How to turn the heating on without a thermostat

Can a Worcester boiler run the heating with no thermostat?

Not on its own, no. Worcester Bosch is direct about it in the Greenstar i System ErP operating instructions: “A room thermostat / programmer must be fitted to control the central heating. This controls the times and temperatures of the central heating, preventing the appliance from firing unnecessarily.” Something has to ask the boiler for heat. Left with no demand at all, it sits there doing nothing except protecting itself from frost.

This trips people up because the boiler has temperature buttons on the front. Those buttons do something different. On the Greenstar i ErP they are the “Central heating settings increase button” and “decrease button”, and what they set is “the desired flow temperature”, meaning how hot the water going out to your radiators is. They do not set room temperature and they do not switch the heating on.

So the better question is what else in your system can call for heat while the thermostat is out of action. On most Worcester setups there are two or three answers, and one of them will usually get the radiators warm again.

The short version

If you only read one part of this page, read this.

• The boiler will not fire for heating unless a control asks it to. Worcester's own user instructions say a room thermostat or programmer must be fitted.
• The buttons on the boiler front set the flow temperature to the radiators, not the room temperature.
• A wireless Greenstar Comfort II RF receiver has an override push button that keeps the boiler running when it has lost the signal from the thermostat.
• A programmer or timer set to constant will call for heat on its own if the thermostat is bypassed or already linked out.
• Flat batteries are a common reason a wireless Worcester thermostat stops talking to the boiler, and they are the quickest thing to rule out.
• Do not join thermostat wires together yourself. Worcester's user instructions state it is forbidden to interfere with the appliance beyond the actions they document.
• If you smell gas, stop and call the National Gas Emergency Service on 0800 111 999.

Five things to try, in order

Work down this list and stop at the first one that gets the radiators warm. Everything here is a control you are meant to operate, and none of it involves opening the boiler.

1. Put the programmer or timer on constant. If your heating has a separate timer, a wall programmer, or a clock built into the boiler front, set the central heating channel to constant, continuous or ON rather than a timed programme. On some systems the thermostat is the only thing between the programmer and the boiler. On others there is no separate room thermostat at all and the programmer is what calls for heat. Worcester's summer advice runs the other way, telling you to “Set the programmer to OFF” or turn the room thermostat down to minimum.

2. Use the override button on a wireless receiver. If your thermostat is a Greenstar Comfort II RF, there is a small receiver box, usually clipped to or sited near the boiler, with a single button and an LED. Worcester describes what it does: if the radio signal between the receiver and the thermostat is lost, the LED flashes once a second, and “the override push button can be briefly pressed once (less than 3 seconds) to override the Programmable room thermostat and maintain the boiler function for heating and hot water.” Pressing it again cancels the override. That is the closest thing to a legitimate no-thermostat mode Worcester provides.

3. Change the batteries and re-pair. Wireless thermostats fail quietly. Worcester's Comfort II RF instructions treat a battery symbol on the display as the sign that “the batteries require replacing”, and a depleted set as the reason for a blank screen. If the display comes back but the boiler still ignores it, the pairing between the thermostat and the receiver has been lost. Re-pairing is not one press of the override button. It is a set procedure that starts at the receiver and is finished from the thermostat's own installer menu, so follow the pairing steps in the Comfort II RF instructions for your unit, or ask an engineer to do it.

4. Open the radiator valves. A thermostatic radiator valve turned down does not stop the boiler firing, but a house full of valves on frost setting feels identical to a dead thermostat. Worcester recommends valves on all but one radiator, and that the remaining one, in the room where the thermostat lives, “must be uncontrolled and left open”. Turn them up while you sort the control out.

5. Turn the boiler's heating temperature up and watch the demand light. Use the central heating increase button on the boiler front to raise the flow temperature. On the Greenstar i ErP there is a blue indicator that “illuminates during a central heating or hot water requirement demand”. That light is a useful free clue, with one caveat. Turn the programmer or thermostat up. If the light stays off, that points at nothing reaching the boiler from the control or its wiring. It is not proof, because an installer can switch the demand light off, and on a boiler set up that way the light does not come on for a heating or hot water demand at all. Treat a light that never comes on as a clue and let an engineer confirm it. If it lights and the radiators stay cold, look at pressure or the pump instead.

What is actually stopping your heating

Why you should not link the thermostat wires yourself

Search this question and you will find people telling you to twist the two thermostat wires together, or to fit a link in the boiler's external controls terminals so it runs permanently. It is true that a link is sometimes used. Worcester's own wiring guide for Greenstar gas-fired boilers refers to the link between Live and Common, notes that it “may already be fitted as part of the Programmable Thermostat, if not then a link must be fitted”, and shows third-party room thermostats connecting to the external controls terminals of the boiler.

