The heating is not coming on. You have turned the thermostat up. Nothing happens. This is one of the most common calls we receive — and in the majority of cases, the fix is quick. The problem is usually not the boiler, and it is often not even the thermostat itself.
This guide walks through every likely cause in the order you should check them, from the simplest (batteries) to the more involved (wiring faults and system settings). Most households can diagnose and fix this without calling an engineer.
The Most Common Causes
Before working through each cause in detail, here is a quick overview. In roughly this order of likelihood:
Dead or Low Batteries
Wireless thermostats run on AA or AAA batteries. When they fail, the thermostat cannot send a signal to the boiler.
Set Below Room Temperature
The thermostat only calls for heat when the room is colder than the set point. If the room is already warm enough, nothing fires.
Programmer or Timer Issue
The boiler programmer may be set to OFF, or the clock may have reset after a power cut and is now running on incorrect times.
Boiler Lockout or Fault
The boiler has shut itself down due to low pressure, overheating, or another fault. The thermostat signal is reaching it, but the boiler will not fire.
Lost Wireless Connection
Smart and wireless thermostats can lose pairing with the receiver. The thermostat works but the signal is not getting through.
Wiring Fault
A loose terminal or failed wire has broken the connection between the thermostat and the boiler. Less common, but requires an engineer to fix.
Start Here: The Two-Minute Check
Turn the thermostat to its maximum setting (30°C or as high as it goes). Wait two minutes. If the boiler does not fire and there is no sound from the system at all, the issue is likely between the thermostat and the boiler — not inside the boiler itself.
Step 1 — Check the Batteries
If your thermostat is wireless — meaning it sits on a wall or table without any visible wires — it runs on batteries. This is the most common cause of heating failure and takes 30 seconds to check.
Look at the thermostat display
A blank display, faint display, or a battery indicator icon means the batteries need replacing. Even if the display appears normal, low batteries can prevent the thermostat from transmitting correctly. Replace with fresh AA or AAA batteries (check the back panel for the size), then turn the thermostat up and wait two minutes.
Step 2 — Check the Thermostat Setting
The thermostat calls for heat only when the room temperature is below the set point. If your living room is already at 20°C and the thermostat is set to 19°C, nothing will fire — this is working correctly, not a fault.
Turn the thermostat to 25°C or higher. If the boiler fires within a minute or two, the original set point was simply too low for the current conditions. Adjust to your preferred temperature and leave it.
Many people turn the thermostat up during cold weather expecting it to heat the room faster. The boiler always runs at the same rate — the thermostat only decides when it switches on and off. A higher set point means the heating runs for longer, not harder. Set it to 20–21°C and leave it.
Step 3 — Check the Programmer and Timer
Most central heating systems have both a room thermostat and a separate programmer (or timer). The programmer controls when the system is allowed to run — typically morning and evening slots. If the programmer is set to OFF, or the schedule has lapsed, the boiler will not fire regardless of what the thermostat reads.
What to check on the programmer
- Is the programmer set to ON or AUTO (not OFF)? Many have a manual override button.
- Is the clock showing the correct time? After a power cut, programmers often reset to 12:00 and the schedule runs at the wrong times.
- Are any heating slots set for the current time of day? Check the programme and ensure your desired hours are included.
- Is the hot water set separately from the heating? On two-channel programmers, switching off hot water by mistake can sometimes affect the heating channel too.
If you have a smart thermostat (Hive, Nest, Tado, etc.), open the app and check for any scheduled off periods or holiday modes that may be active.
Step 4 — Check the Boiler
Walk to the boiler and look at the display. If there is a fault code, the boiler has locked out due to an internal problem. Common causes include:
- Low pressure: Most combi boilers need 1.0–1.5 bar at rest. If the pressure gauge reads below 0.5 bar, the boiler will not fire. Repressurise via the filling loop (a short grey flexible hose, usually under the boiler) until the gauge reads around 1.2 bar.
