E5
Worcester Bosch Boiler
Severity: ModerateWhat Does This Error Mean?
Worcester Bosch E5 means the primary heating flow temperature climbed above the safety threshold.
The PCB shuts the burner down to protect the heat exchanger.
Cause is usually low flow — pump airlocked, dirty filter, frozen condensate trap restricting modulation, or the flow sensor itself reading high.
Fix the cause first, then reset.
Repeated E5 events stress the heat exchanger.
Affected Models
- Worcester Bosch Greenstar 30i
- Worcester Bosch Greenstar 25i
- Worcester Bosch Greenstar 8000 Life
- Worcester Bosch Greenstar 8000 Style
- Worcester Bosch CDi Compact
Common Causes
- Pump airlocked or running too slowly
- Magnetic filter clogged with sludge
- Heat exchanger fouled with limescale
- Heating flow NTC sensor drifting high
- All TRVs closed with no bypass open
- Recent power cut left the pump in a stuck state
How to Fix It
-
Wait for the boiler to cool.
E5 trips at high flow temperature.
Wait at least 30 minutes before resetting.
The PCB reads the cooled sensor and the lockout clears more reliably. -
Check pressure.
Pressure gauge on the boiler should read 1.0-1.5 bar cold.
Below 0.5 bar, top up via the filling loop until 1.0-1.2 bar.
Above 2.5 bar, bleed a radiator briefly. -
Bleed the pump.
Most Worcester Bosch combis have a pump with a bleed screw.
Slow turn anti-clockwise with a small flat-blade screwdriver until water trickles out, then close.
Power-cycle the boiler. -
Open at least one radiator fully.
If every TRV is closed, the pump pushes water against a wall and the boiler overheats locally.
Open the bathroom towel rail or a hallway radiator fully so the pump always has somewhere to send water. -
Clean the magnetic filter.
If a Magnaclean or similar filter is installed, clean it.
Iron sludge from old radiators chokes flow and causes recurring E5.
Annual cleaning is the rule of thumb. -
Engineer step: power flush if scaled.
If E5 returns within hours, the heat exchanger is likely scaled.
A power flush is 400-700 GBP and restores flow.
Worth doing on systems older than 8-10 years if it has not been done recently. -
Reset and observe.
Hold the reset button.
Run a heat call.
Watch for E5 returning within 30 minutes.
If it does, the underlying cause is still present and continued resets risk damaging the heat exchanger.
Frequently Asked Questions
Is E5 dangerous?
The lockout protects the heat exchanger.
The danger is repeated E5 resets without finding the cause — every overheat event shortens the heat exchanger's life and a new one is 600-1200 GBP.