Gas Safe registered
Keston specialists
C35, C40, Qudos and the older twin flue models. Twenty years on Keston boilers, rates agreed beforehand, and parts guaranteed for a year.
Answered 24 hours a day by a technical customer service member.
Low pressure has four different codes on Keston depending on the model, so tell us the range as well as the number.
No heating or hot water right now? Call 0203 868 6134. We will tell you the earliest realistic slot rather than the one you want to hear.
Two details to start. Add the fault code and your postcode and we can usually tell you the likely part before we attend.
Three steps, no sales pitch, and you will know where you stand before an engineer is booked.
Keston is the boiler most often written off by somebody who does not work on them. It is a smaller name than Worcester or Vaillant, generic fault code sites barely cover it, and the twin flue arrangement is unfamiliar to engineers who have not seen one. None of that means your boiler is beyond help. It means you want somebody who has opened one before, and we will tell you honestly on the phone if the answer really is replacement.
Keston faults cluster around air, flue and combustion far more than around the water side, which is unusual and it follows directly from how these boilers are built.
Keston number the same fault differently across their ranges, and low pressure is the worst example: it appears as E37, E44, E67 or F1 depending on which boiler you own. Generic code sites barely cover this brand, so a search will often find you nothing or, worse, something confident and wrong.
What the boiler is saying: it ran its ignition sequence and could not establish a flame, so it locked out. The most common Keston code across the range.
Check the gas first. Try a hob. If nothing gas powered in the property will light, top up a prepayment meter or call your supplier, because no gas anywhere is not a boiler fault.
In winter, the condensate. Thaw the outside pipe with warm water, never boiling, then reset once.
What it usually is otherwise: the ignition electrodes, then the gas valve, then a flue or air supply problem.
The Keston specific part: because these boilers can be fitted with very long flue runs, an air supply problem shows up as a failure to light more readily than it would on a boiler flued straight through a wall. If E01 keeps returning after an electrode has been changed, the flue is the next place to look rather than the third.
One reset only. Repeated E01 means the gas supply, ignition components, burner or flue need looking at properly.
Does the hob light? And where does your flue run?
This is the most common Keston fault and it has the most confusing labelling of any code on any make we work on.
The same problem appears as E37, E44 or E67 depending on the model, and as F1 on an S30 system boiler. All four mean the system does not have enough water in it for the boiler to run safely.
What you can safely do: find the filling loop and top up slowly with the system cold until the gauge sits between 1 and 1.5 bar, then reset once. Fill slowly, because overshooting creates a high pressure fault instead.
Before you top up, decide which problem you have. A system that simply has low pressure needs filling. A system that keeps losing pressure has a leak, and topping it up repeatedly means water going somewhere it should not, often into floorboards or plaster, while the sludge problem gets worse each time.
If the gauge reads healthy and the code is still showing, the pressure sensor has failed rather than the pressure. Small part, short job.
Has it needed topping up before? That answer decides everything here.
The water went past the temperature the safety system allows and the boiler stopped. E18 covers the same territory on some models.
What to check before resetting: system pressure, and whether the radiator valves are open. A thermostatic valve seized shut over the summer is a common and free find.
Then let it cool fully before a single reset. An overheat code reset on a hot boiler tells you nothing.
What we normally find: circulation rather than the burner. A jammed pump, a closed valve, sludge or trapped air, all of which leave the boiler making heat faster than the system carries it away.
If it comes back after all that: stop resetting and get it looked at. A repeated overheat is a circulation problem that will eventually take the pump or the heat exchanger with it.
What helps most: not a part. Getting the system properly clean deals with the cause where a component swap usually does not.
Leave it off to cool and tell us whether any radiator was cold beforehand.
E05 on a C36, E28 on other models, F3 on an S30. Three numbers, one part.
Here is the part that catches people out: a fan that has stopped is a fault, and so is a fan that is merely running slowly. The fan creates the draught that carries combustion gases out through the flue. Too slow a draught and the air pressure switch decides the boiler is unsafe and locks it out, even though the fan is clearly turning.
What you will notice: with a failed fan the boiler does not sound like it is even trying to light. With a slow fan it often sounds normal, which is why people insist the fan is fine.
Worth ruling out first: the flue and the air path. On a Keston that is a longer route than usual and it collects more.
Why fan codes deserve respect: the fan and flue are what carry combustion products safely out of the building. This is not a code to reset past.
