Worcester Bosch 2964 Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch 2964 fault code mean?
Fault code 2964 (sometimes displayed as 2964 B) appears on Worcester Bosch boilers when the control board detects that the flow rate through the heat exchanger is too low. In practice, this means the boiler fired but did not detect the expected temperature rise at the flow sensor — a sign that water is not moving through the heat exchanger correctly. This is a blocking error, so the boiler shuts down to protect itself but may attempt to restart automatically. If the underlying cause is not addressed the fault will keep returning. To see the code itself you usually need to press the spanner or return button on the display, as the boiler may simply show a blank or fault indicator during the lockout.
General guidance only — not a substitute for professional advice. 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
- Air trapped in the heat exchanger or system Common
When air pockets build up inside the heat exchanger or pipework, water cannot flow freely and the flow sensor fails to detect a proper temperature rise after ignition. Worcester Bosch specifically advise powering the boiler off and back on to trigger an automatic air purge cycle as a first response to this fault. Bleeding radiators regularly helps prevent air accumulating in the first place.
- Pump failure or reduced pump output Common
The circulating pump pushes water continuously around the system. If it seizes, becomes sluggish from age or debris, or loses efficiency, circulation drops and the heat exchanger overheats. This triggers the flow-rate safety check and the 2964 fault. A failing pump is one of the most frequently confirmed root causes when an engineer attends this fault.
- Sludge or debris restricting circulation Common
Over time, rust particles and pipe debris break down into a thick sludge that coats pipe interiors, blocks the magnetic system filter, and reduces flow through the heat exchanger. Older systems or those that have never had a magnetic filter fitted are particularly vulnerable. Even a partially blocked inline filter can be enough to trigger this fault.
- Faulty or poorly seated flow temperature sensor Sometimes
The sensor that monitors primary flow temperature must be properly clipped to the heat exchanger. If it has worked loose, corroded, or drifted out of calibration, it can report an out-of-range temperature even when flow is actually adequate, generating a false 2964 fault.
- Limescale build-up or damage inside the heat exchanger Sometimes
In hard-water areas, limescale deposits gradually coat the inside of the heat exchanger, acting as insulation and reducing heat transfer efficiency. This can cause localised overheating and abnormal sensor readings. A heavily scaled or physically damaged heat exchanger may need chemical descaling or, in severe cases, replacement.
How to fix it
- Power cycle the boiler to trigger an air purge DIY safe
Turn the boiler off at the on/off switch or programmer, wait around 30 seconds, then switch it back on. Worcester Bosch's own guidance recommends this as the first step because the restart sequence initiates an automatic air purge that can clear minor airlocks in the heat exchanger without any dismantling.
- Check the system pressure and top up if low DIY safe
Look at the pressure gauge on the front of the boiler. It should read between 1 and 1.5 bar when the system is cold. If it is below 1 bar, top up using the filling loop (usually a braided flexible hose with one or two valves beneath the boiler). Open the valve(s) slowly until the gauge reads around 1.2–1.3 bar, then close them fully. A full air purge is also triggered automatically when the system is repressurised after a low-pressure episode.
- Bleed the radiators to remove trapped air DIY safe
If any radiators feel cold at the top but warm at the bottom, they contain trapped air. Use a radiator bleed key on the bleed valve at the top corner of each radiator. Hold a cloth underneath and open the valve slowly until water (not air) trickles out, then close it. Work from the ground floor upwards. After bleeding, recheck the system pressure and top up again if needed.
- Reset the boiler DIY safe
Once you have carried out the above checks, press the reset button (consult your model's handbook — it is usually a flame symbol with a cross, or the return/spanner button). Hold it for around 3 seconds. If the fault clears and the boiler fires normally, monitor it over the next hour or so. Do not reset more than 2–3 times in total; repeated resets without a fix can stress components.
- Check that the inline system filter is not blocked Gas Safe engineer
Many Worcester Bosch installations include a magnetic system filter on the return pipework. A clogged filter is a very common trigger for this fault. An engineer can isolate, remove, and clean the filter cartridge safely — this is often all that is needed. Do not attempt to remove the filter yourself unless you are confident isolating the relevant valves and have the necessary skills, as incorrect removal can cause a water leak or introduce more air.
