What the E9 276 code means
On the Greenstar i and similar models, Worcester's installation manual describes cause code 276 as "primary flow temperature exceeded 95°C". The primary flow is the hot water leaving the boiler on its way to the radiators or the hot water heat exchanger. The flow temperature sensor measures it constantly, and if it goes past 95°C the boiler stops.
In normal use the flow should be well below that figure, so the boiler is telling you that heat is building up faster than it is being carried away. That almost always comes back to how well water is moving round the system.
E9 276 is closely related to E9 224, which is a safety thermostat tripping. The difference is that 276 is detected by the flow sensor, while 224 comes from a separate high limit or flue thermostat. They share several causes, so the same basic checks apply.
Common causes of E9 276
Low system pressure
With too little water in the system, circulation suffers and the heat exchanger heats up quickly. Worcester lists system pressure as the first thing to check.
Air in the system
Air pockets block the flow of water through radiators and pipework, and air in the pump reduces how much water it can move.
Nowhere for the heat to go
If most radiator valves are closed or turned right down, and there is no bypass to keep water moving, the boiler has very little to heat and the flow temperature shoots up.
Pump problems
A sticking, slow or failed pump cannot move enough water. This often shows up alongside the A1 281 pump fault.
Sludge or a faulty sensor
Sludge in older systems restricts flow, and a faulty flow sensor or poor connection can give a false high reading.
Hot water use on a combi
On a combi, some people notice E9 276 during or just after a hot water draw. If the plate heat exchanger that heats your tap water is partly blocked with scale, heat is not passed on quickly enough and the primary flow temperature can spike. Hard water areas are more prone to this. It is worth telling the engineer if the fault only appears when taps or the shower are running.
| Cause | Typical sign | Who fixes it |
|---|---|---|
| Low pressure | Gauge below 1 bar when cold | You, via the filling loop |
| Air in radiators | Cold at the top, gurgling | You, by bleeding radiators |
| Radiator valves closed | Fault when most rooms are turned down | You, by opening valves |
| Pump fault | Humming, poor heat, A1 codes | Gas Safe engineer |
| Sludge or blocked filter | Cold patches at the bottom of radiators | Gas Safe engineer |
| Faulty flow sensor | Fault with good pressure and circulation | Gas Safe engineer |
Safe checks you can do yourself
- Turn the heating off and let the system cool before checking anything.
- Check the pressure gauge. If it is below 1 bar, top up to 1 to 1.5 bar with the filling loop, then close both valves fully. Our guide on how to repressurise a boiler shows how.
- Bleed any radiators that are cold at the top, then check the pressure again.
- Check your thermostat, timer and radiator thermostatic valves. Make sure several radiators are open so the boiler has somewhere to send the heat.
- Press and hold reset as your manual describes. Stop after two or three attempts if the code comes back.
Do not remove the casing or touch the pump, sensors or heat exchanger. If you hear loud banging from the boiler or smell burning, switch it off at the fused spur and call a Gas Safe registered engineer. If you smell gas, call the National Gas Emergency Service on 0800 111 999.
When to call a Gas Safe engineer
If the pressure is correct, the radiators are bled and open, and E9 276 still returns, call a Gas Safe registered engineer. Worcester's guidance points engineers to the sensor wiring and connector, circulation through the heating system and the operation of the pump. They may also check the filter and look for sludge.
When you book, tell the engineer the full code, the pressure reading, whether you have bled any radiators and whether the fault happens on heating, hot water or both. Noises such as kettling or banging are also worth mentioning, as they suggest poor flow through the heat exchanger.
You should also call an engineer straight away if you have to top up the pressure more than once or twice in a few weeks. A system that keeps losing water will keep drawing in fresh, oxygen-rich water, which speeds up corrosion and sludge, and that in turn makes overheating faults more likely.
Typical repair costs in the UK
Many E9 276 faults are cleared by topping up and bleeding, which costs nothing. If a repair is needed, these ranges are typical. Prices vary by region and by engineer.
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £80 to £150 |
| Flow temperature sensor replacement | Typically £80 to £160 |
| Pump replacement | Typically £250 to £450 fitted |
| Magnetic filter fitted | Typically £150 to £250 |
| System clean or powerflush | Typically £400 to £700 |
How to stop E9 276 coming back
Keep the pressure between 1 and 1.5 bar when cold and check it monthly in winter. Avoid shutting every radiator off at once while the heating is running. If you often find air in radiators or keep losing pressure, there may be a leak, and our guide on a boiler leaking water can help you spot it. A yearly service lets an engineer check the pump, filter and sensors, and our boiler service cost guide explains what that typically costs.