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Last updated: 15th April, 2024


Worcester Bosch A21 Error Code - How To Fix

Worcester Bosch A21 Error Code - How To Fix

Key takeaways

  • The Worcester Bosch A21 error code indicates your boiler and wireless thermostat are no longer effectively communicating via RF signal.
  • The signal loss could be caused by: dead batteries in boiler or thermostat, a faulty component within either or the thermostat being out of effective range.
  • Always arrange a qualified, Gas Safe engineer to carry out a full boiler service.

If Your Worcester Bosch boiler is displaying an A21 error code it can be a vexing issue, especially if your home isn’t being heated as it should.

Here at iHeat we’re dedicated to assisting our customers in any way we can, whether it be stocking a premium boiler collection or giving helpful heating tips and advice. 

In this blog we’re going to be exploring the Worcester Bosch A21 error code, including: what this means, what causes this, how to fix and more.

What Is An Error Code?

A Worcester Bosch error code is simply a warning message to homeowners that their boiler isn’t functioning as it should.

Error codes (or fault codes) are alphanumeric and often 3 digits in length (A21), they are shown on a boiler’s digital display screen, usually located in the centre of the unit or hidden beneath a folding plastic cover lower on the boiler.

Due to the nature of this particular error code however, the A21 fault code may also appear on your room thermostat.

What Does The A21 Error Code Mean?

The A21 error code indicates your Worcester Bosch boiler and wireless thermostat are no longer effectively communicating, this occurs when there is a lack of Radio frequency (RF) signal.

RF is how a boiler and wireless thermostat transmit data to each other in order to effectively and safely heat up your home.

When room temperature drops below the desired level while the heating is switched on, the thermostat detects this and via RF signal alerts your boiler it needs to start emitting heat once again to bring the temperature of the room to satisfactory level..

RF Signal Loss Causes

Thermostat batteries have run out and require replacing

Find a flat blade screwdriver to carefully remove the thermostat from its wall mounting, replace faulty batteries, line up with wall mounting and re-attach, then more than likely the thermostat will need resetting to your desired temperature  settings.

Thermostat needs to be in closer proximity to the boiler

Remove the thermostat from its wall mounting as described above, move to a new location and carry out signal tests as instructed by manufacturer guide/manual, and if necessary reattach wall mounting and thermostat in new location (considering performance and safety).

Thermostat or boiler’s receiver are faulty

Component error or faulty performance in this case may need to be assessed and remedied by a qualified Gas Safe registered engineer.

Other RF signals interfering with central heating appliance communication

If the source of interference is discovered, and if safe to do so, this or the thermostat should be relocated to allow for efficient communication with the central heating system.


Arranging for your boiler to be annually checked over or ‘serviced’ by a qualified engineer can be a fantastic way of analysing the state of your heating system in yearly increments, identifying and rectifying any faults before they have a chance to progress.

Be Proactive With iHeat

The best way of ensuring your boiler and thermostat are operating as they should, is by proactively replacing ageing or faulty boilers at the earliest signs of degradation, this preventative step is far more prudent than reactive maintenance.

It’s a mechanical and engineering fact that due to some of the technological advancements in recent times, modern boilers are more robust and efficient than anything made previously. 

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Frequently Asked Questions

How Do I Fix My Worcester Boiler Error Code A21?

Error code A21 on a Worcester boiler typically indicates an issue with overheating. This can be caused by several factors, including a problem with the pump, blockages in the system that restrict water flow, or issues with the thermostat. To fix this:
Check the Pump: Ensure the pump is operating correctly and that it's not blocked or faulty.
Bleed Radiators: Air trapped in the system can cause overheating. Bleed your radiators to remove any air pockets.
Inspect the System: Look for any blockages or closed valves that might be restricting water flow.
Reset the Boiler: After checking these issues, reset the boiler to see if the error clears.
Consult a Professional: If the error persists, it's advisable to contact a qualified heating engineer to diagnose and resolve the issue.

How Do I Clear the Error Code on My Worcester Boiler?

To clear an error code on a Worcester boiler:
Reset the Boiler: Most Worcester boilers have a reset button or function. Press the reset button and hold it for a few seconds or until the boiler restarts.
Power Cycle: If the reset doesn't work, turn off the boiler at the mains, wait a few minutes, and then power it back on.
Check the Manual: Refer to your boiler's manual for specific instructions related to clearing error codes, as the process can vary by model.

What Does Fault 224 Mean on Worcester Boiler?

Fault code 224 on a Worcester boiler typically indicates a loss of communication between the boiler's control board and the user interface. This could be due to a wiring issue, a fault in the user interface, or a problem with the boiler's control board itself. Checking the connections and ensuring all wiring is intact can be a first step. If the problem persists, contacting a professional technician is recommended.

How Do I Reconnect My Worcester Comfort RF Thermostat?

To reconnect a Worcester Comfort RF thermostat:
Check Batteries: Ensure the thermostat's batteries are fresh and installed correctly.
Re-establish Connection: Follow the pairing instructions specific to your Worcester Comfort RF model. This usually involves pressing a pairing or reset button on the thermostat and sometimes on the boiler.
Consult the Manual: Refer to the thermostat's manual for detailed instructions on re-establishing a connection.

What Does A21 Mean on Thermostat?

On many thermostat systems, including some connected to Worcester boilers, an A21 error code might indicate a communication problem between the thermostat and the boiler or a similar fault. It generally points towards an issue in sending or receiving commands, potentially due to a range issue, interference, or a malfunction in the thermostat itself.

Why is My Boiler Not Communicating With My Thermostat?

If your boiler is not communicating with your thermostat, it could be due to:
Distance and Interferences: Make sure the thermostat is within range of the boiler and that there are no physical obstructions or electronic interferences.
Battery Issues: Low batteries in a wireless thermostat can cause communication failures.
Configuration Errors: Ensure that both devices are configured to communicate with each other, following manufacturer's settings.
Faulty Components: There could be a fault in the thermostat, the receiver, or the boiler’s control panel.

What to Do if Thermostat is Not Communicating?

If your thermostat is not communicating with your heating system:
Check Batteries and Power Supply: Replace batteries if necessary and ensure all devices are powered.
Reset Devices: Reset both the thermostat and the boiler to clear any temporary communication issues.
Check Range and Interference: Ensure there are no range issues or interferences blocking the signal.
Re-pair Devices: Attempt to re-pair the thermostat with the boiler, following the manufacturer’s instructions.
Consult a Technician: If problems persist, it may be necessary to have a professional diagnose and repair the issue.

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Article by
Stephen Day | Co-founder
Gas Safe registered and FGAS certified engineer with over 20 years experience in the heating and cooling industry.