Central Heating Not Working: Causes and What to Check

Hot water works but the heating doesn't

This is one of the most common combi boiler faults, and the usual suspect is the diverter valve, the component that switches the boiler's output between the central heating circuit and the hot water circuit. If the diverter valve sticks in the hot water position, taps will run hot but radiators stay stone cold no matter what the thermostat is set to.

Before assuming it is the diverter valve, check the room thermostat is calling for heat (set it a couple of degrees above room temperature and listen for the boiler firing), check the programmer or timer is set correctly, and check any zone valves or motorised valves in the pipework, since a stuck valve actuator produces exactly the same symptom and is often cheaper to fix than a diverter valve.

Heating works but there's no hot water

On a combi boiler this is less common, since the same heat exchanger usually serves both, but it can still happen if there is a fault with the plate heat exchanger or flow sensor dedicated to hot water production. On a system boiler or conventional boiler with a hot water cylinder, this pattern more often points to a faulty cylinder thermostat, a motorised valve stuck in the heating-only position, or an immersion heater issue if that is being relied upon instead.

Check first whether the cylinder thermostat is set correctly and the airing cupboard controls have not been knocked or turned down, since this simple check resolves a fair number of these calls without any parts needing replacement.

Neither heating nor hot water is working

When both fail together, the fault usually sits with the boiler itself, its power supply, or the system pressure. Start with the basics: check the boiler display for a fault code, check the power supply and any fused spur switch, and check the pressure gauge is within the normal range, usually 1 to 1.5 bar when cold.

Low or zero pressure will cause most modern boilers to lock out completely. If topping up the pressure via the filling loop restores operation but it drops again over the following days, there is a leak somewhere in the system that needs finding and fixing rather than repeatedly topping up.

Motorised valves, zone valves and wiring centres

Many heating systems, particularly those with a separate hot water cylinder, use one or more motorised valves (sometimes called zone valves) to direct water to the heating circuit, the hot water circuit, or both. These valves have a small electric actuator that can fail mechanically or electrically, and a wiring centre (sometimes called a junction box) that coordinates signals between the thermostat, programmer, valves and boiler.

A fault at the wiring centre can produce confusing symptoms, such as heating that comes on at the wrong time or valves that do not respond correctly to the thermostat, and diagnosing this properly usually needs a multimeter and a good understanding of the wiring diagram for that particular system, which is why this is generally best left to a qualified engineer.

Thermostat and programmer faults

A room thermostat that has failed, lost calibration, or has flat batteries (on wireless models) will simply never call for heat, leaving the boiler idle even though everything else is working correctly. Try turning the thermostat well above room temperature to see if the boiler fires; if it does, the thermostat or its batteries are the likely culprit.

Programmers and timers can also fail or be set incorrectly, particularly after a power cut resets the clock. Many modern smart thermostats have a manual override or boost function that bypasses the schedule entirely, which is a useful way to test whether the fault lies with the programming or with the heating system itself.

Frozen condensate pipes in winter

During cold snaps, one of the most frequent causes of a combi boiler cutting out is a frozen condensate pipe, the plastic pipe that carries slightly acidic waste water from the boiler outside to a drain. If this pipe freezes, usually where it is exposed to the cold on an external wall, the boiler senses a blockage and shuts down, often displaying a specific fault code for this condition.

If you suspect this, and it is safe to reach, gently warming the external section of pipe with a warm (not boiling) water-filled hot water bottle or warm cloth can thaw the ice and get the boiler running again. Once thawed, reset the boiler according to its instructions. Longer term, insulating the condensate pipe or having it rerouted internally reduces the chance of this happening again.

Frequently asked questions

What should I check first if the heating stops working?

Check the boiler display for a fault code, check the pressure gauge, and check the thermostat is calling for heat by turning it above room temperature. These three checks resolve or explain a large proportion of heating faults before any parts need replacing.

Can I fix a diverter valve myself?

No. The diverter valve sits inside the boiler casing and working on it involves the gas appliance itself, so this must be done by a Gas Safe registered engineer. Attempting it yourself risks both safety and any warranty on the boiler.

Why does my boiler keep losing pressure?

Pressure loss usually means a small leak somewhere in the system, sometimes at a radiator valve, a pipe joint, or occasionally within the boiler itself. Topping it up repeatedly without finding the source is only a temporary fix and can mask a developing problem.

What does it mean if my boiler shows a fault code?

Fault codes vary by manufacturer but generally point to a specific area, such as ignition failure, low pressure, or a frozen condensate pipe. Checking the boiler manual or manufacturer's website for the exact code is a quick way to narrow things down before calling an engineer.

I can smell gas, what should I do?

If you smell gas, turn off the gas supply at the meter if it is safe to do so, open windows and doors, do not use electrical switches or naked flames, leave the property, and call the National Gas Emergency Service on 0800 111 999 immediately.