Strom E-Boiler E5 Error Code Explained & How to Reset!
Last reviewed: 17 July 2026
On Strom E-Boilers covered by the V6.2 Rev 2 manual, the E5 error code means there is inadequate water flow through the central-heating circuit. The boiler requires a minimum flow rate of 3 litres per minute before it can operate correctly.
Quick answer: E5 is normally caused by restricted circulation, an incorrectly operating pump, air in the system, a bypass-valve problem or blocked strainers. Correct the cause first, then press and hold the boiler’s power button until it beeps to clear the code.
The 3 litres-per-minute figure relates to water circulating around the heating system. It is not the flow rate coming from a hot tap.
This guide follows the Strom V6.2 installation and user guide for Rev 2 system and combination boilers. Strom’s V2.2 heat-only boiler manual gives the same E5 definition and minimum flow requirement. Check the model, revision and manual supplied with your own boiler before carrying out any work because controls and fault-code definitions can vary between versions.
What does the Strom E-Boiler E5 error mean?
The boiler uses a dedicated heating flow sensor to confirm that water is circulating through the heat exchanger. When the detected flow falls below the required level, the boiler displays E5 and prevents normal heating operation.
This protects the boiler from attempting to transfer heat into water that is not moving around the system quickly enough. Resetting the display without restoring circulation will normally result in the E5 code returning.
The manufacturer’s fault-finding information identifies four initial areas to check:
- The circulating pump and its speed setting.
- Air trapped within the heating system.
- Whether an automatic bypass valve has been installed and set correctly.
- Blockages in the inlet and return-line strainers.
How do you reset a Strom E-Boiler after an E5 fault?
Once the cause of the inadequate flow has been addressed, press and hold the boiler’s power button until the boiler beeps. The error should clear and the heating can then be tested again.
- Turn down the room thermostat so that there is no demand for heat.
- Check the external, user-accessible items described below.
- Do not remove the boiler casing or touch internal wiring.
- Press and hold the power button until the boiler beeps.
- Restore the heating demand and watch the display.
- If E5 returns, stop resetting the boiler and arrange further fault-finding.
A reset clears the displayed fault. It does not repair a circulation problem, unblock a strainer or correct an unsuitable pump or bypass setting.
Safe checks a homeowner can make
Check the heating-system pressure
Look at the pressure gauge without opening the boiler. The V6.2 Rev 2 specification gives a heating-pressure range of 0.5 to 1.5 bar, while the filling instructions describe bringing a cold system to between 1 and 1.5 bar.
Follow the instructions for your exact boiler and installation. Do not continue adding water if you are uncertain how the filling arrangement works or if the pressure repeatedly drops.
Low water content or insufficient pressure is identified separately as an E0 fault in the V6.2 manual, rather than E5. However, pressure, air and circulation are connected, so the gauge is still worth checking when diagnosing a heating problem.
Read the related guide covering the Strom E-Boiler E0 error code if that is the code shown on your display.
Check that user-operated radiator valves are open
Confirm that the normal thermostatic radiator valves have not all been turned off. A correctly designed system should maintain an adequate circulation path through its automatic bypass arrangement, but closing every radiator may expose an existing bypass or control problem.
Do not adjust lockshield valves, pump settings or the automatic bypass valve unless you are responsible for commissioning the system and have the necessary information and measuring equipment.
Look and listen for signs of trapped air
Gurgling pipework, radiators that remain cold at the top and an unusually noisy pump can indicate air within the system.
A homeowner who is familiar with bleeding radiators may release air using the normal radiator vent. The system pressure must then be checked again and restored according to the boiler and heating-system instructions. Stop if the pressure falls rapidly, water does not appear at the vent or repeated bleeding is required.
Note what happens when there is a heating demand
When the thermostat calls for heat, note whether the pump symbol appears on the display and whether any external pump, zone valve or pipework can be heard operating. This information can help an engineer identify whether the fault occurs before circulation starts or after the pump has begun running.
Common causes of a Strom E5 error code
Incorrect pump operation or pump speed
The pump must move enough water through the boiler to satisfy the minimum flow requirement. An unsuitable speed setting, a stalled pump, poor electrical control or an incorrectly selected pump can all result in inadequate circulation.
Increasing the pump speed should not be treated as an automatic fix. The correct setting depends on system resistance, pipework, valves, emitters and the required flow-and-return temperature difference.
An engineer should confirm that the pump starts when expected, runs in the correct direction and produces the required circulation under all operating conditions.
Air trapped in the boiler or heating circuit
Air can reduce or interrupt the water passing through the flow sensor. It is particularly common after filling, draining, radiator work or alterations to the heating system.
The complete circuit may need to be purged, including radiators, high points, cylinder-coil pipework and any automatic air vents. Once the air has been removed, the system pressure and water treatment should be checked again.
An automatic bypass valve that is missing or incorrectly set
The manufacturer’s installation guidance states that an automatic bypass valve should form part of the system. Its purpose is to maintain a circulation path as radiator valves, zone valves or actuators close.
If the bypass is missing, fitted in the wrong position, stuck or set incorrectly, the available flow through the boiler may fall below 3 litres per minute when heating zones begin to shut down.
The bypass should be commissioned against the system design. Simply opening it fully can create short-circuiting between the flow and return, while closing it too far may leave the boiler without sufficient circulation.