- Flame failure: The boiler tried to ignite and failed. Reset by pressing the reset button (usually marked with an arrow or the word RESET). If it fails again, do not keep resetting — call a Gas Safe engineer.
- Overheating lockout: The boiler shut down to protect itself. If the pipework above the boiler is extremely hot, there may be a circulation problem. Call an engineer.
If you can smell gas near the boiler, do not attempt to reset or operate it. Leave the property, leave doors open as you go, and call the National Gas Emergency line on 0800 111 999. Do not operate any electrical switches.
Step 5 — Check the Wireless Receiver or Wiring
Wireless thermostats send a radio signal to a small receiver unit, which is usually wired directly to the boiler or a control panel nearby. If this pairing is lost, the thermostat appears to be working but no signal reaches the boiler.
For wireless thermostats
Look at the receiver unit
Find the small receiver — usually a white box near the boiler or on a nearby wall. Check that it has power (an LED or display). Look for any indicator that shows it is receiving a signal from the thermostat. Many receivers have a manual boost button; pressing this should fire the boiler directly, which confirms the boiler itself is fine and the issue is the thermostat-to-receiver connection.
Re-pair the thermostat
Most wireless thermostats can be re-paired to their receiver by following the instructions in the manual. This typically involves pressing a button on the receiver to put it into pairing mode, then confirming on the thermostat. If you no longer have the manual, search for your thermostat model number (printed on the back) online — most manufacturers publish the pairing procedure.
For wired thermostats
A wired thermostat uses low-voltage wiring — typically two or three cables — that connect it to the boiler or programmer. These connections can work loose over time, particularly if the thermostat has been bumped or if the property has had work done nearby.
Remove the thermostat from its backplate (most clip or unscrew off) and check that all terminal screws are tight and that no wires have pulled free. If everything looks secure and the heating still does not respond, the thermostat itself may have failed — a heating engineer can test this quickly with a simple bypass check.
When the Problem Is the Thermostat Placement
A thermostat that is working correctly but positioned incorrectly will give unreliable results. Common placement problems:
- Near a heat source: If the thermostat is close to a lamp, radiator, or sunny window, it reads the room as warmer than it is and cuts the heating off too early.
- In a draught: Near an external door or air vent, the thermostat reads colder than the rest of the house and runs the heating longer than needed.
- In an unused room: If the thermostat is in a hallway or spare room that is kept cooler by closed doors, it may keep calling for heat even when the rest of the house is warm.
The ideal position is on an internal wall, in the room where you spend most time, at roughly shoulder height, away from direct sunlight and heat sources.
Thermostatic Radiator Valves (TRVs) and the Thermostat Conflict
One common source of confusion: if TRVs on every radiator in a room are turned down or closed, no heat enters that room even if the thermostat is calling for it. The boiler may fire, but the hot water circulates to rooms where TRVs are open instead.
Always ensure at least one radiator in the thermostat's room has its TRV fully open (or removed — TRVs should not be fitted in the same room as the main thermostat at all, as they create a conflict). If the thermostat room has a TRV, it should be replaced with a lockshield valve set to fully open.
When to Call a Heating Engineer
Call an engineer if any of these apply
- The boiler shows a fault code that does not clear after a single reset
- The boiler fires manually (via receiver boost) but will not respond to the thermostat
- The wiring behind the thermostat backplate looks damaged, corroded, or has been disturbed
- You have re-paired the wireless thermostat and it still does not control the boiler
- The heating runs continuously and cannot be turned off via the thermostat
- Multiple rooms are cold despite all TRVs being open (may indicate a pump or zone valve fault)
- You can smell gas anywhere near the boiler or gas meter
Thermostat Replacement Costs
Typical Pricing Guide
Basic wired room thermostat (supply and fit): £80–£150
Programmable digital thermostat: £100–£200
Wireless thermostat and receiver kit: £120–£220
Smart thermostat — Hive, Nest, or Tado (supply and installation): £200–£350
Diagnostic call-out (to identify wiring or system fault): £60–£100
Prices are guide rates for Slough and Berkshire. Cost depends on system type and whether additional wiring is needed. All Manako Plumbing quotes are fixed price before work begins.