That is installer documentation, not a homeowner instruction, and the difference matters. The terminals sit inside the case of a mains-powered gas appliance. Worcester's user instructions put it plainly under user caution: “It is forbidden for any interference with the appliance other than those actions allowed in this document.” Nothing in the user manual allows you to open the case.

There is a legal backstop too. Regulation 3(1) of the Gas Safety (Installation and Use) Regulations 1998 says: “No person shall carry out any work in relation to a gas fitting or gas storage vessel unless he is competent to do so.” A registered engineer can fit the correct control and leave the system safe and controllable. Guessing at it can leave the boiler firing with no way of stopping it, which costs money every hour and leaves a house dangerously hot for anyone who cannot move away from the heat.

If you smell gas at any point, stop troubleshooting. Do not touch electrical switches, do not smoke, put out naked flames, open the doors and windows, and call the National Gas Emergency Service on 0800 111 999. That is the first instruction in Worcester's own user manual and it comes before any of this.

The proper fix, and what the rules expect

Running on an override or a timer is a stopgap. Without a room thermostat the boiler heats to a flow temperature and keeps going, so the house drifts warmer and you pay for every degree of it.

The rules point the same way. Under the Boiler Plus standards, which the government introduced for England, boiler installations from April 2018 have to include time and temperature control. The government's own factsheet says the standard “will also make timers and room thermostats an explicit requirement for all gas and oil systems, putting beyond doubt the need to give consumers the power to achieve the greatest comfort in the home”, and confirms the standards apply “whenever a boiler is installed in an existing heating system anywhere in England”. If your boiler was fitted from April 2018 and there is no working room thermostat, something has gone missing since installation.

One thing worth doing while you wait for an engineer, especially in a cold snap. Worcester's advice for an unoccupied property is to leave the programmer on constant and the room thermostat set to 15°C. With no working thermostat you cannot do the second half, so leave the heating running low with the radiator valves open. Be careful with this, because it is the state the section above warns about: with no room thermostat nothing tells the boiler to stop. Keep the flow temperature low, check on anyone who cannot move away from the heat, and switch the programmer off if the house gets too warm. The boiler's own frost protection runs the pump below 8°C and fires below 5°C, but it protects the appliance, not your pipework.

When you are ready to get it sorted properly, you can book a one-off repair and a Gas Safe registered engineer will come and diagnose the control, or call us on 0333 772 6247. One-off appointments run Monday to Friday, 09:00 to 17:00. If you would rather have future breakdowns handled for you, boiler and central heating cover is a separate product, and the short version of how it works is in the box below.

Turning the heating on without a thermostat: FAQs

Heating still off? Book a Gas Safe registered engineer.

For the fault you have now, book a one-off repair and a Gas Safe registered engineer will come and diagnose the control. One-off appointments are booked between 09:00 and 17:00, Monday to Friday, and we work with a national network of thousands of Gas Safe registered engineers. Cover is a different product, and it looks forward rather than back. It applies to the equipment listed on your cover sheet, a fault already happening counts as pre-existing, and the boiler and central heating module rules out any repair or replacement of wifi enabled or internet based equipment. Where it does pay, parts and labour are covered up to £500 in a 12-month period on a boiler under 7 years old, or up to £200 once the boiler is over 7. A £95 call-out fee is paid in advance. It is charged during your first 30 days, on faults the plan does not cover, on visits where nobody is home, and where you ask for the annual service early. Cover itself still runs during those first 30 days, so only the fee changes. Attendance is a target and not a promise: we aim to get an engineer out to you within two working days, and cover-plan call-outs are booked Monday to Friday, 08:00 to 18:00. A 12-month agreement period starts once you first use a service. You have 14 days to change your mind, that window closes the moment a service is carried out, and leaving early after that costs the remaining months or 75% of the outstanding balance. Smart Plan is a service plan, not insurance, from UK Boiler Company Ltd, trading since 2014.

Helpful links

• Wireless thermostat not working (start here for RF and pairing faults)
• Thermostat not controlling the boiler
• Room thermostat problems
• How to set a Worcester thermostat
• Worcester Bosch fault codes
• Boiler repair
• Boiler & central heating cover
• Worcester Bosch boiler manuals
• Check an engineer on the Gas Safe Register
• Gas Safety (Installation and Use) Regulations 1998, regulation 3