Blocked inlet or return-line strainers
Debris caught in a strainer can restrict the heating flow even when the pump appears to be running. The restriction may build gradually or appear after older pipework and radiators have been disturbed.
Strainer inspection normally involves isolating and opening part of the heating system. This should be completed by a competent heating engineer who can isolate the electricity and water safely, clean the component and test the boiler afterwards.
System debris or a restricted heating filter
Magnetite, installation debris and contaminated system water can restrict pipework, valves, heat exchangers and filters. A heavily loaded system filter can also reduce circulation.
For a practical example of filter maintenance, see the guide on how to clean a central-heating magnetic filter. Follow the instructions for the particular filter fitted because isolation and servicing methods differ.
Partially closed or restrictive isolation valves
The Strom installation guide specifies full-bore isolation valves on the heating flow and return. A valve that has been left partially closed, has failed internally or is unnecessarily restrictive can reduce the available flow.
An engineer should check the complete water path rather than assuming that a valve is fully open because its handle appears to be in the correct position.
Zone valves opening too late
The heating controls should allow the relevant zone valve to open before the boiler begins heating. Where the boiler receives a demand before a water path has been established, the flow sensor may detect inadequate circulation.
Fault-finding may involve checking valve actuators, end switches, wiring centres, thermostats and the boiler’s switched-live signal. This is electrical work and should not be attempted with the boiler energised.
A flow-sensor or connection problem
If a measured flow of at least 3 litres per minute is passing through the boiler but E5 remains, attention may turn to the heating flow sensor, its connection and the relevant control circuit.
The casing must not be removed by a homeowner. Electric boilers can draw a substantial current, and safe isolation must be proved before internal checks begin. The related article on electrical safe isolation explains why switching a control off is not the same as proving that equipment is electrically dead.
A practical E5 fault-finding sequence for engineers
- Confirm the boiler model, revision and correct installation manual.
- Verify the displayed code and establish when it appears.
- Check the system pressure and water condition.
- Confirm that a heating demand reaches the boiler correctly.
- Verify that all required zone valves are fully open before the boiler starts.
- Confirm pump operation, direction and appropriate speed.
- Purge air from the boiler, emitters and system high points.
- Check that the automatic bypass valve is present and commissioned correctly.
- Inspect the flow and return isolation valves for restrictions.
- Inspect and clean the relevant strainers and system filter.
- Measure the actual heating flow rather than relying only on pump noise.
- If adequate flow is confirmed, test the flow-sensing circuit according to the manufacturer’s information.
- Clear the code by holding the power button until the boiler beeps, then test the complete operating sequence.
The boiler should be tested with the heating zones fully open and again as valves begin to close. A system that only works with every radiator open may still have an incorrectly commissioned bypass or control arrangement.
What should you do if E5 returns after a reset?
Do not repeatedly reset the boiler. Record when the fault occurs and arrange for the circulation problem to be diagnosed.
Useful details to provide include:
- The complete boiler model and revision.
- The pressure shown when the system is cold.
- Whether E5 appears immediately or after the system has heated.
- Whether the fault occurs on every heating zone.
- Whether radiator or zone valves have recently been changed.
- Whether the system has recently been drained, filled or serviced.
- Whether the pump symbol appears on the boiler display.
- Any noise from the pump, boiler or pipework.
Contact a competent heating engineer or the boiler manufacturer when the fault requires internal electrical checks, pump adjustment, system draining, strainer cleaning, control testing or flow measurement.
Frequently asked questions
What does E5 mean on a Strom electric boiler?
On boilers covered by the V6.2 Rev 2 manual and the V2.2 heat-only manual, E5 means inadequate flow through the central-heating circuit. The required minimum is 3 litres per minute.
How do I clear the E5 code?
After correcting the cause, press and hold the boiler’s power button until it beeps. Restore the heating demand and test the system. If E5 returns, further fault-finding is required.
Is Strom E5 a low-pressure error?
Not directly. The V6.2 manual identifies E0 as the code for insufficient water or heating-system pressure. E5 relates to inadequate circulation detected by the heating flow sensor.
Is the 3 litres-per-minute figure the hot-water flow?
No. It is the minimum circulation rate through the central-heating side of the boiler. Domestic hot-water performance is a separate measurement on combination models.
Can air in the radiators cause E5?
Yes. Air can interrupt circulation and reduce the flow detected by the boiler. The system may need to be purged and its pressure checked afterwards.
Can a blocked filter cause a Strom E5 fault?
Yes. A blocked strainer, magnetic filter, valve or section of pipework can reduce heating circulation below the boiler’s required flow rate.
Why does E5 appear when radiator valves close?
This can indicate that the automatic bypass valve is missing, incorrectly set or unable to maintain sufficient circulation. The sequencing of zone valves and the boiler demand may also need checking.
Can I remove the boiler cover to check the pump?
No. Internal inspection exposes electrical and hydraulic components and should be completed by a competent person after the supply has been safely isolated and proved dead.
Will repeatedly resetting the boiler fix E5?
No. Resetting only clears the displayed code. The fault will return if circulation remains below the required level.
Related Allen Hart guides
- Strom E-Boiler E0 error code explained
- How to clean a central-heating magnetic filter
- Electrical safe isolation when working on boilers