Does it sound like it is trying to light, or silent? Different answers.
E07 means the temperature in the flue has gone above what the boiler will allow, so it stopped.
Why Keston have this code when many makes do not: these boilers are designed around a flue system that can run a very long way, and the system has a working temperature limit. Monitoring flue temperature protects that flue, so the boiler stops rather than pushing heat into it.
What it usually means: heat that should be going into your water is going up the flue instead. That points at combustion being set up wrongly, a fouled heat exchanger, or a sensor reporting incorrectly.
Why it is worth taking seriously: like all flue codes it concerns how combustion products leave the building, and unlike most it is also telling you the boiler is wasting fuel before it ever locks out.
What it needs: a proper combustion analysis rather than a parts swap. The numbers from a flue gas analyser tell an engineer whether the appliance is burning correctly, and there is no substitute for measuring it.
When was it last properly analysed rather than just looked at?
Three codes sit together here and all of them involve safety critical components.
E02 is false flame detection. The boiler is reading a flame signal when there should not be one, which is the opposite of E01 and a different order of problem.
E08 is a flame circuit fault, so the boiler cannot trust what the sensing side is telling it.
E09 is a gas valve error.
E26 sits alongside them, covering flame dropout, where the boiler lights and then loses the flame during operation.
What to do with any of them: not repeated resets. These involve the components that decide whether gas is released and whether it is burning, and a boiler declining to run on that basis is doing its job. Call somebody.
Leave it switched off at the spur and call us rather than resetting it.
Which Keston you own decides what you are looking at, and one of the three is genuinely misleading.
C35, C40 and C40P from around 2018 onwards have a large digital display showing E or L codes. Straightforward to read.
Qudos 28 and QSpa have a small LED screen showing E codes.
The older twin flue models alternate between the temperature and the fault number, or on some just flash lights. That alternation is the problem: people watch the display, see a number, look it up, and have actually read the water temperature rather than the fault.
What to do if yours alternates: watch it through two full cycles and write down both numbers. One of them is a temperature and one is the fault, and telling us both lets us work out which is which.
If yours only flashes lights: note what the panel shows while the fault is up. That reading is the entire diagnosis on those models.
Does the number change every few seconds? Then note both.
| Code | What the boiler is saying | What it usually turns out to be |
|---|---|---|
| E01 | Ignition failure | Gas supply, then electrodes, then the flue |
| E02 | False flame detected | Sensing electrode or moisture. Do not reset repeatedly |
| E03 / E18 | Overheat | Circulation, not the burner |
| E05 / E28 / F3 | Fan fault | Fan, wiring, or a restricted air path |
| E07 | Flue overheat | Combustion set up wrongly, or a fouled exchanger |
| E08 | Flame circuit fault | Sensing side. Engineer rather than reset |
| E09 | Gas valve error | Safety critical. Engineer rather than reset |
| E26 | Flame dropout during operation | Gas pressure under load, or the flue |
| E37 / E44 / E67 / F1 | Low system pressure | Same fault, four numbers. Check the model |
The reason you have a Keston is almost certainly the flue. Their twin flue system can be extended up to 27 metres, horizontally or vertically, which is far beyond what most boilers allow and is why they go into flats, converted properties and anywhere the boiler cannot sit against an outside wall. That same length is why a Keston service is not quite like anybody else's: there is a great deal more flue to check, and it collects dust and cobwebs in places a short flue never would.
Almost everything unusual about owning a Keston comes back to the same thing, which is where the flue goes and who can reach it.
In a flat a Keston flue frequently runs through a ceiling void, a boxed section, or a communal riser shared with other properties. From your hallway there is nothing to look at.
Why it matters when something goes wrong: a fault in a section of flue you cannot reach is a different conversation from one at a terminal on your outside wall. Access may need a managing agent, and in a communal riser the flue may not be yours alone to work on.
What to establish early: whether your flue exits your own wall or disappears into the building. If it disappears, find out who holds access to the riser before an engineer attends rather than after.
The regulation nobody mentions until it bites: a flue in a void needs to be inspectable. If yours was boxed in without inspection hatches, that is a finding a Gas Safe engineer has to record, and it is worth knowing before somebody raises it on a Friday afternoon.
Does your flue exit your wall, or vanish into the building? Call 0203 868 6134.
On the older twin flue Keston models the display alternates between the water temperature and the fault number, cycling every few seconds.
