Heating not working? Checks to do before calling an engineer

Cold radiators are often caused by straightforward issues like an incorrect thermostat setting, low boiler pressure, or a valve that hasn’t fully opened, rather than indicating a serious breakdown. Before calling an engineer, work through these basic checks step-by-step. For example, your thermostat might be set too low or the timer could be off, preventing the heating from turning on. Low boiler pressure—usually below 1 bar—can stop hot water circulating properly, so check the pressure gauge and top up if needed. Also, zone valves control the flow of hot water to different parts of your home; if they’re stuck closed, radiators will stay cold.

Remember to never remove boiler covers or tamper with gas components yourself, as this can be dangerous and should always be left to a Gas Safe registered engineer. Following this order of checks can often resolve the issue quickly and safely without the need for costly repairs.

Start with the controls

Set the room thermostat higher than the current room temperature and check that its display is working. If it uses batteries, replace them if the screen is blank or faint. For a wireless thermostat, make sure it is communicating with its receiver and has not been moved out of range.

Next, check the programmer or timer. The heating should be set to “on” or to a current timed period, rather than hot water only. Some systems have separate heating and hot water channels, so confirm the heating channel is active and that any holiday, frost or override setting has not changed the schedule.

• Turn the thermostat up and listen for a click or a call for heat.

• Check the boiler has electrical power and that its local switch is on.

• Confirm radiator thermostatic valves are not set to zero or frost protection.

• Check whether a recent power cut has reset the clock or heating programme.

Check the boiler and heating system

Look at the pressure gauge

Most sealed combi and system boilers operate at around 1 to 1.5 bar when cold, although the manufacturer’s instructions take priority. If the gauge is very low, the boiler may refuse to run or may lock out. You may be able to top up the pressure using the filling loop, but only follow the appliance instructions and close the valves fully afterwards.

If pressure repeatedly falls, topping it up is only a temporary measure. The system may have a leak, a faulty pressure relief valve or an expansion vessel problem, which needs professional diagnosis. Do not continue repressurising it without finding the cause.

Consider the system type

A combi boiler heats radiators and domestic hot water directly, without a separate hot water cylinder. A system or regular boiler normally works with a cylinder, and may also have a loft tank, motorised valves and a separate pump. This affects the likely fault: controls, valves and pumps are particularly relevant where the heating and hot water circuits are separate.

When hot water works but radiators stay cold

If taps produce hot water but the heating does not come on, the boiler itself may be able to ignite, making the thermostat, programmer, motorised zone valve or circulation pump more likely suspects. On a system or regular installation, a valve may be stuck in the hot water position and fail to send heat to the radiators.

With a combi boiler, check the heating controls first, then look for a fault code or unusual boiler behaviour. If the boiler fires briefly and switches off while radiators remain cold, poor circulation, trapped air or a pump problem could be involved.

Radiators, air and circulation faults

Bleeding a radiator is useful when the top is cold while the bottom is warmer, or when you hear gurgling and bubbling. Switch the heating off, allow the radiator to cool, protect the wall and floor, then use a radiator key carefully. Check the boiler pressure afterwards, as bleeding releases a small amount of water as well as air.

A radiator that is cold throughout is less likely to need bleeding. It may have a closed valve, a stuck thermostatic valve pin, a balancing issue or no circulating hot water. Avoid forcing a valve or opening boiler components.

A failing circulation pump can cause the boiler to heat rapidly and shut down, noisy pipework, uneven radiator temperatures or a lack of heat throughout the home. Pump sounds alone are not conclusive, so professional testing is sensible if the controls and pressure are normal.

Check for condensate and fault-code clues

On a condensing boiler, a frozen or obstructed condensate pipe can prevent ignition and cause a lockout. Typical signposts include a fault code, a gurgling noise, or a boiler that attempts to start and then stops. Only inspect the accessible external pipe visually and follow the boiler manufacturer’s guidance for safe thawing. Never use boiling water or work on inaccessible pipework.

Fault codes vary by manufacturer, but common patterns include ignition or flame-failure codes, low-pressure warnings, overheating or circulation codes, and sensor or fan faults. Record the exact code, any flashing lights and what the boiler does when you reset it. A single reset may be permitted by the instructions, but repeated lockouts indicate an unresolved problem.

Safe checks and when to stop

You can usually check the thermostat, programmer, power supply, radiator settings, pressure gauge and visible pipework without touching gas components. Read the appliance manual, keep note of symptoms and avoid removing the boiler casing.

• Stop immediately if you smell gas, avoid switches and flames, leave the property and contact the appropriate emergency gas service.

• Call a Gas Safe registered engineer if a carbon monoxide alarm activates or anyone develops possible carbon monoxide symptoms.

• Arrange professional help for repeated lockouts, persistent low pressure, leaks or suspected pump and valve faults.

• Do not attempt repairs inside the boiler, adjust gas settings or bypass safety controls.

What an engineer will test

A qualified engineer will trace the fault rather than simply reset the boiler. They may check electrical signals from the thermostat and programmer, measure flow and return temperatures, test pump operation, confirm that motorised valves actuate and inspect pressure, sensors, condensate drainage and combustion safety.

This testing helps distinguish a control fault from poor circulation, a failing component or a wider system problem. It also reduces the risk of replacing parts unnecessarily and confirms that the appliance is operating safely.