Thermostat not controlling your boiler

The short answer

The thermostat asks for heat. The boiler answers. Between them sits a pair of wires, or on a wireless system a receiver box near the boiler. When a thermostat stops controlling the boiler, the break is in the asking, the link, or the answering.

On a wireless system that receiver light is the most useful thing in the house. Worcester Bosch's MT10RF instructions give the normal pattern: the receiver LED “flashes twice each time a signal is received from the transmitter”, and is “continuously on when there is a demand for heating”. If it never does either while you turn the thermostat up, nothing is arriving.

First, rule out the symptom that is not a fault. Honeywell Home's T4R guide lists “Thermostat shows heat demand but boiler doesn't respond” with the cause “Boiler (water) already at temperature” and the fix “Normal behaviour - wait for water temperature to drop”.

Key facts

The short version first.

• Batteries first. Honeywell Home puts a blank T4R display down to batteries that are exhausted, installed incorrectly, or a pull tab never removed.
• Losing Wi-Fi does not usually stop your heating. Google says a Nest thermostat and Heat Link “communicate over their own proprietary wireless network”.
• Wiring at the boiler is not a homeowner job. Worcester Bosch's MT10RF instructions say the receiver “MUST BE FITTED BY A COMPETENT PERSON”, with the mains switched off.

Wired, wireless or smart: work out which you have

A wired thermostat has cables running back to the boiler or a wiring centre. Nothing to pair, no batteries to flatten. If it stops working, the fault is the thermostat, the wiring or the boiler itself.

A wireless thermostat is battery powered and sends a radio signal to that receiver. The pairing is normally done for you: Worcester Bosch says the MT10RF thermostat and receiver “are pre commissioned (paired) at the factory”, and Honeywell Home says “The thermostat and receiver box in this pack are pre-bound.” It can still be lost, and set up again.

A smart thermostat such as Nest, Hive or Wiser adds an app and usually a hub, but the heating link is still thermostat to receiver. Keep the complaints apart: a broken app and unresponsive heating are often unrelated.

What you can see, and what it points to

Checks you can safely make yourself

Work through these in order and stop where the fault clears. None of them involves opening the boiler.

1. Ask for heat properly. Set the target several degrees above the room temperature shown, and wait a few minutes. If a programmer is fitted, check it is inside a heating period, not hot water only.

2. Change the batteries. Check they are the right way round, then fit fresh ones of the same type. Worcester Bosch is blunt: “Do not use rechargeable batteries.” Hive's advice for a thermostat showing no signal is to “Remove the batteries and immediately reinsert them”, which restarts it and helps re-establish the connection.

3. Read the receiver light, and check its power. Find the box near the boiler or cylinder and compare its light with the maker's own table. Hive treats solid red as a lost connection and no light as no power. Where there is no light at all, the only checks that are yours to make are visible ones: whether the switch on the fused spur beside the boiler is on, and whether a breaker in the consumer unit has tripped. Opening the spur to inspect or replace its fuse is a job for an electrician or engineer. Do not open the receiver, the boiler casing or the wiring centre.

4. Move the thermostat and try again. Honeywell Home's fix for lost wireless communication is to “Move thermostat to a location where wireless communication is reliable”. Worcester Bosch asks for the thermostat to sit on an inside wall, roughly 1.5 metres above the floor, away from heat sources, curtains, draughts and damp.

5. Use the override to stay warm until an engineer can come. The MT10RF receiver has an emergency mode: “If a signal is not received for 60 minutes LED [A] continues to flash rapidly and the central heating is switched off”, and you can “Press button [B] and release for manual override and to switch the central heating on.” Google's equivalent is the Heat Link button. Treat it as a short-term measure and nothing more. It runs the central heating with no room thermostat controlling it, so it will keep heating until you switch it off by hand. Turn it off when the room is warm enough, and do not leave it running unattended or overnight.

Re-binding the receiver to the thermostat

Binding, sometimes called pairing, tells one receiver to listen to one thermostat and ignore everything else. It survives a power cut: Worcester Bosch says of the MT10RF radio link that “This link remains even in the event of a power loss.” So a link that has genuinely gone usually went for a reason, such as a replacement unit, a reset, or a receiver put into bind mode by accident. If it is the thermostat itself that looks dead rather than the link, start on our page about a wireless thermostat not working and come back here for the receiver and wiring side.

The procedure is model specific. Worcester Bosch's MT10RF method is to hold the receiver button for about five seconds until its LED is on continuously, at which point “The Receiver is now ready to accept a set up signal from the Transmitter for up to two minutes”, then press the reset button on the thermostat. When it works, “the LED will briefly flash twice and extinguish”. Honeywell Home's T4R uses a menu and warns you to “Keep at least 1m between devices during binding”.

Two cautions. Hive warns that a full reset erases any heating and hot water schedules you have set up. And on some boilers the receiver sits inside the control panel, which is the competent-person work quoted above. If the procedure sends you behind a casing panel, stop there.

When it is not the thermostat at all

Wiring looks exactly like a thermostat fault from the hallway. Honeywell Home's remedy for it, “Check wiring at thermostat and boiler terminals”, is engineer work.

Siting causes the rest. A thermostat above a radiator, in a draught, or in a hallway nobody heats reads a temperature that has little to do with the room you are in. That feels like a control fault and is really a siting fault.

When to call an engineer

Once the receiver has power, the batteries are fresh, the settings call for heat and the link is proven, anything left is inside the boiler or the wiring. The law is not vague about who touches the gas side: the Health and Safety Executive states that “Anyone employed to work on gas appliances in domestic premises must be a Gas Safe registered engineer and competent in that area of gas work.”

One safety point comes before every step on this page. If you smell gas, the HSE says to call the National Gas Emergency Service on 0800 111 999, open all the doors and windows, and shut off the gas supply at the meter control valve if you know where it is. Do not use electrical switches or create sparks.

To book a repair, call us on 0333 772 6247. One-off appointments run Monday to Friday, 09:00 to 17:00. Work on the boiler itself goes to a national network of thousands of Gas Safe registered engineers, because Gas Safe registration certifies gas work. Most of the faults on this page are control wiring rather than gas, and that side goes to vetted engineers and tradespeople qualified for it.

Thermostat and boiler communication FAQs

Boiler cover from Smart Plan, and what it leaves out

Be clear on the limits before you buy. Thermostats and heating controls sit outside the module, internet-connected ones included, and so does any fault your system is already showing today. What the module does pay for is parts and labour on the boiler and central heating equipment listed on your cover sheet: up to £500 in any 12-month period while the boiler is under 7 years old, dropping to £200 once it is over 7. When something covered breaks, we aim to get an engineer out to you within two working days, booked inside our cover-plan call-out hours of Monday to Friday, 08:00 to 18:00. There is a fee to know about too: £95, paid in advance, on call-outs in your first 30 days, on no-access visits, on annual services asked for early, and on faults the plan does not cover. Cover itself is live from day one, so those first 30 days change the fee and nothing else. Once you use a service, a 12-month agreement period starts. Smart Plan is a service plan, not insurance, from UK Boiler Company Ltd, trading since 2014.

Helpful links

• Wireless thermostat not working
• Worcester Bosch thermostat not working
• No heating? Book a repair
• Boiler fault codes hub
• Boiler & central heating cover
• HSE gas safety advice for homeowners
• Check an engineer on the Gas Safe Register
• Worcester Bosch MT10RF instructions
• Google: when the Heat Link light is yellow