What A21 means on Worcester Bosch controls
If you see A21 on a Worcester Bosch system, look at your heating control rather than the boiler. A21 is shown by two Worcester controls, the Greenstar Comfort I RF and the Greenstar Sense I, and the code means something different on each. On the Comfort I RF it is a lost wireless signal. On the Sense I it is a configuration problem.
Both controls pair A21 with the sub-code 1001, so the screen may flick between "A21" and "1001". We did not find A21 in any of the Greenstar boiler fault tables we checked, which is another reason to start with the thermostat. If your boiler display is showing its own code as well, look that code up separately on our Worcester Bosch fault codes page.
| Control showing A21 | What kind of control it is | What A21 1001 means | Where to start |
|---|---|---|---|
| Greenstar Comfort I RF | Wireless room thermostat working with a twin channel programmer/receiver | No RF signal between the thermostat and the receiver | Batteries, signal strength, thermostat position |
| Greenstar Sense I | Wired room thermostat on the boiler's two-wire BUS | Sense I in heating circuit 1 set up incorrectly | The A.1 setting, checked by whoever fitted it |
Not sure which you have? The Comfort I RF thermostat runs on batteries, while the Sense I takes its power from the boiler wiring and has none.
A21 on the Greenstar Comfort I RF: no radio signal
Worcester's installation and operating instructions for the RF twin channel programmer/receiver and room thermostat (document 6 720 810 964) list A21 1001 simply as "No RF signal". The room thermostat sends its readings by radio to the programmer/receiver, which is wired to the boiler. When the receiver cannot hear the thermostat, the system can no longer use the room temperature in the normal way.
A21 straight after fitting or a battery change
The same manual says the thermostat shows A21 at first switch-on until the two units have set up their radio link. The units are paired at the factory and connect automatically, so a brief A21 after new batteries or a fresh installation is expected. It only needs attention if it stays.
Checks you can make on the Comfort I RF
- Batteries. Worcester's A21 entry assumes every unit is powered with no low battery symbol. If the thermostat screen is blank or shows the battery symbol, fit new AAA 1.5V alkaline cells, which is the type the manual specifies.
- Signal strength. Press the knob to open the settings, choose S 2 and read the number from 0 to 10. Worcester rates 0 as no signal, 1 to 3 as weak, 4 to 6 as acceptable and 7 to 10 as very good.
- Position. Keep the thermostat at least 300mm from metal objects, including metal wall boxes, and away from direct sunlight, draughts and radiators. The manual suggests about 1.2 m above the floor. Move it, wait a moment, then read S 2 again.
- The programmer/receiver. If its display is blank, the manual links that to a lost connection to the boiler's EMS bus. That is a wiring matter for the installer, not a battery or signal issue.
If A21 turns into A22 or A28
A22 1002 means "No RF signal or pairing fault" and the manual's remedy is to repeat pairing. A28 1008 means the units are not paired at all. Pairing sits in the installer menu under the radio settings, both units must be in the same pairing mode, and a replaced unit needs the old one unpairing first. If that is unfamiliar, ask the installer to do it.
A21 on the Greenstar Sense I: configuration fault
Worcester's installation and operation manual for the Sense I wired room thermostat (document 6 720 648 748) gives A21 1001 as "Sense I in heating circuit 1 incorrectly configured". The Sense I connects to the boiler over a two-wire BUS using the EMS or EMS 2 protocol and is powered through it, so there is no battery or radio side to check. The code points to a setting.
The manual's remedy is to check the A.1 setting, which tells the Sense I what job it is doing:
- Fb (room thermostat) when a Sense II is the main control.
- SC (zone controller) when a recognised Greenstar Wiring Centre is fitted.
- CO (controller) when there is no higher-level control and only one heating circuit.
In practice this code tends to follow a change: new controls, a Sense II added or removed, or a wiring centre fitted. A.1 is an installer setting, so if A21 appeared after recent work, call the installer back. Worcester also says the Sense I must not be used with its older FX controls (FR10, FW100 and FR110), so mention it if one of those is still on the wall.
Why the boiler reset button will not clear A21
The reset button on a Greenstar clears boiler lockouts. A21 lives in the control, so pressing reset on the boiler does nothing for a lost radio link or a wrong setting. Fix the cause at the thermostat and the code clears once the control is happy. Our Worcester boiler reset guide covers resets for genuine boiler codes, and if the heating ignores the thermostat with no code showing, see boiler not responding to the thermostat.
When to call the installer, an electrician or a Gas Safe engineer
Call for help if the Comfort I RF still shows A21 with fresh batteries and a good S 2 reading, if pairing has failed, or if a Sense I keeps showing A21 after the A.1 setting has been checked. When you ring, give the control's name, the full code including 1001, the S 2 number if you have one and anything that changed recently.
Do not open the boiler casing to look at the control terminals. Control wiring away from the boiler is a job for a qualified electrician or the heating installer, and any work at the boiler end needs a Gas Safe registered engineer.
Typical repair costs in the UK
Most A21 cases cost nothing beyond batteries or a better spot for the thermostat. If someone has to visit, these ranges are typical.
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £60 to £150 |
| Labour to re-pair or reconfigure controls | Typically £50 to £80 per hour (London and the South East £90 to £110) |
| Minor wiring repair on the control circuit | Typically £100 to £180 |
| Replacement room thermostat, fitted | Typically £75 to £200 |