Common faults and what causes them
Worcester Bosch fits two very different families of control, and the fix depends on which is on your wall. The Greenstar Comfort RF controls are battery powered room thermostats that talk to a receiver at the boiler over radio. The Bosch EasyControl (CT200), and the older Worcester Wave it replaced, are mains powered smart controls using your Wi-Fi and a phone app. Work out which you have, then check the instructions for your exact model: codes and menus differ across the Comfort RF range.
Now match the symptom to the likely cause:
- Blank or dim screen on a battery unit: flat batteries. Go to changing the batteries.
- A21 or A22 showing: the radio link has dropped. Go to Comfort RF signal faults.
- No app connection, or a dark screen: Wi-Fi, or the proximity sensor. Go to EasyControl.
The temperature shown is not the temperature you set
Check the settings first. A thermostat that lost power can fall back to its default programme, and a schedule set below the current room temperature will never fire the boiler. Set the target at least two degrees above the room temperature and wait. On a Comfort II RF the burner symbol shows the boiler firing, and Worcester Bosch warns there can be a six minute delay before the programmer shows it. Unsure which button does what? Read our walkthrough of how to set a Worcester thermostat.
If the settings are right, look at where the thermostat lives. Worcester Bosch's siting instructions for the Comfort II RF are precise: an inside wall roughly 1.2 metres above the floor, no closer than 300 mm to metal objects including wall boxes, and clear of sunlight, draughts, radiators and anything else giving off heat. A stat above a radiator reads warm and switches the heating off early.
The boiler is not responding to the thermostat
A wireless system has two halves, the control on the wall and the receiver at the boiler, so the fault can sit in either. Confirm the control is calling for heat, then check the boiler has power, shows no code of its own and has not locked out. Reset it once using your boiler manual. If the code comes straight back, book an engineer. Our guide to no heating and what to check sets out the order.
Switched to Off, a Comfort II RF still shows a frost symbol and fires the boiler below 5°C. That is frost protection, not a fault.
Changing the batteries on a Worcester thermostat
The Greenstar Comfort range runs on LR6 (AA) 1.5V alkaline batteries. Once the battery low symbol appears, Worcester Bosch says they must be replaced within 60 days or the programmed settings will be lost.
To change them:
- Insert a flat bladed screwdriver into the slot on the bottom edge and twist gently until the bottom catches release.
- Separate the thermostat from its wall plate.
- Fit new batteries of the same type, positive end into the positive terminal.
- Hook the top edge onto the wall plate first, then snap the bottom edge into place.
If a fresh set does not wake the display, clean the battery contacts. That is Worcester Bosch's own next step for a blank display or a low battery symbol that will not clear. Allow a minute for the thermostat and receiver to re-establish their link. A control still blank on new batteries has failed.
The EasyControl and the Wave have no batteries, so a dark screen there is never a battery problem. Read the EasyControl section instead.
EasyControl and Wave: app control, dark screens and Wi-Fi
Advice specific to the Bosch EasyControl CT200 and CT200B, and to the Worcester Wave it replaced.
EasyControl is the smart control Worcester Bosch sells today, and the usual replacement for a Wave. Do not assume the two swap over. Check the instructions supplied with the control and your boiler, and have a qualified heating engineer change it over, because the unit is wired at the boiler.
The touch screen is dark
A proximity sensor lights the screen when someone comes near. If it detects nothing for five minutes the screen switches off, and Bosch states it can then only be woken by touching it. A screen that stays black when touched, or arrived cracked, is a hardware fault: go back to whoever supplied it.
The app cannot reach the thermostat
Work through Bosch's published causes in this order before you call anyone:
- Wi-Fi band. EasyControl needs broadband over WLAN 802.11 b/g/n, 2.4 GHz. A 5 GHz only network will not do.
- A dollar sign in the password. Bosch states the network password must not contain a $ character. Change it if it does.
- A hidden or weak network. EasyControl connects to visible networks only, so switch SSID broadcast back on to pair. If the router is too far away, Bosch's fix is a WLAN amplifier.
- Wrong login details. The serial number and access code are case sensitive, and both are on the sticker on the back of the unit.
- Brand new out of the box. The software updates itself automatically over several hours, so not every function works on day one. Push it under Info, then About, then Checking & Update.
Soft reset and hard reset are not the same thing
A soft reset keeps your settings: undock the EasyControl briefly from its plinth, then dock it again. A hard reset restores factory settings and deletes your Wi-Fi details. It uses the wastepaper basket symbol on screen, or a paper clip in the reset button, and offers two choices: wipe everything including consumption data, or keep that data.
