What F27 means, and which manual this guide uses
Vaillant's homeowner fault-code page explains F.27 as a flame signal detected when the burner is off and directs the problem to an expert. That description concerns the boiler's monitoring and safe operation. It is different from a simple demand for more heat or a message that the room thermostat is satisfied.
The ecoTEC exclusive installation manual 0020193968_05, covering the 835 and 843 combi models, labels F.27 as a safety switch-off for flame simulation. Its fault table lists moisture on electronics, a defective flame-monitoring function and a leaking gas solenoid valve among possible causes. Those alternatives require investigation; they do not establish which component has failed in your boiler.
Check the exact model before applying a code explanation. Photograph the model information available without removing the casing, or use installation and service paperwork. An online page headed only “Vaillant boiler” may combine several generations, while a component or procedure described for one model may not apply to another.
F27 and F.27 are common written forms of the displayed fault. Record the full message and any additional code rather than shortening everything to “an ignition problem”. That wording can wrongly send the enquiry towards a different fault, especially when several related numbers appear in a search result.
First decide whether there is an emergency
A displayed fault and a suspected gas escape are different observations. If you smell gas, suspect fumes or have a carbon-monoxide alarm, leave the affected building and use the emergency route. Do not spend time comparing online fault-code lists before responding to those signs.
The HSE's domestic gas guidance gives the National Gas Emergency Service number for Great Britain as 0800 111 999. Avoid flames and operating electrical switches where a gas escape is suspected; call from a safe place outside and follow the emergency service's instructions. Seek urgent medical help if anyone is unwell.
Without those emergency signs, arrange qualified boiler investigation and leave internal components untouched. Do not open the case to see whether the burner is really alight. The displayed wording refers to a signal detected by the controls; a homeowner visual check cannot validate the sensing circuit or gas-valve operation.
If you cannot tell whether an unusual smell or noise is significant, describe it to the appropriate emergency or technical contact. Do not use the absence of a visible flame, a normal-looking pressure gauge or a quiet boiler as a safety certificate.
Why F27 does not automatically mean a new circuit board
A fault table gives the engineer a set of possible explanations for a reported condition. Replacing the most frequently mentioned part in online discussions skips the question of which explanation fits this appliance. A useful diagnosis should connect the observed fault to findings from the manufacturer's procedure.
If a board is proposed, ask what evidence supports that conclusion and whether another issue could damage a replacement. This is a request for a plain-language explanation, not a demand that the engineer perform unsafe demonstrations or expose live components to you.
For example, an engineer might report that an internal electrical assembly is affected by moisture. The next questions are what was observed, what source was identified and what needs addressing before normal operation resumes. It would be incomplete to describe that only as “the code needs clearing”.
Equally, do not reject a justified board replacement simply because another possible cause appears in the manual. The aim is a supported repair, not a preferred cheap component. Ask for the diagnosed fault, proposed scope and relevant follow-up checks in the estimate or job record.
The useful evidence you can collect without opening the boiler
Take a clear photograph of the fault display before anything is changed. Note the date, time and whether the appliance was providing heating, hot water or apparently waiting between demands. Record what you actually saw rather than inferring an internal operating stage.
| Record | Useful detail | Avoid |
|---|
| Appliance identity | Model from accessible label or paperwork | Removing casing to find a serial number |
| Fault sequence | Codes in the order observed, with photographs | Combining several messages into one remembered number |
| Timing | First start, hot-water use, heating cycle or overnight occurrence | Running repeated tests to force the shutdown |
| Visible water | Location and safe photograph of an external drip or stain | Touching wet electrics or tracing water inside the case |
| Previous work | Service date, repair invoice and changes since then | Assuming a recent visit caused the fault without evidence |
A short record is sufficient. A booking message containing the model, exact code, recurrence and any emergency signs is more useful than a long copied list of possible parts. Keep any normal-user pressure reading alongside the fault, but do not turn it into a diagnosis of flame monitoring.
Do not repeatedly reset F27 to keep the boiler running. The manufacturer identifies it as a problem for expert attention. If technical support gives instructions for your exact appliance and circumstances, follow those instructions and report what happens; do not adapt a reset sequence from a different code or model.
A temporary return to operation does not identify or repair the original cause. Keep the first fault photograph and record any reset already attempted. If the display becomes normal before the appointment, tell the engineer about both states rather than cancelling solely because the screen is clear.
Avoid switching the appliance off and on repeatedly to recreate a fault. Intermittent events can be difficult to reproduce, and deliberate cycling can remove the useful record of what happened during ordinary use. Ask the engineer how they want the appliance left while waiting.
Do not enter installer menus, change diagnostic parameters, bypass a control or follow a video showing how to bridge connections. None of those is a homeowner reset. The professional manual is a source for understanding the fault definition, not permission to undertake its internal test procedures.
