What is the A21 fault code and how do you fix it? Here we explain the most common scenarios.
Curious about what Worcester’s A21 fault means? This guide will explain the A21 error, its potential causes, and how to fix it.
Fault codes help homeowners identify issues with their systems. The manual provides an overview of the problem indicated by each fault code.
Each fault code can have multiple causes and solutions. So, what specifically does the A21 fault mean?
Let’s dive in and find out!
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What Does the Worcester A21 Fault Mean?
First and foremost, when an A21 fault appears on a Worcester Bosch boiler it means that there has been a loss of radio frequency (RF) signal.
This is the method used by a wireless thermostat to communicate with your boiler when you want it to fire up.
This signal would occur when your thermostat is showing a temperature lower than the one you’ve selected (during a time when you have the heating scheduled to be on, of course).
In other words, when the room drops below the desired temperature, the RF signal would let the boiler know it’s time to fire up again and generate more heat.
The component that will receive the RF signal from your thermostat is the receiver. It tends to be located on or near the boiler.
The receiver will then pass the signal from the thermostat onto your boiler.
Once the signal is received by the boiler, it will fire up and generate more heat.
When the signal from the thermostat to the receiver is experiencing an interruption, the boiler won’t be able to know when it is to fire up.
As a result, the boiler will lock out and an A21 fault code will be triggered.
What Can Cause a Loss of RF Signal?
If your Worcester Bosch boiler has an A21 fault, there are several potential explanations, namely:
Thermostat Batteries Have Ran Out
The most obvious reason for no RF signal is that your thermostat has simply run out of batteries. Of course, if your thermostat was installed only recently then there isn’t a high chance that this is the cause but either way, it’s worth checking.
Thermostat or Receiver is Faulty
Of course, it’s also possible that either your receiver or thermostat is acting up. It’s possible that the receiver or thermostat was not set up correctly or in the correct location.
Moreover, as with any appliance, they could run into issues at any time. Also, there is a case to make that not all households are suited to wireless technology of this nature.
Thermostat Needs to Be in Better Range of the Receiver
Don’t forget that your thermostat must be in a good location. Otherwise, it may not be in the range of the receiver and thus you can see how an A21 fault could arise.
Additional Radio Frequencies Affecting the Signal
Lastly, there’s also a risk that other radio frequencies are interfering with the RF signal from the thermostat to the receiver.
You may be interested in Gas Engineer Allen Hart explaining the most common boiler problems and how you can resolve them in the video below:
How to Fix the F1 Fault
Can I Fix the A21 Error DIY?
In some cases, it’s possible to resolve an A21 error by yourself.
However, it would depend on the cause and your skills/knowledge among other potential factors.
Thermostat Needs New Batteries
If your thermostat requires fresh batteries, you can try to fix this DIY without the help of a professional.
That of course assumes that you’ve diagnosed the problem correctly. Either way, there’s no harm in replacing the batteries and seeing if it works.
You should always follow the manufacturer's instructions when replacing batteries for a thermostat. However, we’ve provided a broad overview of what’s required below.
Note: You’ll probably find that your thermostat must be reprogrammed back to the settings you prefer after you’ve replaced the batteries.
Here are the steps usually needed to replace a thermostat’s batteries:
Fit a flat-bladed screwdriver into the slot that can be found below the bottom of your thermostat
Twist the screwdriver carefully until you release the thermostat from its mounting plate
Fully remove the thermostat from the wall and replace its batteries
Line the top of the thermostat up with the wall mounting location. Then push gradually until you hear/feel it snap back into position.
If this approach has worked, then the error code should subsequently disappear.
Thermostat Not in Range of Your Receiver
Another scenario where a DIY solution may work is if your thermostat needs to be placed in a better range than your receiver.
First and foremost, here’s how you could determine where this is the case:
Remove the thermostat from the mounting plate that keeps it on the wall. You can achieve this by fitting a flat-bladed screwdriver into the slot beneath the bottom of the thermostat before twisting it carefully until it releases from the mounting plate.
Carry your thermostat closer to the boiler (or wherever your receiver is situated). Check to see whether the signal has improved. To test the signal please follow the manufacturer’s instructions.
Should you find that the receiver is not located near your boiler, you need to hire a professional.
They might be able to relocate your receiver to another location if possible. Otherwise, you’d need to switch to a wired thermostat.
If you find a spot that works better, you can relocate the thermostat with the following steps:
Remove the wall mountain plate per the manufacturer’s instructions.
Carefully attach to a new suitable location that is in better range of the receiver. Make sure to carefully consider exactly where you are going to relocate it from a practical and safety point of view. Also, don’t forget that your thermostat measures temperatures based on where it is, so by relocating it, your thermostat may not work as well as before.
Related boiler guides:
Cost of Hiring a Heating Engineer to Fix an A21 Fault
It costs approximately £30 to £60 per hour to hire a heating engineer in the UK.
Of course, this will depend on various factors (including your location).
Whether or not supply costs will apply would depend on the nature of the fix.
Here are the estimated costs of specific repairs/replacement jobs if hiring a professional to solve an F21 error:
Replacing thermostat batteries
£35 to £125Moving a thermostat closer to the receiver
£30 to £120Relocating the receiver and/or thermostat *
£30 to £120Replacing a faulty receiver
£80 to £170Replacing a faulty thermostat
£150 to £300Installing a brand-new boiler
£800 to £2,800These jobs might also apply as a solution should you find that other radio frequencies are interfering and causing an issue.
Read our Worcester Bosch vs Viessmann Boilers review if you need some help in choosing between these two popular brands.
Will my Warranty Cover Repairs or Replacement Work for an A21 Fault?
How long Worcester Bosch guarantees would last for would depend on the boiler model.
As for whether the guarantee would cover repairs/replacement work, unfortunately, this is unlikely.
According to Worcester Bosch’s Ts and Cs, their guarantee would apply to repairs or replacement of components if they are defective as a result of the manufacturing process itself or the materials used.
Of course, it’s important that your guarantee is still active.
What’s more, a Worcester Bosch guarantee would only apply to domestic heating/hot water appliances and components.
Beyond that, they must specifically be those manufactured by Bosch or/and branded ‘Worcester’, ‘Bosch', or ‘Buderus’.
They must be supplied from the UK, the Republic of Ireland, the Isle of Man, and/or the Channel Islands.
As you can see, it’s overall unlikely that a warranty would apply to repair or replacement work to fix an A21 fault.
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