Protherm F.23 Fault Code: Causes, Fixes & Repair Costs
What does the Protherm F.23 fault code mean?
The F.23 code on Protherm boilers signals that the temperature gap between the flow pipe (hot water leaving the boiler) and the return pipe (cooler water coming back) has grown dangerously large — exceeding 35°C (35K). The boiler interprets this as a sign that water is not circulating properly, raising the risk of localised overheating. Protherm's control board will tolerate a brief excursion, but once the fault has been logged five times it escalates to a full safety lockout requiring a manual reset. Because Protherm is part of the Vaillant Group, the F.23 definition is consistent across the Gepard (Cheetah), Panther and Leopard ranges and shares the same logic as Vaillant's own F.23 code. The underlying problem is nearly always related to poor water circulation, low system pressure, or a sensor giving inaccurate readings — not a gas-side fault — but it still demands prompt attention to protect the heat exchanger from damage.
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
- Weak or seized circulation pump Common
The pump is responsible for pushing heated water around the system and back to the boiler at a steady rate. If the pump motor is running slowly, its impeller shaft has seized through inactivity, or its speed setting is too low, water lingers in the radiators and returns to the boiler much cooler than expected — widening the flow/return gap beyond the 35°C threshold that triggers F.23. This is the single most common root cause of this fault.
- Sludge or scale restricting circulation Common
Over time, rust particles from steel radiators and limescale from hard water accumulate as a dark, gritty deposit throughout the pipework, radiators and heat exchanger. This narrows the internal bore of pipes and restricts how freely water can move, slowing circulation enough to create the large temperature differential the boiler detects. Systems that have never been power-flushed or that lack a magnetic filter are particularly vulnerable.
- Airlock in the heating circuit Common
Pockets of trapped air in radiators or pipework interrupt the continuous flow of water around the circuit. Even a partial airlock can slow circulation significantly, causing the boiler to overheat the water sitting near the flow sensor while the return runs cold. Airlocks often develop after system pressure has dropped, after radiators have been bled without topping pressure back up, or following work on the pipework.
- Low system pressure Sometimes
Protherm boilers require water pressure of roughly 1.0–1.5 bar to circulate correctly. If the pressure has fallen below 0.5 bar — visible on the boiler's pressure gauge — there is simply not enough water in the circuit for the pump to maintain adequate flow. This can produce the same dangerous temperature split that triggers F.23.
- Faulty flow or return NTC thermistor Sometimes
The boiler relies on two NTC temperature sensors (thermistors) to measure the flow and return temperatures and calculate the differential. If either sensor has drifted out of calibration, developed a loose connection, or failed outright, the boiler may calculate a false temperature gap and lock out even when circulation is perfectly normal. An engineer can confirm this with a resistance check against the expected values.
How to fix it
- Reset the boiler (up to 2–3 attempts) DIY safe
Locate the reset button on your Protherm boiler — on most models it is a dedicated button labelled 'Reset' or shown with a flame-and-arrow symbol. Hold it for 3–5 seconds until the display clears, then allow the boiler a full ignition cycle. If the boiler runs normally for a sustained period the fault may have been a one-off glitch. Do not reset more than two or three times; repeated resets without fixing the cause can mask a worsening problem.
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the front of the boiler (or on the pipework below it). A healthy cold reading is 1.0–1.5 bar. If the needle is below 1.0 bar, repressurse the system via the filling loop — a braided silver hose usually found beneath the boiler connecting two valves on the pipework. Open both valves slowly, watch the gauge climb to around 1.2 bar, then close both valves. If pressure keeps dropping after topping up, there is likely a leak that needs professional attention.
- Bleed the radiators to clear airlocks DIY safe
Starting with the radiator furthest from the boiler and working back towards it, use a radiator bleed key to open the bleed valve (the small square fitting at the top corner of each radiator) just a quarter-turn. Hold a cloth beneath it. When a steady stream of water replaces the hissing air, close the valve. After bleeding all radiators, recheck the boiler pressure and top up again if it has dipped below 1.0 bar, as releasing air will cause a small pressure drop.
- Check that the pump is running and not seized DIY safe
With the heating on, carefully place your hand on the pump body (usually a cylindrical unit on the pipework near the boiler — avoid touching very hot surfaces). A working pump produces a faint hum and very slight vibration. Complete silence may mean the pump has seized. Do not attempt to dismantle or replace the pump yourself — isolating the heating circuit, draining sections of pipework and fitting a replacement pump must be done by a Gas Safe registered engineer.
- Look for obvious signs of sludge or blockage DIY safe
Check whether some radiators are cold at the bottom but warm at the top — a classic sign of sludge settling. Also check whether any radiator lockshield or thermostatic radiator valves have been accidentally closed, as this reduces flow through part of the circuit and can indirectly widen the temperature differential. Open any valves that appear fully closed. Do not attempt to dismantle pipework or radiator connections.
- Call a Gas Safe registered engineer to diagnose and repair Gas Safe engineer
If the fault persists after the checks above, a qualified engineer needs to carry out: resistance tests on both the flow and return NTC thermistors to confirm they are reading accurately; a pump performance check including speed setting and flow rate; an assessment of whether a power flush is needed to clear sludge; and replacement of any faulty components such as the pump, thermistors, or motorised valves. Do not continue to reset and run the boiler if the underlying cause has not been identified, as repeated overheating can damage the heat exchanger.
Parts you may need
- Circulation pump (suitable for Protherm Gepard/Panther) · from £85
- NTC flow thermistor (temperature sensor) · from £25
- NTC return thermistor (temperature sensor) · from £25
- Magnetic system filter (e.g. Adey MagnaClean or equivalent) · from £70
- Power flush chemical pack (central heating cleaner and inhibitor) · from £35
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 my Protherm F.23 fault and keep using the boiler?
A single reset is worth trying — occasionally the fault is triggered by a brief circulation hiccup and clears itself. However, F.23 becomes a full lockout after five occurrences, and continuing to reset without fixing the root cause risks damaging the heat exchanger through repeated overheating. If the code returns within a short time, stop resetting and arrange for an engineer to investigate.
How much does it cost to fix a Protherm F.23 fault in the UK?
Most homeowners pay somewhere between £120 and £380 all in. A simple fix such as clearing an airlock or adjusting the pump speed setting sits at the lower end. Replacing the circulation pump — parts plus an hour or two of labour — typically comes in at £180–£280. A professional power flush to remove sludge usually costs £250–£400 depending on the size of the system and your location. Replacing a thermistor is generally one of the cheaper repairs at £100–£180 including labour. A complete heat exchanger replacement, which is only necessary in extreme cases where overheating has caused damage, can cost £500–£900 — but this outcome is avoidable if the fault is dealt with promptly.
Why does my Protherm show F.23 only in cold weather?
In very cold conditions your boiler fires harder and more frequently to meet the heating demand. If your pump is already marginal or there is a partial sludge restriction in the system, the extra heat input exposes the weakness — the pump simply cannot shift water fast enough to keep the flow/return differential within the 35°C limit. The fault may disappear in milder weather only to return each winter, so it is worth having the system inspected before the next cold spell rather than waiting for a full lockout.
Is F.23 the same on Protherm and Vaillant boilers?
Yes. Protherm is a Vaillant Group brand and the two ranges share the same control and fault-code architecture. F.23 means an excessive flow-to-return temperature differential (triggered above 35°C differential) on both makes. An engineer familiar with Vaillant fault codes will therefore have no trouble working on a Protherm showing F.23, and the same parts — pumps, thermistors — are often interchangeable between equivalent models.