If your thermostat says the heating is on but the boiler stays quiet, the two devices may not be communicating. Sometimes the cause is as simple as flat batteries, a changed schedule or the thermostat being too close to a heat source. At other times the receiver, wiring or boiler controls need professional attention.
This guide explains the safe checks you can make without opening the boiler or touching fixed electrical wiring. Thermostats and boilers vary, so keep the user manuals for both devices nearby and follow the model-specific instructions where they differ.
First, check whether this is really a communication fault
Turn the requested room temperature several degrees above the temperature shown on the thermostat. Look for a heating, flame or demand symbol. Then give the system a few minutes: some controls deliberately delay firing to avoid rapid on-and-off cycling.
Next, look at the boiler. A normal display with no fault code suggests the boiler has power, but it does not prove that it is receiving a heating request. A warning symbol, error code or blank display points you towards a different problem. Record exactly what each screen shows rather than repeatedly resetting either device.
If hot water still works on a combi boiler but the radiators do not heat, the gas and electrical supplies to the boiler are probably present. The fault may lie in the heating controls, but that symptom alone is not a diagnosis. If neither heating nor hot water works, start with the boiler’s user-safe supply and fault checks.
Safe checks to try in order
1. Check the thermostat setting and schedule
Confirm that the thermostat is in heating mode, not frost-protection, holiday or summer mode. Check the day, time and programmed periods, especially after a power cut or clock change. Temporarily raising the target temperature above the measured room temperature is the clearest way to create a demand.
Do not assume a flame icon always means the boiler is burning. On many controls it means the thermostat is asking for heat. Whether that request has reached the receiver is a separate question.
2. Replace removable thermostat batteries
If the thermostat uses replaceable batteries, fit a fresh matched set of the type stated in its manual. Observe the polarity markings and replace the battery cover properly. A screen can remain visible even when the batteries are too weak for reliable wireless communication.
If the control has a sealed rechargeable battery or is mains powered, do not dismantle it. Check its user guide for the correct power indication instead.
3. Check that the boiler and receiver have power
Make sure the boiler display is lit and that any separate wireless receiver has its normal indicator lights. You may check an accessible consumer-unit switch or fused-spur switch visually, but do not remove covers, replace internal fuses or disturb wiring. If a circuit breaker repeatedly trips, leave it off and arrange an electrical or heating-engineer diagnosis.
4. Reduce possible wireless interference
For a portable wireless thermostat, take it back to its usual position and keep it away from televisions, routers, large metal objects, direct sunlight, radiators and draughts. Do not place it inside a cupboard or directly above a heat source. Thick walls, foil-backed insulation and long distances can weaken a radio signal.
Some Worcester Bosch wireless controls show A21 or A22 for loss of radio-frequency signal. Worcester’s Comfort I instructions say to check power and thermostat batteries and consider the thermostat position. Codes are model-specific, so never apply one manufacturer’s meaning to another control.
5. Separate Wi-Fi trouble from boiler communication
A smart thermostat may use two links: one between the app or cloud service and its hub, and another between the heating control and the boiler. Losing home internet can stop remote app control while local heating continues. Conversely, a working app does not guarantee that the boiler receiver is responding.
Check whether the thermostat works locally at the wall unit. Confirm that your router is operating and inspect the app’s service status if one is provided. Avoid deleting the device or factory-resetting it as a first step; that can remove schedules and turn a small connection problem into a complete setup job.
6. Re-pair only when the correct manual tells you to
Many wireless controls have a user pairing or radio-link procedure. Use only the instructions for the exact thermostat and receiver model. Worcester Bosch provides model-specific control manuals and a support video for re-establishing radio-frequency links. Baxi’s AmbiZen guidance likewise describes re-pairing for its own no-thermostat-detected alert.
Do not experiment with installer menus, remove the boiler casing or alter receiver wiring. If the manual’s normal user pairing process fails once, record the lights or codes and call for help.
