineer to find and fix the root cause.
How much does it typically cost to fix a Protherm F.20 fault in the UK?
For most households the repair falls between £120 and £380, covering call-out, diagnosis, and the most common fixes such as freeing or replacing a pump (£150–£250 including labour), replacing an NTC sensor (£100–£180), or a system flush with inhibitor (£150–£300). If the heat exchanger is heavily scaled and needs full replacement, costs can rise to £400–£600 or more, but this is less typical and worth discussing with your engineer before committing.
My Protherm boiler is making a kettling noise as well as showing F.20 — what does that mean?
Kettling — a rumbling or boiling sound coming from the boiler — strongly suggests limescale build-up on the heat exchanger, which is particularly common in hard water areas across the Midlands, South East, and East of England. The scale creates hotspots where water almost boils locally, pushing the flow temperature up and tripping the F.20 limiter. A Gas Safe engineer can chemically flush or power-flush the heat exchanger and add scale inhibitor to slow future build-up.
How can I prevent the F.20 fault from coming back?
Annual servicing by a Gas Safe engineer is the single most effective measure — they will check pump operation, system pressure, inhibitor levels, and heat exchanger condition before problems develop. Fitting a magnetic system filter (such as a Fernox TF1 or Adey MagnaClean) and keeping it topped up with inhibitor protects against sludge and scale year-round. Balancing your radiators so water flows evenly around the system also reduces the risk of hotspots that can trigger overheating faults.