What goes wrong: somebody glances at it, sees 60, searches for Keston fault code 60 and finds nothing, or finds something that has no bearing on their boiler at all. They have read the temperature.
What to do instead: watch the display through two complete cycles and write down both numbers in the order they appear. Tell us both. One is a temperature and one is a fault, and which is which becomes obvious to somebody who knows the range.
If yours has no numbers at all, only indicator lights, note what the panel shows while the fault is up. On those models that reading is the diagnosis, and resetting destroys it.
Why this matters more here than elsewhere: generic fault code sites barely cover Keston, so a wrong number sends people down a much longer wrong path than it would on a mainstream make.
Keston is part of Ideal Heating, which matters more than a change of ownership usually does, because Keston's reputation for aftersales was formed before that happened.
What it means in practice: the support and parts network behind your boiler is a considerably larger one than the Keston badge on the front suggests, and that is the relevant fact when somebody tells you a part will take weeks.
What to ask: which part specifically, and which supplier was checked. A genuine availability problem names both. A vague answer about Keston being an unusual make is not a diagnosis.
What has not changed: the boiler on your wall. The twin flue arrangement, the code sets and the parts inside are what they always were, so an engineer still needs to have worked on one.
We repair Ideal boilers alongside Keston for exactly this reason.
A Keston service is genuinely not the same job as a service on a wall hung boiler flued straight through a wall, and the difference is length.
What accumulates: a long twin flue draws air across a much greater distance, and dust, debris and cobwebs collect in the air path in places a short flue simply does not have. Over a few years that restricts the air the boiler can draw.
How it shows up: not as a dramatic failure but as intermittent ignition faults, fan codes that come and go, and a boiler that struggles more in some weather than others.
Why the annual service is not optional on this make: on most boilers a skipped service costs you efficiency. On a Keston it also lets the air path silt up, and that is the route to E01 and the fan codes.
What to ask for: that the air path and the full flue run are checked, not only the appliance. A proper service on this brand takes longer than on a conventional boiler and it should.
When was the flue itself last checked, rather than the boiler? Call 0203 868 6134.
The flue is why it is here, and where it fails
A Keston is in your flat because its twin flue runs where nothing else can, up to 27 metres. That same length is where a disproportionate share of its faults live, and it collects dust in places a short flue does not have.
The label is behind the front panel. On this make the model decides the code numbering, the display type and often the flue arrangement too.
Keston name the range and the output together, so a C35 is a C range boiler rated 35 kilowatts.
If yours is an S or a Heat, there is stored hot water somewhere in the property and the cylinder is as likely to be the fault as the boiler is.
C35, C40 and C40P from roughly 2018 onwards have a large digital display showing E and L codes. These are the ones most often on London walls now.
Qudos 28 and QSpa use a small LED screen with E codes.
The older twin flue Celsius and Combi models alternate the temperature with the fault number, or on some simply flash lights.
S30 and the system range use F numbers rather than E numbers, which is why an S30 shows F1 for the same low pressure fault a C40 calls E44.
The practical test: if your display shows F followed by a single digit, you are on a system model. If it shows E followed by two digits, you are on a combi.
With four different numbers for low pressure and three display types across the range, the name on the front is not enough to work from on this brand.
The serial number and GC number on the label identify exactly which boiler and revision is on your wall, which decides both the fault table and the parts list.
Read us the whole label when you ring, and if you can see where the flue runs, mention that too. On a Keston those two pieces of information together do more than anything else to get the right engineer with the right part. We work on every make of boiler and this is the one where the model matters most.
London is a hard water city almost everywhere, and that is the single biggest difference between repairing a Keston here and repairing one in Manchester or Glasgow.
The supply comes off the Thames and the Lea, both of which run through chalk in the Chilterns and the North Downs. London water measures roughly 277 milligrams of calcium carbonate per litre, and across the city's postcode districts the figure runs from about 250 to over 350. Inside a combi that has one consequence: the plate heat exchanger furs up, and it does it faster than the service intervals in the manual assume.
It is the same story in Notting Hill and Holland Park as it is in Sydenham and Forest Hill. The water is hard, the plate heat exchanger scales, and the only question is whether anybody checks it before condemning the boiler.
Hardness is not uniform, and it does not follow borough boundaries. It varies by supply zone and can change over a short distance, so your supplier publishes a figure for your own postcode and it is worth reading before anybody guesses.
