How long does a boiler diagnosis take?

A straightforward fault is usually identified inside the first hour, because the sequence of checks rules out whole branches quickly. Intermittent faults take longer by their nature, since the appliance has to be run under the conditions that make it fail before anything can be measured. We will say on the visit if it is turning into the second kind.

Can you tell me what is wrong over the phone?

We can often tell you what it is likely to be, and that is worth knowing because it tells us what to carry. What we will not do is quote for a part on the strength of a symptom or a code, because the commonest cause of a given code is not necessarily yours, and the wrong part fitted is money spent twice.

Can it be repaired on the same visit?

Often, where the part is a common one and the engineer is carrying it. Where a component has to be ordered, or only the manufacturer supplies it, we tell you what it is and arrange the return visit. Nothing gets fitted without explaining what is involved first.

My boiler keeps locking out and I keep resetting it. Does that matter?

It is worth taking seriously. A lockout is the boiler protecting itself, and resetting does nothing about the condition that caused it. Manufacturer guidance on most appliances is to stop after a small number of attempts and have the fault investigated. If it will no longer accept a reset at all, stop and book it in.

The heating works but there is no hot water. Is that the boiler?

On a combi it usually is, with a diverter valve stuck in the heating position or a scaled plate heat exchanger the two usual causes. On a system with a hot water cylinder it is just as likely to be the motorised valve, the cylinder thermostat or the immersion circuit rather than the boiler at all. Which you have changes what the engineer needs to bring, so say when you book.

Someone already fitted a part and it did not fix it. Will you start again?

Yes, from the beginning, and that is the right thing to do. Previous work is information rather than a shortcut, and telling us exactly what has been replaced saves the time we would spend proving those parts. A fault that survives a part change usually means the first diagnosis stopped one step short.

Boiler Diagnosis in London

A boiler diagnosis is a visit to find out why a boiler is misbehaving when it has not obviously died. It locks out and resets. It heats the radiators but gives you nothing at the taps. It runs for ten minutes and drops out. The engineer reads whatever the boiler is reporting about itself, then works through the electrical, gas and water sides in a fixed order until the fault has been proved rather than guessed at. You are then told what is actually wrong, what it takes to put right, and whether this boiler is worth the repair.

What a diagnostic visit is for

Most boilers booked in for this are not dead. A dead boiler is easy. It is the awkward middle ground that costs people money: the unit that works until it does not, the one that has had three parts thrown at it by three different people, the one that has been resetting happily for a month and has now stopped accepting the reset.

The difference between a diagnosis and a repair is that a diagnosis has to end in proof. Anyone can look at a boiler that will not fire, say it needs an ignition electrode, fit one and be wrong. An engineer who has measured the gas working pressure under load, watched the fan prove, seen the spark and seen no flame signal returned knows which part has failed and which is fine. So we arrive with test gear rather than a box of parts and a theory: manometer, multimeter, flue gas analyser and the literature for that specific model, because test points and component values differ between appliances in the same range.

Check these yourself before you call anyone

There is no sense paying an engineer to press a button you could have pressed. A fair share of the calls we take are one of a handful of things that take two minutes to rule out, and you are better off knowing that.

If all of those are in order and the boiler still will not behave, the fault is inside the appliance or in the controls wiring, and that is the point at which a visit is worth booking.

The gas. If you have a hob, light it. No gas at the hob means the supply or a prepayment meter, not the boiler.
The electrical supply. Check the fused spur next to the boiler and the breaker in the consumer unit. Boilers are often on their own way and it trips like anything else.
The programmer and the room thermostat. Flat batteries in a wireless room stat are one of the commonest reasons a boiler goes quiet, and a programmer that lost its time in a power cut is doing exactly what it has been told.
The pressure gauge on the front. If the needle sits in the red at the bottom the boiler will refuse to fire, and that is a separate subject rather than a fault with the appliance.
The condensate pipe, where any of it runs outside. In a cold snap it freezes, the boiler locks out, and it clears once the pipe is thawed.

