Worcester Boiler A21 Fault – Error Code Guide

When A21 appears on the display screen of your boiler or room thermostat, an issue has occurred which will prevent your heating system from working consistently.

Fortunately, this fault can often be fixed on a DIY basis, without excess costs. However, any work on the gas boiler itself and the final connection of the water pipework to the boiler, must be done by a Gas Safe registered engineer.

This guide will help you to understand the issue, how to address it, and when you may need to call an engineer for further repairs.

What Does the Worcester Boiler A21 Fault Mean?

Worcester Bosch gives the meaning of this code as ‘Loss of RF signal’. RF stands for radio frequency, which is the method by which a wireless thermostat communicates to the boiler when it needs to fire up.

This signal happens when the thermostat records a temperature lower than the one you have programmed, which is why they are usually placed in the area of the house where you spend the most time. 

Once this temperature is recorded, the thermostat sends a signal to its partner component, known as a receiver.

This is usually placed on or close to the boiler unit, and passes on the signal from the thermostat to the boiler. The boiler will start the heating process until the room thermostat records an appropriate temperature.

The A21 code appears when something has interrupted the signal between the room thermostat and receiver, which in turn stops the boiler from knowing when to turn on. 

What Causes the Worcester A21 Fault?

Thermostat Needs New Batteries

As with any battery-powered appliance, the batteries will occasionally need to be replaced when they run out of power.

You may also have noticed a low battery symbol on the display before the A21 code appeared. This symbol indicates that you have sixty days to replace the battery before it dies and the thermostat’s settings are lost.

Low batteries aren’t likely to be causing the issue if you’ve recently had your thermostat installed.

However, if you’ve had it for a while then it’s worth checking if new batteries will solve the issue before calling out an engineer.

Thermostat is Out of Range of the Receiver

If the system has recently been installed, it’s possible that the thermostat isn’t in an optimal position to communicate with the receiver.

The thermostat’s job is to record the temperature in its location, which then informs whether the boiler needs to turn on. If the signal is interrupted or inconsistent, this function won’t take place.

While not all engineers agree on the truth behind this claim, Worcester Bosch advises that some buildings just aren’t suited to their wireless products.

This particularly applies to modern housing that includes steel components such as rolled joists and layers within walls, as well as foil-lined plasterboard.

Other Radio Frequency Signals are Interrupting the Connection

If you have a lot of wireless gadgets in the house, it’s possible that they’re responsible for interrupting the signal between thermostat and receiver.

This is because any devices that communicate through radio frequency will be competing for the ‘airspace’.

Faulty Thermostat or Receiver

It’s not impossible that either of the parts has simply broken.

As both parts are required for the connection with the boiler to take place, A21 could indicate that only one of these is broken.

If the code has appeared on your boiler’s display screen, and the room thermostat’s screen is blank, this is a good indication that the thermostat is the faulty part.

How to Fix a Worcester Boiler A21 Fault

In many cases you can clear the A21 code from your thermostat’s display screen on a DIY basis. There are a few steps you can try to safely diagnose and fix the thermostat, before calling an engineer.

If you’ve exhausted all of the DIY options, the best course of action is to arrange a visit from an engineer.

If your Worcester Bosch boiler is still in the warranty period, make sure to take advantage of any free repairs and replacements.

If the Thermostat Needs New Batteries

DIY Fixes to Try

To access your thermostat’s batteries you will need to remove the thermostat from the mounting plate that attaches it to the wall.

How you do so will vary slightly depending on which model you have. For most models of Worcester Bosch thermostat you can do the following:

  • Insert a flat bladed screwdriver into the slot under the bottom of the thermostat
  • Twist gently until the thermostat is released from the mounting plate
  • Remove completely from the wall before replacing the batteries
  • To reattach, line up the top of the thermostat with the wall mounting place and push gently until it snaps into place

Most models will require a similar process, however you may want to refer to the thermostat’s manual if you can’t find an opening slot.

If the batteries were causing the A21 code, it should now be cleared from the display. You will likely need to reprogramme the thermostat to your preferred settings once this has been completed.

When to Call a Gas Safe Engineer

If your boiler is in an awkward space such as the attic or garage, it may be that the receiver is always going to be too far away from the thermostat.

In this case, an engineer may move the receiver downstairs to a more convenient location.

It may be that a wireless thermostat isn’t suitable for you if the distance between the desired room and the receiver is too great, or interrupted by structural features such as steel joists.

In this case, an engineer may suggest a replacement to a wired thermostat. These usually cost under £100, plus labour costs of installation.

If the Thermostat is Out of Range of the Receiver

DIY Fixes to Try

To test whether this is the case, remove the thermostat from the mounting plate that holds it onto the wall.

Carry it towards the boiler, where the receiver is most likely located, and see if the signal improves.

If you have a Worcester Bosch branded thermostat then checking the signal is usually achieved by this process, or similar.

  • Press the menu button for more than three seconds.
  • Turn the knob to select Info and press the knob.
  • Turn the knob to select Signal strength and press the knob to enter the function.
  • The signal is measured out of 10, with anything over 4 being acceptable but over 7 is best for consistency.

If you have found an area with a stronger signal, you can relocate the thermostat by removing the wall mounting plate and attaching it in the new area.

It’s important to remember that the heating system will be controlled by the temperature of the space that the thermostat is located in.

For most people, it makes sense for this to be in the living room, where they spend the most time. It should also not be in direct sunlight or near another heat source, as this can prevent accurate readings of the room’s temperature.

You can always test a position by leaving the thermostat there, before making the commitment of moving the wall mounting plate.

When to Call a Gas Safe Engineer

An engineer can use a tool called a radio frequency reader to determine whether other devices are responsible for the interference.

If it’s not possible to move these devices or the thermostat to a position that prevents the issue, they may suggest switching to a wired thermostat. 

If Other Radio Frequency Signals are Interrupting the Connection

DIY Fixes to Try

You may be able to isolate a particular device that’s causing the A21 issue.

For example, if the code usually occurs in the evenings when a games console has been turned on then it’s likely to be the culprit.

In this case it might be possible to avoid moving the thermostat, if it’s possible to move the other device further away, or ensure it’s only used at times when the heating isn’t due to turn on.

When to Call An Engineer

After ruling out other potential causes, your engineer may advise that a faulty component is causing the issue.

In some cases the parts can be repaired, such as if there are damaged connections which can be easily replaced. It is likely that if a whole replacement is required, it will need to be both the thermostat and receiver.

This is because Worcester Bosch produces many models where both parts come factory-paired together, so they can’t be paired up with a new part.

The cost for this will depend on which model of thermostat you choose, with more recent or complex models costing up to £200.

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