What the receiver lights may tell you
A receiver fitted near or within the boiler is the bridge between many wireless thermostats and the heating system. Its lights may indicate power, a successful radio link, a heating request or a fault. The colours and button functions are not universal. Read the label and manufacturer manual before pressing anything.
A thermostat showing demand while the receiver shows no demand suggests a link, battery or pairing issue. A receiver showing demand while the boiler does nothing may indicate a control, wiring or boiler fault. That distinction is useful information for an engineer, but it is not a reason to open either unit.
Brand-specific points
Worcester Bosch controls
Worcester Bosch’s Comfort range uses a wireless room thermostat with a boiler-mounted programmer or receiver. The manufacturer’s literature identifies low batteries, loss of RF signal, poor thermostat positioning and pairing as possible causes of communication problems. It also notes that the units can take a short time to establish communication after power-up.
Use the exact Comfort, MT, Wave or Bosch EasyControl manual because their displays and recovery steps differ. Worcester’s support library includes separate control manuals and troubleshooting videos.
Baxi controls
Baxi’s current AmbiZen information describes an H0081 alert as no thermostat being detected and says re-pairing may be needed. The same official material notes that exhausted thermostat batteries can also be involved. That meaning belongs to the compatible Baxi system described by the manufacturer; it should not be assumed for every Baxi boiler or third-party thermostat.
Ideal controls
Ideal boilers may be paired with Ideal-branded or third-party wired and wireless controls. Check both product names before searching for instructions. A boiler fault code and a thermostat communication message are not necessarily the same thing. Photograph both displays and use the appropriate user guides.
When the thermostat is probably not the whole problem
Arrange a professional diagnosis if the receiver appears to call for heat but the boiler does not respond, the display shows a persistent boiler fault, wiring or a control enclosure looks damaged, the system repeatedly loses the connection, or the problem began after electrical or building work. An engineer may need to test the receiver output, control wiring, compatibility, boiler input and internal components.
Controls can also appear faulty when the boiler is locked out, system pressure is outside its permitted range or another safety condition prevents firing. Do not bypass a thermostat, bridge terminals or try to force the boiler to run.
Gas and carbon-monoxide safety
A communication fault by itself is not normally evidence of a gas leak. However, if you smell gas or suspect carbon monoxide, stop troubleshooting, move to fresh air, avoid operating electrical switches or flames, and call the National Gas Emergency Service on 0800 111 999 in Great Britain. Gas Safe Register advises householders not to repair gas appliances themselves and to use a registered engineer.
Information to collect before booking a repair
- The boiler make, model and exact fault code.
- The thermostat and receiver make and model.
- What the thermostat, receiver and boiler each display.
- Whether hot water still works.
- Whether the fault followed a power cut, battery change, router change or building work.
- What safe checks you tried and whether the fault is intermittent.
A short video showing the thermostat demand followed by the receiver and boiler displays can be especially helpful. Never include passwords, Wi-Fi credentials or access codes in material you send.
Frequently asked questions
Will my heating stop if the internet goes down?
It depends on the system. Many smart controls continue operating their local schedule but lose app access; others have features that rely on an online connection. Worcester states that its Wave’s timed heating and hot-water programme will not function until internet communication is restored. Check your own model’s manual.
Can I move a wireless thermostat closer to the boiler?
You can temporarily move a portable unit to test signal strength if its manual permits it. Its normal location still needs to represent the room temperature accurately, away from radiators, sunlight and draughts.
Should I keep pressing reset?
No. One user reset performed exactly as the boiler manual describes may be appropriate for some faults. Repeated lockouts or a failed reset need diagnosis rather than repeated attempts.
Can any thermostat work with any boiler?
No. Controls can use simple on/off switching, manufacturer communication protocols or digital standards, and compatibility varies. Confirm compatibility before buying or replacing a control.
Related boiler help
See also boiler not firing up, how to reset a boiler safely, normal boiler pressure and a boiler that keeps turning off.
Thermostat still not talking to the boiler?
Book a clear, fixed-price boiler repair visit for your Baxi, Worcester Bosch or Ideal boiler.