Quick answer: The A21 fault code means your Worcester Bosch boiler has lost the radio signal between your wireless room thermostat and the receiver on the boiler. Nine times out of ten it’s flat batteries in the thermostat. This is one of the very few Worcester fault codes most homeowners can genuinely fix themselves, without an engineer and without touching the boiler casing.
Written by the engineering team at Edinburgh Boiler Company, Scotland’s number one boiler installer. We fit Worcester Greenstar systems across Edinburgh and the Lothians every week, and A21 is the code we most often talk customers through over the phone rather than charging them a call-out for.
What does A21 mean on a Worcester boiler?
A21 means loss of RF communication. Your wireless thermostat and the receiver module on or near the boiler talk to each other over a radio frequency link. When that link drops for long enough, the boiler flags A21 and falls back to whatever default behaviour it’s programmed with, which usually means your heating either stops responding to the thermostat or runs at odd times.
The important thing to understand is that the boiler itself is almost certainly fine. Nothing has failed in the burner, the pump or the gas side. The boiler has simply stopped hearing its instructions.
| What A21 is | What A21 is not |
|---|---|
| A communication fault between two devices | A gas fault |
| Usually a battery or range problem | A combustion or safety lockout |
| Often fixable in five minutes | A reason to stop using the boiler |
What causes an A21 fault?
Batteries are the headline answer, but there are four realistic causes and they’re worth ruling out in order.
| Cause | How common | Tell-tale sign |
|---|---|---|
| Flat or low thermostat batteries | Very common | Faint, blank or flickering thermostat display |
| Thermostat out of range or blocked by walls | Common | Fault appeared after moving the stat, or works in one room but not another |
| Radio interference from another device | Occasional | Fault comes and goes, often after new equipment was installed |
| Failed thermostat or receiver | Less common | Fresh batteries and close range still won’t pair |
Batteries are worth a second mention because of how they fail. A thermostat doesn’t go from working to dead. It fades. The display stays lit while the transmitter, which draws far more current, starts dropping packets. So you can be standing in front of a thermostat that looks perfectly alive while the boiler hasn’t heard from it in hours.
Why does A21 happen so often in Scottish homes?
Scotland’s building stock is unusually hostile to radio signals, and that’s not a throwaway line. Traditional Scottish construction is solid stone, not cavity brick. A Victorian tenement in Marchmont or a granite terrace in Aberdeen can have external walls 600mm thick, with internal walls of lath and plaster over dense timber studwork. Radio signals lose a lot of strength passing through that.
Add a few Scotland-specific complications:
- Foil-backed insulation. A huge amount of Scottish housing has been retrofitted with insulated plasterboard as part of energy efficiency work. The foil layer is very effective at blocking RF. We’ve seen A21 appear for the first time within days of a room being insulated or drylined.
- Boilers in odd places. Tenement flats often have the boiler in a hall cupboard, a bathroom cupboard, or a back kitchen press, surrounded by pipework and sometimes a metal-lined enclosure. Metal around the receiver kills range.
- Cold hallways. Battery chemistry performs worse in the cold. A thermostat on a draughty stair or unheated hall in January will flag low battery months before the same stat would in a warm living room.
How do I fix an A21 fault myself?
Work through these in order. Most people are sorted at step one.
1. Replace the thermostat batteries. Use fresh alkaline batteries from a sealed pack, not ones from a drawer. Match the type exactly, usually AA. Avoid rechargeables, because their lower voltage can leave the transmitter underpowered even when the cell reads as full.
2. Give it a few minutes. The thermostat and receiver won’t always re-link instantly. Leave it five to ten minutes before deciding it hasn’t worked.
3. Test the range. Carry the thermostat to within a metre or two of the boiler and see whether the link restores. If it does, you have a range or obstruction problem rather than a hardware failure, and the fix is repositioning rather than replacing.
4. Power-cycle both units. Take the batteries out of the thermostat for a minute. Switch the boiler off at the fused spur for the same period, then back on. This forces both devices to restart the pairing process.