- Have a Gas Safe engineer inspect and test the circulation pump Gas Safe engineer
An engineer will check whether the pump is running, spinning freely, and delivering adequate flow. They may clean or free a seized pump impeller, or replace the pump entirely if it has failed. Pump replacement on most Worcester Bosch models is a straightforward job for an engineer and is one of the most cost-effective fixes for this fault.
- Ask the engineer to inspect and test the flow temperature sensor Gas Safe engineer
The engineer will check that the sensor is correctly clipped onto the heat exchanger, that its electrical connections are sound, and that it is reading within the expected range on the service tool. A faulty sensor is replaced as a complete unit and is relatively inexpensive to source.
- Consider a power flush or chemical system cleanse if sludge is suspected Gas Safe engineer
If the engineer finds thick sludge in the filter or evidence of poor flow across the system, a power flush or chemical flush may be recommended. This involves pumping a cleaning agent and fresh water through every radiator and the boiler circuit at high velocity to remove deposits. Following a flush, a corrosion inhibitor should be added and a magnetic filter installed (or an existing one cleaned and reinstated) to prevent recurrence.
- If the fault persists, call a Gas Safe registered engineer for full diagnostics Gas Safe engineer
If none of the DIY steps resolve the issue and an engineer has not yet attended, book one promptly. Persistent 2964 faults that do not clear after a power cycle and pressure top-up almost always have a mechanical or component cause — pump failure, sludge, a sensor fault, or in rarer cases a scaled or damaged heat exchanger — that requires professional diagnosis and repair.
Parts you may need
- Circulation pump (compatible replacement) · from £85
- Flow temperature sensor (NTC thermistor) · from £25
- Magnetic system filter (e.g. Fernox TF1, Adey MagnaClean) · from £55
- Inhibitor treatment (e.g. Fernox F1, Sentinel X100) · from £18
- Heat exchanger (model-specific) · from £320
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 £150–£450, depending on the underlying cause.
Frequently asked questions
Can I fix Worcester Bosch fault 2964 myself?
You can safely try a power cycle (off-on) to trigger an air purge, top up system pressure to 1–1.5 bar, bleed the radiators, and reset the boiler once or twice. These steps resolve the fault in a meaningful number of cases, particularly where the cause is an airlock or low pressure. However, if the fault returns after those checks, you need a Gas Safe registered engineer — the underlying cause is almost certainly a pump issue, sludge blockage, sensor fault, or heat exchanger problem, none of which are safe or practical for a homeowner to tackle.
How much does it cost to fix a Worcester Bosch 2964 fault?
Most people with this fault pay somewhere between £150 and £450 all-in. A pump service or replacement typically costs £200–£400 including labour; a sensor replacement is usually £100–£200; cleaning a blocked magnetic filter is often included in a standard call-out. If the heat exchanger itself needs replacing the cost rises significantly — typically £500–£900 or more once parts and labour are included — but this is a less common outcome. If your boiler is over 10–12 years old and facing a heat exchanger replacement, it may be worth getting a new boiler quote at the same time.
Why does fault 2964 keep coming back after I reset it?
Repeated returns of this fault mean the root cause has not been fixed. The boiler is detecting genuinely low flow through the heat exchanger each time it fires. Common culprits are a pump that is gradually seizing, a magnetic filter that is becoming clogged with sludge, or significant limescale inside the heat exchanger. Continuing to reset without addressing the cause can put extra strain on the heat exchanger and pump. Book a Gas Safe engineer to diagnose the problem properly.
What is the difference between fault code 2964 and 2964 B?
They refer to the same fault. Worcester Bosch technical documentation uses both notations — '2964' is the numeric code and 'B' denotes the fault category (blocking error) in their coding system. A blocking error means the boiler shuts down for safety but the fault is not necessarily permanent; it may attempt to restart automatically. The cause, diagnostic steps, and repairs are identical whichever way the code is written on your boiler's display.