Andrews A21 Fault Code: Causes, Fixes & Repair Costs

Common causes

• Dead or weakened thermostat batteries Common

  Even when batteries are low rather than flat, the RF signal strength drops enough to break communication. Many thermostat displays remain lit on low charge, so the warning is easy to miss. This is the single most common trigger for A21.

• Thermostat too far from the receiver or obstructed by building materials Common

  Thick concrete walls, steel joists, or simply too great a distance between the thermostat and its boiler-side receiver will attenuate the RF signal. Wireless thermostats typically need a reasonably clear line of sight and should ideally be within the range stated in the thermostat manual.

• Thermostat and receiver have lost their pairing Common

  The RF link can drop out after a power cut, a battery change, or a firmware reset, leaving the two devices unable to find each other. The boiler then raises A21 because it is expecting a signal that never arrives.

• Interference from other wireless devices Sometimes

  Wi-Fi routers, baby monitors, smart home hubs, and cordless phones all operate in frequency bands that can disrupt the RF link. Placing the thermostat or receiver close to any of these devices increases the chance of random A21 appearances.

• Original Worcester Bosch receiver still fitted after a third-party thermostat installation Sometimes

  If the boiler's RF receiver was left in place when a replacement thermostat (e.g. Hive, Nest, or similar) was installed, the receiver continues to search for the original Worcester handset. Finding nothing, it reports A21. The receiver needs to be properly removed or bypassed.

• Faulty thermostat handset or RF receiver Rare

  Physical wear, moisture ingress, or an internal component failure in either the thermostat or the wall-mounted receiver can permanently break the RF link. Frequent or random A21 appearances that return quickly after re-pairing suggest a hardware fault.

How to fix it

1. Replace the thermostat batteries with a fresh set DIY safe

   Remove the thermostat handset from its wall bracket — usually a gentle upward lift or a flat-blade screwdriver is all that is needed. Fit new alkaline batteries of the correct size (check the battery compartment lid). Reattach the handset, allow it to reconnect, and check whether A21 clears from the boiler display. Keep a spare set at home during winter so you can restore heat quickly.

2. Check the position of the thermostat relative to the receiver DIY safe

   Move the thermostat closer to the boiler-side receiver and check whether the A21 code disappears. Avoid locations near large metal objects, mirrors, thick masonry walls, or steel structural elements. Once you find a position where the link is stable, that is where the thermostat should live permanently.

3. Remove potential sources of RF interference DIY safe

   Temporarily switch off nearby Wi-Fi routers, baby monitors, cordless phone bases, and smart home hubs. If A21 clears after doing this, interference was the culprit. Relocate the thermostat or receiver away from those devices as a long-term fix.

4. Re-pair the thermostat to the receiver DIY safe

   Consult the pairing section of your specific Worcester Bosch thermostat manual (models such as the Wave, Sense, or older RF thermostats each have slightly different pairing sequences). The general process involves pressing and holding a pairing button on the receiver until an indicator light flashes, then confirming on the thermostat. Once paired, check that the boiler fires when a heating demand is set.

5. Check whether a redundant original receiver is still fitted DIY safe

   If a third-party smart thermostat was installed at some point, look for the original Worcester Bosch RF receiver — often a small box plugged into or wired near the boiler. If the original receiver is still active and searching for its lost handset, it will keep triggering A21. Inform your engineer so they can remove or correctly isolate it.

6. Reset the boiler DIY safe

   Once you have worked through the above checks, press and hold the reset button on the boiler for a few seconds until the display changes. Do this no more than two or three times in total. If A21 returns promptly after resetting, the signal issue has not been resolved and further investigation is needed.

7. Call a Gas Safe registered engineer if A21 persists Gas Safe engineer

   If the fault code keeps returning despite fresh batteries, re-pairing, and repositioning, there is likely a hardware problem with the thermostat handset, the RF receiver, or the wiring connecting the receiver to the boiler. An engineer can test the controls, advise whether replacement parts are needed, and confirm that any new thermostat is correctly commissioned. They may also suggest switching to a wired thermostat, which eliminates RF issues entirely and typically costs under £100 for the unit plus a labour charge.

Parts you may need

• Worcester Bosch RF thermostat handset (e.g. Wave or Sense) · from £120
• Worcester Bosch RF receiver unit · from £55
• Replacement alkaline batteries (pack) · from £5
• Wired room thermostat (alternative to RF) · from £45

The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.

Typical repair cost

Expect to pay roughly £150–£350, depending on the underlying cause.

Frequently asked questions