Boiler fault codes
Worcester A21 Fault Code: Wireless Thermostat Out of Reach
Published
Worcester Bosch display
A21
What it means
On Worcester Comfort RF wireless controls, A21 means the room thermostat has lost its radio link with the receiver, so the boiler is no longer getting heating demands. On a wired Sense I control, A21 with sub-code 1001 means the control has been set up wrongly instead.
Safe to check yourself
- Replace the batteries in the room thermostat
- Check the boiler has power and whether the receiver light is flashing once a second
- Wait a minute after power returns for the two units to reconnect
- Move the thermostat away from metal, mirrors and thick walls
- Press the receiver override button briefly to keep the heating running
Call an engineer if
- A21 stays after new batteries and a repositioned thermostat
- The receiver light stays dark even when you press override with the boiler powered
- The boiler itself shows a separate fault code or will not light
- You smell gas (ventilate, shut the meter valve if safe, no switches, then call 0800 111 999 from outside)
A21 on a Worcester Bosch wireless thermostat means the thermostat and its receiver at the boiler have stopped talking over radio. The boiler is usually fine; it simply is not being told to heat. Check the thermostat batteries first, then the signal path, then power to the receiver.
Why is A21 showing on my Worcester thermostat?
Worcester's installation manual for the Greenstar Comfort II RF lists three causes for A21 and A22: the room thermostat is placed where the signal cannot reach the receiver, the receiver or the thermostat has no power, or the units have just been switched on and have not yet linked. The manual says A21 shows at first switch-on until the radio link is made, which can take up to a minute.
A21 is a controls code, so it normally appears on the wall thermostat or programmer rather than as a lockout on the boiler. If the boiler display shows its own code as well, such as EA 227 for no flame, deal with that one first, because a boiler in lockout will not heat whatever the thermostat says.
Not every Worcester control uses A21 the same way. On the wired Greenstar Sense I, Worcester lists A21 with sub-code 1001 as the control being configured for the wrong role, for example set as the main controller when a Sense II is also fitted. That is an installer setting, not a signal problem.
If the display shows A21 with 1001 on a wired Sense I, new batteries will not help. The control needs its A.1 setting changed by whoever installed it.
What to check, in this order
Start with the batteries. A blank screen or low battery symbol on the thermostat means they are spent. Worcester specifies AA 1.5V alkaline cells for the Comfort+ II RF control unit; fit a fresh set, not ones taken from another device.
Next, check the boiler has power: the display should be lit and the fused spur switched on. The receiver needs that power too. On the Comfort II RF, a receiver LED flashing once a second means it has lost the thermostat, which Worcester describes as a local error, not a boiler fault.
Then look at where the thermostat sits. Metal radiator covers, mirrors, foil-backed plasterboard and a boiler in a loft or garage all weaken the signal. Worcester suggests moving the thermostat for better signal and, where the boiler is in a loft or garage, wiring the receiver to a wall plate closer to the living space.
Only after those checks is it worth re-pairing. Follow the pairing steps in your control manual exactly. Worcester warns against pairing the same units twice, because it affects how they work.
How to keep the heating on tonight
On the Comfort II RF receiver, a short press of the override button (under three seconds) switches the boiler on for heating and hot water without the thermostat. The LED stays on with brief flashes off while override is active. Press it again to cancel once the thermostat reconnects.
Override has no temperature control, so keep an eye on the house and switch it off if rooms get too warm. It is a stopgap for a cold night, not a fix.
When A21 needs an engineer
If fresh batteries, a better position and a minute to reconnect do not clear A21, the fault is usually in the receiver, the thermostat unit or the bus wiring between the receiver and the boiler. An engineer checks power and the EMS connection at the boiler, reads the control fault history (the Comfort II RF keeps the last five boiler and control faults), and re-pairs or replaces the faulty unit.
Our Worcester boiler repair visits start with the £60 call-out, and the repair price is agreed on site before any work starts. See pricing for how that works.
Questions people ask
Is the Worcester A21 fault dangerous?
No. It is a lost radio signal between the thermostat and the receiver. The boiler keeps its own safety controls. If you smell gas at any point, that is a separate emergency: open windows, turn off at the meter if safe, avoid switches, leave and call 0800 111 999.
What is the difference between A21 and A22?
Both mean a lost radio link on the Comfort II RF. One edition of Worcester's manual groups them on one line; another gives A22 as no signal or a pairing fault, fixed by repeating the pairing. On the wired Sense I, A22 to A24 mean something else: a missing bus signal for heating circuits two to four.
Will a boiler reset clear A21?
Usually not, because the boiler has not locked out. Clear the fault by restoring the signal. Press the knob on the thermostat to return to the main screen once it has reconnected.