If you can smell gas, test nothing. Turn the gas off at the meter, open the windows, leave the electrics alone and call the national gas emergency service on 0800 111 999 before you call a plumber.

What the engineer checks, and in what order

The order is not arbitrary. Each stage clears or narrows a whole branch of causes, and jumping to the interesting part is how someone ends up replacing a circuit board to cure a blocked condensate trap.

It starts with what the boiler will say about itself: the current fault code and, on models that keep one, the fault history, which often shows today's lockout has been building for weeks. Then the basics get confirmed at the terminal strip, including permanent live, switched live, earth and whether the demand from the programmer and room stat is actually arriving. A good number of boiler faults turn out to be a control on the wall or a wire behind it.

Next the gas and combustion side. Standing and working pressure are measured at the inlet with the appliance running at full rate, because a pressure that looks fine at rest and collapses under load points at the supply rather than the boiler. The ignition sequence is then watched through: fan proving, spark, flame establishment, and whether the flame sensing electrode returns a signal. The analyser goes in last to confirm the burner is not only lighting but burning correctly with a clear heat exchanger and flue.

Then the water side, because a boiler that keeps cutting out often has nothing wrong with it on gas at all. Flow and return temperatures are read while it runs, and a rapid rise with almost no difference between them means water is not moving: a seized pump, a motorised valve not driving open, a blocked heat exchanger or a system full of debris. On a combi the diverter valve and the plate heat exchanger get looked at specifically, since those two account for most cases where the heating works but there is no hot water, and they are also behind a good share of the calls where the water simply is not getting hot enough rather than not getting hot at all. On a stored system the same complaint points somewhere else entirely, because no hot water but the heating works is far more often a motorised valve that is not driving open, a cylinder thermostat that has drifted or an immersion circuit that has tripped than it is a fault inside the appliance, and the engineer proves which of the two it is before touching the boiler. Last come the cheap parts that quietly shut down expensive ones: the condensate trap and its run, the overheat stat, the pressure sensor and the flow switch.

Fault code and fault history read from the appliance.
Supply, earth, and whether the demand from the controls reaches the boiler.
Standing and working gas pressure measured under full load.
Ignition sequence observed, then combustion read with a flue gas analyser.
Flow and return temperatures under load to prove circulation.
Pump, motorised valves, diverter valve and plate heat exchanger where relevant.
Condensate trap and pipe run, overheat stat, pressure sensor, flow switch.

What a fault code tells you, and why intermittent faults are harder

A code is useful and routinely over-read. A code meaning the boiler saw no flame after ignition does not mean the electrode has failed. It means the boiler asked for a flame and got no confirmation, which can be no gas, a failed gas valve, a wet or cracked electrode, a condensate blockage holding the flue gases back, a fan not proving, or a board not seeing a signal that is genuinely there. Bring us the code, because it tells us what to carry. Be wary of anyone quoting for a part over the phone on the strength of one, because the part they name is the commonest cause, not necessarily yours. We keep a separate page for each of the codes that comes up most often, so if your display is showing an Ideal boiler L2, a Vaillant F28 or F29, or a Baxi E133, there is a walkthrough of what that specific code means and what gets checked for it.

The boiler that fails only when it is cold outside, only after an hour of running, or only when hot water is drawn is the one that gets three visits and stays broken. Intermittent faults are about conditions, not components: something drifts out of tolerance under heat, load or vibration and recovers in time to look healthy. The way through is to reproduce the conditions rather than wait for luck, reading live values while the appliance runs hard. What you have noticed is a real clue here, so tell us whether it fails on heating or hot water, from cold or once warm, and whether it started after cold weather, a power cut or somebody else working on it. No hot water from a combi boiler is the commonest version of this pattern, and saying whether the radiators are still behaving normally tells us which half of the appliance to look at first.

