A1 on a Worcester Bosch means the pump is not moving enough water through the boiler, so the appliance has shut down to protect itself.

There is a check worth doing here, but there is also a trap: topping the pressure up without asking why it dropped just hides a leak.

What A1 actually means

The boiler needs water circulating through the heat exchanger to carry heat away. If the pump runs without enough water passing through it, the boiler stops rather than let the heat exchanger cook.

The first thing the service booklet has an engineer check is whether system pressure is below 1.2 bar.

What you can safely check yourself

Look at the pressure gauge. If it is below about 1 bar, the system is low on water. That alone will bring on A1.

Before you top it up, ask why it dropped. A sealed heating system should not lose pressure. If you are repressurising every few weeks, there is a leak somewhere, or the expansion vessel has lost its charge. Topping up repeatedly puts fresh oxygenated water into the system every time, which is how you get sludge and, eventually, a pump or heat exchanger failure.

Check the radiator valves are open. If someone has closed valves on several radiators, or the system has been drained down for decorating and not properly refilled and bled, flow can drop far enough to trigger it.

If the pressure was low and there is no visible leak, repressurise using the filling loop to about 1.2 bar cold, bleed the radiators, and reset.

When to stop, and why

What causes it beyond the simple checks

A seized or failing pump, an air lock, sludge or debris restricting flow through the heat exchanger, a blocked filter, a closed valve left over from other work, or a failed flow sensor. On older systems that have never been flushed, restricted flow through a silted heat exchanger is common.

How Vantage can help

We are Gas Safe registered under 956889 and cover Hertfordshire, Bedfordshire and Cambridgeshire, with an emergency line answered 24 hours a day. Call us on 01462 506374. See also boiler repair and boiler servicing.

Frequently asked questions

What pressure should a Worcester Bosch be at?

Around 1.0 to 1.5 bar cold for most domestic systems. Check the manual for your appliance. It will rise when the heating is hot, which is normal.

Will a power flush fix A1?

If the cause is sludge restricting flow, then often yes. If the pump has failed or the vessel has gone, flushing will not help. It needs diagnosing before spending money.

Can I keep running the boiler on A1?

It will not run. And you should not keep resetting it, because the fault is there to stop the boiler overheating.

Related fault codes

Other Worcester Bosch codes: EA · E9 · C6 · E2 · all Worcester Bosch codes. See also boiler keeps losing pressure, power flushing and radiator not heating up.

In Worcester Bosch’s own words

Worcester Bosch use different code sets across their ranges and generations. The definition above is taken from the Greenstar CDi Classic service booklet. If your boiler is a different model and the code behaves differently, check the manual for your appliance or ring us and we will tell you what it means on your unit.

When to call an engineer

If the checks above have not cleared it, the next steps are engineer work and the boiler should not be reset repeatedly in the meantime. Call Vantage Plumbing and Heating on 01462 506374. We are Gas Safe registered under 956889, the line is answered by a person at any hour of any day, and you get a price before anyone starts.

Need help today?

Call and speak to a person, any hour, any day of the year. Get a fixed price quote or call 01462 506374.