Worcester Bosch E5 218 fault codewhat it means and how to fix it
The flow temperature sensor has measured water leaving the heat exchanger far hotter than it should be. The boiler shut down to protect itself, and heat is not being carried away fast enough.
Take care
Get a Gas Safe registered engineer to look at your Worcester Bosch E5 218
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What do you notice when it happens?
The right answer depends on what you can see. Pick the one that matches.
Choose the option on the left that matches what your appliance is doing, and the answer for that case appears here.
There is banging or kettling from the boiler beforehand
That noise is water flashing to steam against hot metal, which is what happens when flow through the exchanger has become badly restricted.
Stop using the boiler and have the circuit and heat exchanger examined. Continuing risks the exchanger itself.
Radiators stay cold or only warm at the top
Heat is being made but not distributed, which fits a pump or circulation problem rather than anything wrong with combustion.
Mention which radiators are affected — it helps narrow pump failure against a blockage or a stuck valve.
Hot water is fine but heating trips it
A fault confined to one mode points at the valve or circuit serving that mode, not at the boiler as a whole.
Say so when you book — it saves an hour of diagnosis.
What E5 218 means
E5 218 is a measured overtemperature at the flow sensor, and it is a published pairing: the Greenstar CDi Compact and Si Compact ErP tables list "E5 218 — flow temperature too high — primary flow sensor overheated" as a locking error cleared with the reset button, alongside a second E5 cause code, 332, "primary flow sensor has exceeded 110 °C".
The 105 °C figure once quoted on this page belongs to a different code in the same table — E9 219, the safety sensor on the heat exchanger — and is not Worcester's threshold for 218, so treat the temperature here as well above normal rather than as a number. The Greenstar i ErP table does not use E5 at all; it publishes the measured flow overtemperature as E9 276, "primary flow temperature exceeded 95 °C". In a healthy system neither can happen, because water moving through the exchanger takes heat away as fast as the burner puts it in.
When the water slows or stops, the same amount of gas heats a much smaller volume and the temperature climbs very quickly, and the boiler stops before anything is damaged. Note what this code is not: it is not the safety limiter tripping, which Worcester report as E9 224, and it is not a sensor fault, which is E2. E5 218 says the flow genuinely got too hot, and the question an engineer answers is why the water was not moving — usually the pump, sludge, or a valve that is not opening.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Pump not moving water properly | A pump can spin and still shift almost nothing once the impeller is worn or jammed with debris. | Common |
| Sludge restricting the heat exchanger | Iron oxide narrows the waterways until flow through the exchanger cannot keep pace with the burner. | Common |
| Air trapped in the primary circuit | A pocket of air at the high point of the circuit stops circulation entirely while the gauge still reads normally. | Occasional |
| Zone valve or diverter not opening | If the path the water should take stays shut, the burner fires into a circuit with nowhere to go. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£120–£900
Pump replacement £180–£320. Power flush £400–£700 depending on system size. Magnetic filter £120–£220 fitted. A damaged heat exchanger is considerably more and may prompt a replacement decision.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Central heating circulating pump
CH-PUMP-15-60£90–£180Plumbing merchant
Price checked 28 September 2026
Magnetic system filter, 22mm
MAG-22£65–£120Plumbing merchant
Price checked 28 September 2026
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Worcester Bosch fault codes
- E9 219Stop — call an engineerThe boiler has overheated and the safety thermostat has tripped. Water is not carrying heat away from the exchanger, and it should not be run until that is fixed.Related: overheat and circulation
- E9 224Stop — call an engineerOne of the two safety thermostats has tripped: the flue overheat thermostat or the high-limit stat on the heat exchanger. Worcester's own check list includes the burner gasket for flue gas leakage.Related: overheat and circulation
- A1 229Stop — call an engineerThe boiler has detected a dry-fire condition — it is trying to heat water that is not moving. Turn it off and leave it off until an engineer has seen it.Related: overheat and circulation
- E2 222Get it checkedThe flow temperature sensor is giving a reading the appliance cannot trust. Usually the sensor rather than the temperature.Related: overheat and circulation
- D1 240Get it checkedThe return temperature sensor has short circuited. The boiler has lost its reading of how warm the water is when it comes back from the radiators.Related: overheat and circulation
- D4 341Get it checkedThe water temperature in the boiler is climbing faster than it should, which means very little of it is moving. Worcester's checks are pressure, air, circulation and the pump, in that order.Related: overheat and circulation
Own a Worcester Bosch boiler? Save it and its codes come first next time.
Related guides
Common questions about Worcester Bosch E5 218
How is this different from E9?
E5 is the flow sensor measuring an overtemperature — Worcester publish 95°C as the threshold for their comparable code, E9 276. The E9 group is the safety temperature limiter cutting in, which Worcester describe as the safety circuit and which is the layer of protection behind the measurement. E5 often appears first on the same boiler, and if it is ignored the limiter is what eventually stops it.
Can I just keep resetting it?
No. Every reset lets the burner fire into a circuit that still cannot carry heat away, and repeated overheating is what cracks heat exchangers. A replacement exchanger is one of the most expensive repairs on the appliance.
Would a magnetic filter have prevented it?
It would have helped. A filter catches the iron oxide that would otherwise settle in the exchanger and the pump. It does not undo damage already done, which is why fitting one usually goes alongside a proper flush.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
- Manufacturer support portal
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.