If the fault comes from the boiler rather than the control, the app says so. Our Worcester Bosch technical support page covers what the common boiler codes mean.
Comfort RF: A21, lost signal and pairing faults
Advice specific to the Greenstar Comfort II RF. Other controls in the Comfort RF range use different fault codes and different menus, so check the instructions for your exact model.
These controls have no app and no Wi-Fi. The thermostat on the wall sends a radio signal to a receiver wired into the boiler's EMS bus.
Check the signal strength first
A Comfort II RF will tell you how good its link is. Hold the menu button for more than three seconds, turn the knob to Info and press it, then select Signal strength. A number appears where the temperature normally sits, on the Comfort II RF's own scale:
- 0 = no signal
- 1 to 3 = weak signal
- 4 to 6 = acceptable signal
- 7 to 10 = very good signal
Worcester Bosch's instruction is to try other positions until the number is as high as you can get it. Keep the unit away from other radio devices: thick walls, steel joists and metal boxes all eat the signal.
What the A codes mean
These A codes are the Comfort II RF's own list. Other Comfort RF controls display different codes, so read the instructions for your control.
- A21 and A22, loss of RF signal. Usually the room unit is badly positioned, so check the signal strength and move it. If the boiler is in a loft or garage, the receiver should be wired to a wall plate. Check the boiler has power and the batteries are not low. A21 also shows at switch on, so allow a minute.
- A23, room stat internal sensor failure. The room temperature sensor is out of range and the thermostat needs replacing.
- A24, remote RF unit has a fault. Another room thermostat in the system has a problem, so check the others.
- A25 and A26, pairing faults. Two or more thermostats share a receiver on the same zone, or no zone is free. Unpair and pair again, a different zone each. Up to three zones can be set.
- A27, no communication with the boiler's EMS bus. The boiler is EMS compatible but not talking, and the programmer or receiver needs replacing.
- A28, not paired. Unpair the units and pair them again.
A Comfort II room thermostat also displays the boiler's own fault codes, so a code not on this list is coming from the boiler.
The receiver at the boiler, its EMS wiring and any replacement paired unit are an engineer's job. Repositioning the wall unit and changing its batteries is yours.
Which wireless thermostat works with a Worcester boiler
It depends on your boiler. Compatibility is set control by control, not across the whole range, and Worcester Bosch lists the boiler ranges each control works with on that control's own product page. Check that page and your boiler's instructions before buying, because some combinations need extra parts fitted at the boiler, which is an engineer's job. Broadly, the choice is between a Greenstar Comfort RF room thermostat, which needs no app and no broadband, and the Bosch EasyControl, which gives you app control.
Third party smart thermostats work on many Worcester boilers, but it depends on your model and its wiring, so ask a qualified heating engineer before buying. Wiring a control at the boiler is not a homeowner job. Our guide to wireless thermostat faults covers other brands, and thermostat not controlling the boiler helps you find which half has failed.
Thermostat still unresponsive: who can help?
You have done your part once you have changed the batteries, checked the signal, moved the unit and reset the boiler once. What is left is a failed control or a boiler fault, and both need someone qualified. Anyone working on a gas appliance in your home must be a Gas Safe registered engineer. If you smell gas at any point, do not touch any switches: open the doors and windows, turn the gas off at the meter if you can reach it safely, leave the property and call the National Gas Emergency Service on 0800 111 999.
If the thermostat was supplied with the boiler and the boiler is still in guarantee, check whether the control is included before you pay for a repair. Terms vary, and Worcester Bosch's own guarantee excludes third party controls and consumables including batteries. The dates and the full exclusion list are on our Worcester Bosch guarantee page.
For the manufacturer's own words, the Worcester Bosch literature library holds the full Comfort II RF and EasyControl instructions. The technical steps above come from those documents. Smart Plan is not affiliated with, or endorsed by, Worcester Bosch.
Out of guarantee and need an engineer.
A Smart Plan boiler and central heating module covers parts and labour up to £500 in any 12 month period if your boiler is under 7 years old, or £200 if it is over 7, through a national network of thousands of Gas Safe registered engineers. Cover applies from the day your plan starts. A £95 call out fee, paid in advance, applies to call outs in your first 30 days, to issues outside your cover, and if we cannot get access. Cover plan call outs are booked Monday to Friday, 08:00 to 18:00, and we aim to get an engineer out to you within two working days. It is a service plan, not insurance, and once you use a service a 12 month agreement period applies.
