Boiler Not Firing Up? What to Check Before You Ring an Engineer

A boiler that is not firing up is usually one of two very different problems: either nothing is asking it to run, or something is asking and the boiler cannot light. Work out which of those you have and you will have done half of our engineer’s job before we arrive - and quite often you will not need us at all.

Here is the order our team works through on breakdown calls across Deal, Dover, Sandwich, Canterbury and Thanet, split into the parts you can safely check yourself and the parts you cannot.

First: is anything actually asking the boiler to fire?

A boiler with power, a normal display and no fault code that simply sits there is, more often than not, waiting for a demand that never comes. Before you suspect the boiler, prove the demand.

Turn the room thermostat well past the room temperature and open a hot tap. A healthy boiler should react to one or the other within a few seconds - you will hear a click, a whirr from the fan, and then the burner light. If it fires for hot water but not heating, or heating but not hot water, skip to the section on that below. If it fires for neither, and the stat is clearly calling for heat, carry on down this list.

If you are not sure whether the thermostat is doing its job, our guide to what to do when a thermostat is not working walks through that test properly. Flat batteries in a wireless stat are the single most common “boiler not firing up” we see, and the boiler is entirely innocent.

Checks you can safely make yourself

None of these involve opening the boiler casing. If anything below needs a tool other than your hands, stop.

Power and the fused spur

Every boiler has a switched fused spur nearby, often a small white switch with a red neon. It gets knocked off during decorating and cleaning more often than you would think. If the display is dead, that is the first place to look, followed by the consumer unit for a tripped breaker. A boiler that keeps tripping the breaker after it is reset is an electrical fault and needs an engineer, not a fourth attempt.

Gas supply

Check whether another gas appliance works - a hob is ideal. If the hob will not light either, the problem is upstream of the boiler: a prepayment supply out of credit, a supply interruption, or the emergency control valve knocked to the off position. If you smell gas at any point, stop, open windows, do not operate switches, and ring the National Gas Emergency line on 0800 111 999 before anyone else.

System pressure

Most modern boilers refuse to fire below about 0.5 bar and will show a code when they do. Some older ones just go quiet. Look at the pressure gauge on the front: cold, it should sit around 1 to 1.5 bar. If it is near zero, topping up via the filling loop will usually get you going again, but a boiler that needs topping up repeatedly has a leak somewhere, and our post on why a boiler keeps losing pressure explains where to look.

The condensate pipe

If it is below freezing outside and the boiler has stopped overnight, look for the white plastic pipe running from the boiler to an outside drain. When it freezes, the boiler cannot get rid of its condensate and shuts down rather than lighting - some show a code, some simply gurgle and refuse. Pouring warm, never boiling, water along the outside run frees it. When the first hard frost hits East Kent it becomes the busiest call on our books - we have a full guide to thawing a frozen condensate pipe.

A reset

If the display shows a fault code, note it down before you do anything else - it tells us more than any description. Then press reset once. If it locks out again, repeated resetting will not fix the cause and can mask it. Our boiler fault code guides explain what the common codes mean by brand.

Boiler not firing up for heating but hot water is fine

This is a controls or valve fault far more often than a boiler fault - the boiler proves it can light every time you open a tap.

On a combi, the usual culprit is the diverter valve that switches between heating and hot water - when it sticks in the hot water position, the radiators never see any flow. On a system with a hot water cylinder, it is the motorised valve for the heating circuit or the room stat and programmer chain. Either way it is an engineer’s job, but a comparatively simple one.

Boiler not firing up for hot water but heating works

The reverse case: radiators are warm, the tap runs cold. On a combi this points to the flow sensor or turbine that tells the boiler a tap is open, or again to the diverter valve stuck the other way. Limescale is a real factor here on the East Kent chalk - the plate heat exchanger and flow sensor both suffer in our hard water. We cover this in detail in what it means when the heating works but there is no hot water.

Why a gas boiler will not light: the causes our engineers find

When you have proved the demand is there, the gas is on, the pressure is right and the boiler still will not fire, you are into the ignition sequence. This is not homeowner territory - every component below sits inside the casing and must be handled by a Gas Safe registered engineer. We are registration number 2836.

Roughly in the order we find them across East Kent:

• Ignition electrode or lead. The spark electrode erodes over the years, the gap widens, and eventually the spark is too weak to light the gas. You may hear repeated clicking before it locks out. A cheap part and a common winter repair on older boilers.
• Fan or air pressure switch. The boiler proves the fan is running before it will release gas. A tired fan, a blocked flue or a split air pressure switch tube stops that proof, and the boiler never gets as far as trying to light. On coastal properties in Deal, Walmer and along the Thanet seafront, salt air corroding the fan housing is something we see more than inland engineers would.
• Gas valve. The valve that meters gas to the burner can fail electrically or stick. The fan runs, the spark fires, but there is nothing to ignite.
• Flame detection. The boiler lights, fails to sense its own flame, and shuts down within seconds - it looks like it will not fire when it actually fires and gives up. Glow-worm’s F75 fault code is a pressure-sensor version of this “cannot prove the system is right” logic, and it confuses people the same way.
• Printed circuit board. The PCB is the boiler’s brain, and when it fails the symptoms can be anything. We only reach it after ruling out everything cheaper, and on an older boiler its cost is where we talk honestly about whether replacement makes more sense than repair.

What we will ask when you ring

Tell us what the display shows, whether hot water and heating are both affected, whether you have topped the pressure up, and what happened when you turned the stat up. That usually tells us which of the causes above we are walking into, so the right parts are on the van first time.

Diagnosis and repair are charged hourly rather than at a fixed price, because until the boiler has been tested nobody honestly knows what it needs. Our rate is £90 plus VAT for the first hour, which is always billable, then £45 plus VAT per half hour, with parts on top. The pricing page sets out the full rates. Members on a BOYLA HomeCare plan pay no call-out fees and get their annual service included, which is where most of the electrode and fan problems above get caught before they strand anyone.

Don’t wait for the first cold morning

A boiler that has been off since spring and will not fire in October is our most predictable autumn conversation. If yours has been temperamental, or has not had a service in over a year, now is the time to have it looked at, before the weather forces the issue.

If it will not fire at all, ring us on 01304 381999. A real person answers the phone, and we book same-day boiler breakdown repairs across East Kent within working hours wherever the diary allows. We are Deal-based and cover Dover, Sandwich, Canterbury, Folkestone, Thanet and the villages in between.