🕑 9 minutes read

[Explained by Serviceteam Experts]

If you're a homeowner with a Worcester boiler, you may have encountered the A21 Worcester boiler fault. This error can be frustrating, leaving you without heat or hot water.

This blog will explain what the A21 fault means, its common causes, and how you can fix it.

What Does the Worcester A21 Fault Code Mean?


The A21 fault code on a Worcester boiler shows that there's a problem with the radio frequency (RF) signal between the thermostat and the receiver.

This means your boiler isn't getting the signal it needs from the thermostat to work properly. While the A21 Worcester boiler fault is common, you can often fix it without needing an engineer's help.

In other words, when your room gets too cold, the RF signal tells the boiler it's time to heat up again. The receiver, which catches this signal, is usually near your boiler. It then passes the signal to the boiler, prompting it to generate more heat.

“However, if the RF signal between the thermostat and receiver is damaged, the boiler will not know to start heating. This can cause it to lock out and trigger an A21 fault code.”

What Causes a Loss of RF Signal?

There are two main reasons why your Worcester Bosch boiler may experience a loss of RF signal:

Thermostat batteries are low or dead The-home-Thermostat-is-replaced-with-new-batteries

If you're not getting a Radio Frequency (RF) signal between your thermostat and boiler, it might be because the batteries in your thermostat have run out of power. Even if your thermostat is new, it's a good idea to check the batteries just in case. This could solve the problem and get your heating working again.

The thermostat is out of range The-home-thermostat-is-out-of-range

Sometimes, if your thermostat or receiver isn't working properly, it can cause the A21 error to pop up. It's like when your TV remote doesn't work because it's too far from the TV. In this case, your thermostat needs to be closer to the receiver so they can communicate properly. If your thermostat is too far away from the receiver or there are obstacles (like thick walls) between them, the signal may not reach the boiler.

Make sure your thermostat is in the right place. Otherwise, it might not be able to communicate with the receiver, causing the A21 problem.

The receiver is positioned strategically to communicate with your thermostat, serving as the central connection point for RF signals. It's usually placed near the boiler and is crucial for keeping your home at the right temperature.
The thermostat or receiver might be faulty A-faulty-thermostat

Sometimes, the RF signal between your thermostat and receiver might be affected by other radio frequencies in your surroundings. These frequencies can interfere with the communication between your thermostat and the heating system, causing disruptions in signal transmission.

Challenges with External RF Interference

Sometimes, the RF signal between your thermostat and receiver might be affected by other radio frequencies in your surroundings. These frequencies can interfere with the communication between your thermostat and the heating system, causing disruptions in signal transmission.

Call Serviceteam Heating Professionals

Contact Serviceteam Professionals for all your boiler needs. We offer round-the-clock service in London and handle emergency breakdowns and installations alike.

How To Fix A21 Worcester Boiler Error Code?

The A21 error code can usually be fixed easily by moving the thermostat closer to the boiler.

Can I Fix the Worcester A21 Fault DIY?

In some cases, you may be able to resolve the Worcester A21 fault on your own. Here are two simple checks you can perform:

Thermostat Needs New Batteries

Try replacing new batteries if your thermostat is battery-operated.

Low battery power is a common cause of the A21 error code. Consult your thermostat's manual for instructions on how to change the batteries.
Check the Distance of Thermostat and the Receiver

You need to check the distance between your thermostat and the receiver. If they are too far apart, try moving the thermostat closer. Also, ensure there are no obstacles, such as metal objects or thick walls, blocking the signal.

If these DIY fixes don't resolve the A21 fault, it's best to call a professional heating engineer. They'll use the Worcester for both your boiler and thermostat to find out what's causing the problem.

Plus, they'll explain what they did and how it turned out. If you decide to call an engineer, it's a good idea to schedule an annual service at the same time. That way, you can make sure your boiler is in tip-top shape all around.

Cost of Hiring a Heating Engineer To Fix An A21 Fault

The cost of hiring a heating engineer to fix the Worcester boiler A21 fault will vary depending on the cause of the issue and your location.

A diagnostics visit typically costs between £50 and £100. The cost will be higher if parts need to be replaced, such as the thermostat or receiver.

Serviceteam is certified by Worcester Bosch and serves as an accredited installer.

If you're in London, you can conveniently schedule an online appointment with one of our heating engineers for expert boiler repair service.

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Why Should I Get My Boiler Serviced Annually?


Booking an annual service is a smart move because it allows you to check on your boiler's performance, spot any potential issues, and ensure everything is clean and clear.

A well-maintained domestic boiler can last 10-15 years.
Neglecting maintenance can shorten its lifespan and end up costing you more in the future.

Annual servicing helps boilers run longer and provide consistent heat. And the best part? It only takes about an hour, giving you peace of mind for the year ahead.

Will My Warranty Cover Repairs or Replacement Work For an A21 Worcester Fault?

If your Worcester boiler is still under warranty, repairs or replacement work for an A21 fault may be covered. However, this will depend on the cause of the fault and the terms of your specific warranty. Always check your warranty documentation or contact Worcester directly for clarification.

Don't forget to check your warranty coverage and explore your options for a new, reliable boiler.

Looking For A New Boiler Installation?


If your Worcester boiler is old and keeps experiencing issues like the A21 fault, it may be time to consider a replacement. At serviceteam, we offer top-quality boiler installation service at a competitive price.

Our expert installation services ensure your new boiler runs smoothly and efficiently. Get your free quote today!

For more information on boiler error codes, please visit our blog's dedicated section on boiler error codes.


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