F27 is not the same question as low water pressure
The boiler's water circuit and its flame-monitoring function are separate diagnostic subjects. A low pressure reading may need attention, but adding water does not establish why an F27 signal was reported. Record both symptoms and have them assessed on their own evidence.
If another fault such as F22 appears, write down when it occurred and whether the messages alternate. Do not overwrite the record of F27 with whichever code looks easier to resolve. Our existing Vaillant F22 guide addresses that separate pressure-related enquiry.
Likewise, circulation complaints belong to their own investigation. The F23 guide and F24 guide explain those different messages. A house with several symptoms does not necessarily have one underlying fault, and one successful intervention may not resolve all of them.
Do not bleed radiators, operate a filling loop or add a treatment chemical solely because an internet list calls every boiler fault a pressure problem. If water pressure is repeatedly falling, preserve the reading history for the engineer and ask whether a separate water-loss investigation is needed.
When moisture is mentioned in the diagnosis
The cited model manual includes moisture among its possibilities, but F27 does not prove that there is a hidden pipe leak in your floor. A code cannot identify the source, location or path of water. Ask for the actual findings before booking a separate leak-detection visit.
If you can already see external water, note where it first appears and whether there is damage around the appliance or nearby pipework. Keep clear of wet electrical equipment. Do not remove covers, dry electronics with a hairdryer or restart the boiler because the visible surface seems dry.
Where a qualified boiler engineer identifies a water problem, ask them to distinguish internal appliance work from connected pipework outside the boiler. That distinction helps the next contractor quote for the relevant task and prevents two appointments from each assuming the other will resolve it.
A repair report should state what was found, what was repaired and any remaining uncertainty. “No leak found in the area inspected” is more precise than “there cannot be a leak anywhere”. Keep the scope of each inspection visible when the source of moisture remains unresolved.
Choose and brief the appropriate engineer
For gas-appliance work, use an appropriately qualified Gas Safe registered engineer. HSE explains the registration requirement and the importance of competent work. Confirm that the person attending is qualified for the relevant appliance work, rather than relying only on a business name in an advert.
Ask whether they diagnose your model and this type of intermittent fault. Give them the display photograph and previous repair details before the visit where possible. This may help with preparation, but no responsible booking should promise a specific replacement part purely from your description.
Check what the appointment charge covers: investigation, any authorised repair, parts, further attendance and the written findings. Ask how approval will be obtained if the investigation identifies work outside the original scope. A diagnostic appointment and a guaranteed total repair price are not automatically the same product.
If a manufacturer guarantee or existing repair cover may apply, contact that provider before commissioning unrelated chargeable work. Have the model, installation record and service history available. Eligibility should be confirmed by the provider; the presence of an F27 code alone does not prove that a repair is covered.
How to assess a proposed repair without diagnosing it yourself
Ask the engineer to explain the difference between the symptom, finding and repair. The symptom is what the boiler reported. The finding is what their assessment established. The repair is the work proposed to correct it. Keeping those distinct makes an estimate easier to understand.
A useful question is: “What points to this component, and is there any related fault that must be addressed at the same time?” Another is: “What will you check before handing the appliance back?” These questions help you compare the completeness of proposed work without asking for technical procedures you cannot safely evaluate.
If more than one option remains, ask what further investigation would distinguish them and what that stage costs. Replacing several parts can be appropriate in some circumstances, but the reason should be stated. An unexplained list of expensive components is not made conclusive by the fault number.
Keep the estimate and final invoice together. Where the scope changes during the visit, note what additional work you approved. This is especially useful if another engineer later reviews a recurring problem and needs to know which assemblies were investigated or replaced.
If F27 returns after a repair
Contact the repairer with the previous job reference and the new fault photograph. Describe whether the timing and symptoms are the same. A recurrence during a different operating condition may be relevant, but avoid asserting a new cause before it has been assessed.
Ask about their return-visit process and any repair guarantee. Do not assume a second appointment is automatically free or automatically unrelated to the first repair. The agreement and findings should establish that, and a written chronology helps both parties discuss it.
If you seek a second opinion, provide the earlier report rather than starting with an incomplete history. Tell the new engineer whether a part was replaced, whether moisture was found and whether the appliance was left safe to use. Do not reconnect an appliance that has been isolated or labelled unsafe.
Book water-leak work only when the evidence supports it
Leak Fix London investigates and repairs water leaks; this page does not offer an internal gas-valve or PCB repair service. F27 alone should go to a qualified boiler engineer. That first assessment can establish whether there is also water loss in connected heating pipework.
If such a leak is confirmed or remains a supported concern after the appliance assessment, send us the findings, pressure history and accessible photographs. Our heating-pipework leak repair service can be scoped around the remaining water problem, including necessary access and making good.
The rate for our water-leak work is £150 per hour; if we do not find the leak, you do not pay for time spent looking. See the pricing and scope. This is not a quoted cost to repair F27. The right appointment depends on what needs investigating, and a boiler fault should not be converted into an unnecessary hidden-leak search.