Preventing Thermostat Problems
A few simple habits keep the thermostat and system running reliably:
- Replace batteries annually: Do it at the same time as your boiler service so it does not slip.
- Check the programmer clock after power cuts: Any interruption to the mains can reset the time and throw the schedule off.
- Keep the thermostat clear: Do not place furniture, curtains, or decorative items in front of it — they trap heat and give a false reading.
- Book an annual boiler service: The engineer will check thermostat wiring, programmer settings, and system controls as part of the service — catching minor issues before they become a cold morning.
Related Guides and Services
- Heating Repair — Manako Plumbing — thermostat replacement, zone valve repair, and heating system diagnostics
- Boiler Repair — fault codes, lockouts, and boiler diagnostics in Slough and Berkshire
- Annual Boiler Service — includes thermostat wiring check, programmer review, and system controls
- Cold Radiators? How to Bleed a Radiator — if some rooms are cold but others are warm, bleeding may be needed
- Leaking Radiator? How to Find the Leak and Fix It — system pressure drops can cause boiler lockout
Heating Problem in Slough or Berkshire?
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Get a Free QuoteFrequently Asked Questions
Why is my thermostat on but the heating won't come on?
The most common reasons are: dead or low batteries in a wireless thermostat, the boiler is in lockout (check for a fault code on the boiler display), the thermostat is set below the current room temperature, the programmer or timer is overriding the thermostat signal, or a wiring fault has broken the connection between the thermostat and boiler. Work through these in order — batteries first, then boiler display, then programmer settings — before calling an engineer.
How do I know if my room thermostat is faulty?
Signs of a faulty room thermostat include: the heating turns on and off at seemingly random temperatures, the room temperature does not match the thermostat reading, the display is blank or shows incorrect information, or the heating runs continuously regardless of the thermostat setting. To confirm the thermostat is the issue rather than the boiler or wiring, temporarily bypass it by connecting the two wires directly (short the terminals) and see if the boiler fires — if it does, the thermostat is the fault. Always isolate the power first if attempting this.
Can I replace my thermostat myself?
A like-for-like replacement of a simple room thermostat is a job many homeowners can do — the wiring is low-voltage and straightforward in most cases. However, if you are upgrading to a smart thermostat, have a combi boiler with a modulating connection (OpenTherm), or your existing thermostat is part of a more complex system with a programmer and zone valves, the wiring becomes more involved and a heating engineer should do it. An incorrectly wired thermostat can cause the boiler to run continuously or not at all.
Why does my heating come on at the wrong time?
This is almost always a programmer or timer issue rather than the thermostat itself. Check that the programmer clock is set to the correct time (many revert to 12:00 after a power cut). Review your scheduled on/off times — a single incorrect programme entry can cause heating to fire at unexpected times. On smart thermostats, check for any automated schedules set in the app that might be overriding your manual settings.
What is the correct temperature to set a room thermostat?
The recommended setting for a UK home is 18–21°C when occupied. The thermostat calls for heat when the room drops below the set temperature, then shuts off once it reaches that level. Setting it higher does not heat the room faster — the boiler runs at the same rate regardless. Setting it to 20°C and leaving it there is typically more efficient than constantly adjusting it. If you are away, a frost setting of 7–10°C protects the property without wasting energy.
How much does it cost to replace a room thermostat?
A basic wired room thermostat (parts and labour): £80–£150. A programmable or digital room thermostat: £100–£200. A smart thermostat such as Nest or Hive (supply and installation): £200–£350, depending on model and whether your system requires additional wiring or a Heat Link. If a heating engineer needs to diagnose a wiring fault first, expect to add £60–£100 for the diagnostic call-out.