Boiler not firing up: checks to try first

A boiler that will not fire up is most likely to happen on the coldest morning of the year, which is exactly when you least want to wait. Many of the causes are simple and safe to check yourself. The rest need a Gas Safe registered engineer. This guide helps you tell which is which.

Safety first

If you can smell gas, stop. Do not try to light the boiler, do not use light switches or anything that could spark, open doors and windows, turn the gas off at the meter if you can, leave the building and call the National Gas Emergency Service on 0800 111 999. That number works the same in Sheffield, Dronfield and anywhere in England.

Check 1: does it have power?

• Is the boiler display on?
• Is the fused spur switch next to it on, and has the fuse blown?
• Has a breaker tripped at the consumer unit?
• Is there a power cut? Call 105 to check.

Check 2: is the gas on?

• Does the gas hob light, if you have one?
• Is the lever on the gas meter in line with the pipe, meaning on?
• If you have a prepayment meter, does it have credit, and is the display showing any warning?

If no gas appliances work and the meter is on, contact your gas supplier.

Check 3: is the pressure right?

On most combi and system boilers, the gauge should read around 1 to 1.5 bar when cold. Many boilers will not fire below 1 bar. Top it up using the filling loop, following the manual. If you are topping up often, read our post on boiler losing pressure.

Check 4: is anything asking for heat?

• Is the timer or programmer in an “on” period, or set to constant?
• Is the room thermostat turned up above the room temperature?
• If you have a smart thermostat, is it connected and calling for heat? Check its app.
• For hot water on a cylinder system, is the hot water timer on and the cylinder thermostat turned up?

Replace batteries in wireless thermostats. Flat batteries are a common reason a boiler sits idle.

Check 5: is it freezing outside?

If it is below freezing and your boiler is a condensing boiler, as almost all modern gas boilers are, check the condensate pipe. This is the plastic pipe that runs from the bottom of the boiler to a drain. If part of it is outside, it can freeze. When it does, the boiler locks out and shows a fault code.

Pour warm, not boiling, water along the outside section until it clears, then reset the boiler. Our post on frozen condensate pipes has step by step instructions.

Why Dronfield and high ground see more cold snap failures

Dronfield sits on the slopes of the Drone valley just over the border from Sheffield, and like the higher parts of the city it gets hard frosts. Many homes in Dronfield Woodhouse and Gosforth Valley have boilers on outside kitchen walls with the condensate pipe running outside to a gully. That is a classic setup for freezing. Our Dronfield area page covers other common jobs there.

Check 6: what does the fault code say?

Most boilers show a code or a pattern of lights. Look it up in the manual or on the maker’s website. Common categories:

Code type	What it often means	Can you do anything?
Low pressure	Pressure below the minimum	Top up the pressure
Ignition or flame failure	Boiler tried to light and failed	Check gas, check condensate, try one reset
Condensate blockage	Condensate pipe or trap blocked or frozen	Thaw the pipe, then reset
Overheat	Boiler too hot, often poor circulation	Check radiator valves are open, then call an engineer
Fan or flue fault	Problem with air or exhaust	Call an engineer
Sensor or electronics	Internal part fault	Call an engineer

Check 7: one reset

Once you have checked the basics, try the reset as the manual describes, usually a button or a sequence on the display. If the boiler fires and runs, keep an eye on it. If it locks out again, do not keep pressing reset. Repeated resets can hide a real fault.

When to call an engineer

Call a Gas Safe registered engineer if:

• The boiler still will not fire after the checks and one reset.
• The fault code points to a fan, flue, sensor or electronics.
• You can hear the boiler trying to light repeatedly without success.
• You see water leaking from the boiler.
• The flame looks yellow or orange instead of blue, or there are sooty marks around the boiler. Turn it off and do not use it until checked.

Do not take the casing off yourself. Any work inside a gas boiler must be done by a Gas Safe registered engineer. Ask to see their ID card and check the back of it shows they can work on domestic boilers.

While you wait

• Use plug in heaters safely, away from curtains and furniture.
• If you have a hot water cylinder with an immersion heater, switch it on for hot water.
• Keep doors closed to hold heat in the rooms you are using.
• In a hard frost, keep an eye on pipes in lofts and cellars, as they can freeze with the heating off.

Our boiler repair page explains how a repair visit works. Send us the make, model and fault code and we will tell you what we think the likely cause is.