Scale drops out of solution faster the hotter the water gets. A combi set to deliver taps at 65 degrees furs up its plate heat exchanger noticeably faster than one set to 50, which is still hot enough to scald. Turning that dial down costs nothing and is the most effective thing a householder in a hard water area can do. This applies to the tap setting, not to a cylinder, which must reach 60 degrees for safety.
Larger London houses keep stored hot water, and a combi cannot replace it however good the badge is.
Combi-only is the norm among London firms, and in a city full of large period houses that leaves a lot uncovered. A Keston system or heat only boiler runs a separate cylinder with its own thermostat, immersion element and, if unvented, its own safety devices. So the commonest wrong call-out on this arrangement is the hot water going, the boiler getting blamed, and the boiler turning out to be in perfect health.
The cylinder sensor sits in a pocket on the tank and tells the boiler what the stored water is doing. When it fails, or simply works its way out of that pocket, the boiler stops heating the cylinder while the radiators carry on perfectly, which is exactly why the appliance gets blamed.
The motorised valve decides whether the boiler is heating radiators, the cylinder, or both. A failed motor or microswitch gives you one without the other, and on most installations the head changes without draining the system down.
The immersion element scales and either trips its thermostat or stops heating. Worth testing before anybody condemns a tank, because an element costs a fraction of one. Large houses on stored hot water, of the kind we see across Clapham and Battersea, often have a scale reducer fitted at the same visit.
An unvented cylinder runs at mains pressure and is protected by an expansion vessel, a pressure reducing valve, an expansion relief valve and a temperature and pressure relief valve, discharging through a tundish. If water is running at that tundish, one of those devices has failed. It is not something to leave and certainly not something to cap.
Unvented work is notifiable and needs a G3 qualification alongside Gas Safe registration. Our engineers carry both, which keeps the boiler side and the cylinder side on one visit. That is the reason unvented cylinder repair sits with the boiler work here.
People swap a cylinder for a combi to gain a cupboard, then ask us to put it back. A combi makes hot water as it flows rather than holding a tankful, so two showers at once is beyond it whatever the badge claims. In a three bathroom house, keeping the cylinder is usually the honest recommendation, replacing it if it is tired rather than taking it out.
This is the brand most often written off by people who have not worked on one. Age alone does not decide it, and neither does the badge. Which part has failed decides it.
| What has failed | Under 8 years | 8 to 14 years | 15 years and older |
|---|---|---|---|
| Pressure sensor, temperature sensors | Repair | Repair | Repair if parts exist |
| Ignition and sensing electrodes | Repair | Repair | Repair if parts exist |
| Expansion vessel, pump | Repair | Repair | Repair |
| Fan, air pressure switch | Repair | Repair | Depends on the rest of the boiler |
| Flue components | Repair | Repair | Depends on access as much as on cost |
| Gas valve | Repair | Depends on the rest of the boiler | Replace |
| Control board | Check the guarantee first | Borderline, we will be straight with you | Replace |
| Main heat exchanger | Guarantee claim if registered | Usually replace | Replace |
Every part is current and at this age almost every failure is a single component. An electrode, sensor, fan or pressure switch on a six year old C35 is a repair, and being an unfamiliar badge changes nothing about that.
Check what cover you have before booking anybody, us included. Keston registered guarantees run for a set term from installation, and since the boiler is supported through Ideal Heating the claim route is a larger operation than the badge suggests.
A replacement quote at this age deserves one question: which part, and what does that part cost separately?
Still a clear repair: electrodes, sensors, fan, air pressure switch, pump, expansion vessel. Ordinary wear items that say nothing about the rest of the boiler.
What is different on this make: replacing a Keston is not always a like for like swap, because the flue that made the original installation possible may not suit another manufacturer's boiler. In a flat where the flue runs a long way, or through a void, a replacement can mean a new flue system as well as a new appliance.
Why that matters to the decision: it pushes the balance toward repair for longer than it would on a conventional installation, and any quote for replacing the boiler should say clearly whether the flue is included or extra.
Availability starts to take over from cost here as it does on any make, and the older twin flue models are where it begins.
The flue point applies with more force at this age. A boiler in a flat, fed by a long flue run through a void, can be perfectly repairable while a replacement would be a substantial job involving parts of the building you do not control.
That is a genuine reason to establish availability before anybody writes it off. We will tell you whether the part still exists before you spend anything on a diagnosis, and that question is free. What actually moves the number is set out in our guide to repair and replacement costs.
