Worcester A1 fault: pump running dry or no flow detected
A1 is Worcester's dry-run protection. The boiler has fired or tried to fire without sensing water movement, so it stops before the heat exchanger is damaged.
Things worth knowing about an A1 flow fault
- Air in the pump head is normal after radiator changes and needs venting
- A full magnetic filter starves flow long before it blocks completely
- Check isolation valves under the boiler are fully open
- Seized pumps often restart once and then fail again within days
- Repeated A1s should never be reset-and-ignored
Scope of work
- Domestic boiler installation
- Boiler servicing
- Boiler maintenance and repair
What is actually going on with an A1 flow fault
The most common triggers are trapped air in the pump head after any drain-down, a pump that has seized after a summer of standing still, a blocked magnetic filter that has never been emptied, or closed service valves left shut after work elsewhere on the system. Sensor and PCB faults sit at the end of that list, not the start.
Where we deal with this
Straight answers on an A1 flow fault
Our advice: A1 is a flow message. Filter, valves and air come first; a pump is only replaced once it is proven.
For gas work, use a Gas Safe registered engineer; for oil work, use an OFTEC-registered technician.
Questions we get asked
- Can I clear the air myself?
- Bleeding radiators is safe; venting a pump head involves opening the wet side of the appliance and is engineer work.
- Does A1 mean the pump is dead?
- Not always. Around half the A1s we attend are flow restriction rather than pump failure.
Related help pages
- DiagnosticsIntermittent boiler faults: how to catch a fault that hides
- Vitodens fault codeViessmann F2: flow temperature sensor fault on a Vitodens
- Cycling symptomBoiler turning on and off repeatedly through the day
- Oil boiler diagnosisOil boiler not firing: the checks before an OFTEC callout
- Circulation faultsAirlocks in heating systems: how trapped air actually stops flow
- Older boiler repairParts obsolescence: what happens when a boiler is discontinued
Services that cover this
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