When the honest answer is replacement

Diagnosis is not a route to selling a boiler, and a sound appliance with one failed part should be repaired. Boiler replacement is a separate decision with its own tests, and it should follow a proved fault rather than a hunch. But there is a point where repairing again is the expensive choice, and you are entitled to hear that plainly rather than after the third part. What shifts it is a heat exchanger that has gone, a failed board on a model where boards are no longer supplied, discontinued parts, a unit that has already had several major components replaced, or a boiler simply too old to carry more investment. Where that is the case we explain what has failed and why the repair does not stack up, and leave the decision with you.

The question of which boiler is best is a separate conversation from the one we have just had with a manometer, and it is worth having it on the strength of how an appliance is built and supported rather than on whoever is quoting fastest. The one case with no choice in it is an appliance found to be immediately dangerous. That is not a judgement call. It gets turned off and labelled, and you are told exactly what was found and what it would take to make it safe.

The rate is fixed before we attend and does not rise for an awkward property or an evening visit. The total depends on how long the job takes, and the pricing page carries the figures. Where a diagnosis turns into a repair on the same visit, you are told what is involved before anything is fitted.

Booking a diagnosis, and what to have ready

A diagnosis is booked work rather than an emergency plumber call out, so if there is water escaping or a smell of gas, that is a different call and it comes first. Otherwise book through the contact page and describe what the boiler is doing in your own words. You do not need to work out the cause, and guessing at one sometimes sends the visit the wrong way. What shortens the job is the handful of details that decide what goes on the van, because a fault found and fixed in one visit usually comes down to whether the right part was already on board. We run 20 engineers across London, and the work is booked by appliance and symptom so whoever attends is qualified for the work described and carrying the gear for it.

Make, model and, if you can find it, the GC number from the sticker behind or beside the front panel.
Any fault code on the display, written down exactly as it appears.
Whether it is the heating, the hot water, or both.
What the boiler does when you ask for heat: nothing, fires and drops out, or runs and never gets hot.
What changed just before it started, including cold weather, a power cut, or previous work on the system.
Where the boiler, the stopcock and the gas meter are, and whether the boiler can be reached.

Questions we get asked

How long does a boiler diagnosis take?: A straightforward fault is usually identified inside the first hour, because the sequence of checks rules out whole branches quickly. Intermittent faults take longer by their nature, since the appliance has to be run under the conditions that make it fail before anything can be measured. We will say on the visit if it is turning into the second kind.
Can you tell me what is wrong over the phone?: We can often tell you what it is likely to be, and that is worth knowing because it tells us what to carry. What we will not do is quote for a part on the strength of a symptom or a code, because the commonest cause of a given code is not necessarily yours, and the wrong part fitted is money spent twice.
Can it be repaired on the same visit?: Often, where the part is a common one and the engineer is carrying it. Where a component has to be ordered, or only the manufacturer supplies it, we tell you what it is and arrange the return visit. Nothing gets fitted without explaining what is involved first.
My boiler keeps locking out and I keep resetting it. Does that matter?: It is worth taking seriously. A lockout is the boiler protecting itself, and resetting does nothing about the condition that caused it. Manufacturer guidance on most appliances is to stop after a small number of attempts and have the fault investigated. If it will no longer accept a reset at all, stop and book it in.
The heating works but there is no hot water. Is that the boiler?: On a combi it usually is, with a diverter valve stuck in the heating position or a scaled plate heat exchanger the two usual causes. On a system with a hot water cylinder it is just as likely to be the motorised valve, the cylinder thermostat or the immersion circuit rather than the boiler at all. Which you have changes what the engineer needs to bring, so say when you book.
Someone already fitted a part and it did not fix it. Will you start again?: Yes, from the beginning, and that is the right thing to do. Previous work is information rather than a shortcut, and telling us exactly what has been replaced saves the time we would spend proving those parts. A fault that survives a part change usually means the first diagnosis stopped one step short.