5. Reset the boiler once if the code is still showing after the link is back.
6. Look for new interference. Think about what’s changed. A new router, a baby monitor, a smart doorbell, a wireless speaker or a neighbour’s kit can all sit close enough in frequency to cause problems. Moving the offending device a couple of metres is often enough.
Where should I put the wireless thermostat?
Put it on an internal wall, at roughly 1.5 metres height, in a room you actually use, with clear line of sight to the boiler where possible. Avoid these:
| Avoid | Why |
|---|---|
| Directly on an external stone wall | Maximum signal attenuation |
| Inside or on top of a metal enclosure or radiator cover | Metal blocks RF outright |
| Next to a router, TV or wireless speaker | Interference |
| In direct sunlight or above a radiator | False temperature readings as well as signal issues |
| An unheated hall or porch | Cold batteries, poor control |
If the thermostat needs to live somewhere with a genuinely weak signal, the answer is usually a wired stat or a mains-powered smart control rather than fighting the radio link forever.
When should I call a Gas Safe engineer?
Call an engineer when fresh batteries and a close-range test haven’t restored the link. At that point you’re looking at a failed thermostat, a failed receiver, or a pairing that won’t re-establish, and all three need someone with the right kit and the pairing procedure for your specific model.
You should also call if the boiler is showing A21 alongside any other fault code. A21 on its own is a communication issue. A21 plus an EA, C6 or U9 is a different conversation, and the other code is the one that matters.
How much does it cost to fix an A21 fault in Scotland?
Most A21 faults cost the price of a pack of batteries. When they don’t, here’s what to expect at 2026 Scottish rates.
| Fix | Guide cost | Notes |
|---|---|---|
| Replacement batteries | £3–£8 | Do this first, always |
| Diagnostic visit | £70–£130 | Often deducted from any repair |
| Re-pairing thermostat and receiver | £70–£130 | Usually within the diagnostic visit |
| Replacement wireless thermostat | £120–£220 fitted | Must be compatible with your receiver |
| Replacement receiver module | £150–£280 fitted | Less common than stat failure |
| Upgrade to a smart control | £220–£400 fitted | Worth considering if you’re replacing anyway |
If you’re paying to replace a wireless thermostat, it’s worth asking about an internet-connected control at the same time. The price gap is smaller than most people expect, and you get remote control and scheduling out of it. That’s a genuine benefit in a Scottish winter when you want the heating on before you get home.
Will my Worcester guarantee cover it?
Controls supplied with the boiler and installed at the same time are usually covered by the boiler guarantee, which runs up to 10 or 12 years on Greenstar models when fitted by an accredited installer and registered properly. Batteries aren’t, for obvious reasons.
If your thermostat was added later, or came from a third party rather than Worcester, it’ll have its own separate warranty. Check who supplied it before you assume.
How do I stop A21 coming back?
- Change the batteries once a year on a schedule rather than waiting for a failure. Do it when the clocks change and you’ll never think about it again.
- Keep the thermostat in the same place once you’ve found a spot with a solid link.
- Think about the signal before drylining a room or fitting foil-backed insulation near the thermostat or the boiler.
- Get your annual service done. The engineer can check signal strength and battery condition while they’re there.
The bottom line
A21 is the friendliest fault code Worcester Bosch produces. Fresh batteries fix the large majority of them, and a range test tells you almost everything else you need to know. Scotland’s thick stone walls and retrofitted foil insulation mean we see range-related A21 faults more than most of the UK, so if new batteries don’t do it, think about what’s physically between the thermostat and the boiler before you assume anything has broken.
Still showing A21 after all that? Edinburgh Boiler Company covers Edinburgh, the Lothians and central Scotland with Gas Safe registered engineers and clear written pricing before any work starts. Get in touch and we’ll get your controls talking to each other again.
This guide is general advice. Refer to your specific Worcester Bosch model’s user manual for reset and pairing instructions, and never remove the boiler casing. Gas work must be carried out by a Gas Safe registered engineer.