Local boiler repair specialists
Clear advice. Careful repairs. Family-friendly service.
Call 0203 868 6134London, north and south of the river. These are the districts we work in most.
These are where we work most, not a boundary. An engineer attends anywhere in north London and south London. Tell us where you are and we will say plainly when we can get to you.
If you smell gas: do not touch any switches. Open the windows, leave the property, then call the National Gas Emergency Service on 0800 111 999. That line is free and open 24 hours. Call them before you call us.
If a carbon monoxide alarm is sounding: get everyone out immediately, then call 999 if anyone feels unwell. Do not go back in until the property has been ventilated and checked.
If the boiler locks out repeatedly: try one reset. If it locks out again straight away, stop. Repeated resets hide a fault that is still there, and on a gas appliance that is not a risk worth taking.
Under the Gas Safety (Installation and Use) Regulations 1998, only a Gas Safe registered engineer may work on gas appliances. Never open the boiler casing yourself.
Because Keston number the same fault differently across their ranges. Low system pressure appears as E37, E44 or E67 depending on the model, and as F1 on an S30 system boiler. All four mean the same thing. Generic fault code sites barely cover this brand, so a search often finds nothing or something confidently wrong. Tell us the number and the model together, because on a Keston either one alone is only half an answer.
E01 is ignition failure, meaning the boiler could not establish a flame and locked out. Check the gas first by trying a hob, because no gas anywhere in the property is not a boiler fault, and in winter thaw the outside condensate pipe with warm water. Otherwise it is usually the ignition electrodes or the gas valve. On a Keston it is also worth looking at the flue and air path early, because these boilers run long flues and a restricted air supply shows up as a failure to light.
E07 is flue overheat, which means the temperature in the flue has gone above what the boiler allows and it has stopped. Keston boilers are built around a flue system that can run a very long way and has a working temperature limit, so the boiler protects it. What it usually means is that heat which should be going into your water is going up the flue instead, pointing at combustion set up wrongly, a fouled heat exchanger or a sensor reading incorrectly. It needs a proper combustion analysis rather than a parts swap.
On the older twin flue models the display alternates between the water temperature and the fault number every few seconds. People read one, look it up and find nothing, because they have read the temperature. Watch it through two complete cycles, write down both numbers in the order they appear and tell us both. Telling which is which is straightforward for somebody who knows the range. If yours only flashes lights, note what the panel shows before resetting, because that reading is the diagnosis.
The flue. Keston's twin flue system can be extended up to 27 metres, horizontally or vertically, which is far beyond what most boilers allow. That makes them possible in flats, converted properties and anywhere the boiler cannot sit against an outside wall. It also means a Keston service is not the same job as a service on a conventional boiler, because there is a great deal more flue to check and it collects dust and cobwebs in places a short flue never would.
Less than the badge suggests. Keston is part of Ideal Heating, so the support and parts network behind your boiler is a considerably larger operation than the name on the front implies, and much of Keston's older reputation for aftersales was formed before that. If you are told a part will take weeks, ask which part specifically and which supplier was checked, because a genuine availability problem names both while a vague answer about Keston being unusual is not a diagnosis.
Yes, and it is worth establishing before an engineer attends rather than after. In a flat a Keston flue often runs through a ceiling void, a boxed section or a communal riser, which affects who can reach it and sometimes whose responsibility it is. A flue in a void also needs to be inspectable, so if yours was boxed in without inspection hatches that is something a Gas Safe engineer has to record. Find out who holds access to the riser in advance if you can.
No honest figure exists until somebody knows which part has gone. A price quoted before anybody has asked you a question is a number, not a quote. The figure comes from three things, the part, whether it is in stock on the van, and the labour time. On a Keston the flue arrangement affects the job as much as the part does, so it is worth telling us where it runs. The rate is agreed beforehand and parts we fit are guaranteed for a year, so nothing is carried out on the assumption that you will approve it afterwards. There is no charge for telling you a repair is not the right call. What actually moves the number is set out in our guide to repair and replacement costs.
Keston is part of Ideal Heating, which is why the support behind these boilers is larger than the name on the front suggests.
No gas supply? Plenty of London flats run without one, and we also cover electric boiler repair.
On a Keston the flue matters as much as the part does. Have the code, the model from the label and, if you know it, where the flue runs and whether it exits your own wall.
Gas Safe registered. Unvented cylinder qualified. Parts guaranteed for a year. Covering London, north and south of the river, 24 hours a day.
