Heatline F27 Fault Code: Causes, Fixes & Repair Costs
What does the Heatline F27 fault code mean?
The F27 fault code appears when the boiler's flame supervision system detects an anomaly with the burner flame — specifically, the boiler has opened the gas valve and attempted to light, but the flame detection circuit either reports no stable flame or registers a flame signal at the wrong time. The result is an immediate lockout: both central heating and hot water stop working until the fault is cleared. Because Heatline is part of the Vaillant Group, the F27 code carries the same meaning across Heatline and Vaillant boilers — if you have a Capriz, Advance, or Kalder model, the diagnostic approach is identical to the Vaillant ecoTEC range.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Moisture on the PCB or electrical connections Common
On boilers over seven or eight years old, condensate from the sump or plastic condensate trap can drip onto the circuit board below. Even a small amount of moisture can bridge connections and generate a false flame signal, or prevent a real flame signal from being read correctly. Inspect underneath the boiler for evidence of drips from the white plastic condensate trap — this is the single most common trigger for F27 on Heatline boilers.
- Worn or damaged ignition electrodes and spark leads Common
The electrodes and their connecting leads degrade over time through repeated heat cycling. If the electrode tip is cracked, the gap is incorrect, or the lead insulation has deteriorated, the spark may be too weak or erratic to establish a reliable flame, causing the boiler to lock out on F27.
- Faulty or drifting gas valve Sometimes
The gas valve controls exactly how much gas enters the burner. A valve that is sticking, drifting out of calibration, or failing to open or close fully can disrupt the gas-to-air ratio enough to prevent stable ignition. In some cases a valve that does not fully close allows a trace of gas to seep through when the boiler is off, which can also trigger a spurious flame detection signal.
- Faulty flame sensor (ionisation probe) Sometimes
The flame sensor monitors the burner flame by detecting the small electrical current that a gas flame conducts. If the probe is contaminated with carbon deposits, cracked, or has a damaged connection lead, it will either fail to confirm a genuine flame or, less commonly, report a flame where none exists. This is a less frequent cause than electrode or moisture issues but worth checking at the same time.
- PCB failure Sometimes
If moisture damage is extensive, or if the board has simply aged, the PCB itself may have failed rather than just been temporarily compromised. A failed PCB can send incorrect signals to the gas valve or misread input from the flame sensor, producing F27 even when all other components are functioning correctly.
- Flue blockage or combustion fan fault Rare
Adequate airflow through the flue is required for stable combustion. A partially blocked flue terminal, a bird nest, or a fan that is running slowly or intermittently can prevent the burner from establishing a clean flame, leading to repeated ignition failures and an F27 lockout.
How to fix it
- Reset the boiler once or twice DIY safe
Press and hold the reset button (usually marked with a flame or reset symbol on the display panel) for approximately three seconds. Wait for the boiler to complete a full start-up cycle. If it fires up and runs normally, monitor it over the next few hours. A one-off ignition glitch can occasionally clear this way. If the F27 returns promptly, do not keep resetting — repeated resets will not fix an underlying fault and can mask a developing problem.
- Confirm your gas supply is working DIY safe
Check that other gas appliances in the property — such as the hob or gas fire — are lighting normally. If nothing gas-powered is working, contact your gas supplier (National Gas Emergency: 0800 111 999) rather than attempting any boiler work. If other appliances are working fine, the supply to the property is not the issue.
- Inspect underneath the boiler for condensate drips DIY safe
Look at the white plastic condensate trap and the area directly below the boiler casing for any signs of water staining, drips, or wet marks. You do not need to open the boiler casing to do this — just look at what is visible from outside. If you can see active dripping onto or near the boiler, note its location and mention it to the engineer, as this is a strong indicator that moisture is the root cause of the F27.
- Do not attempt to open the boiler casing or touch internal components Gas Safe engineer
All work inside the boiler — including inspecting or cleaning the flame sensor, testing the ignition electrodes, checking the gas valve, or examining the PCB — must be carried out by a Gas Safe registered engineer. Opening the casing and working on gas or high-voltage components without Gas Safe registration is illegal and dangerous.
- Have a Gas Safe engineer inspect and test the ignition and flame detection system Gas Safe engineer
An engineer will remove the casing and use a multimeter to test the ignition leads, electrodes, and flame sensor for correct resistance and continuity. They will check the electrode gap and condition, clean or replace the ionisation probe, and inspect all wiring connections to the PCB for corrosion or moisture damage.
- Gas Safe engineer to assess the gas valve and combustion Gas Safe engineer
If the ignition and sensor components check out, the engineer will test the gas valve for correct operation and gas rate, and may carry out a combustion analysis to confirm the burner is performing within safe parameters. A drifting or faulty gas valve will need to be replaced — this is a gas-carrying component and is not DIY-repairable under any circumstances.
- Gas Safe engineer to assess the PCB if other components are serviceable Gas Safe engineer
If moisture damage to the PCB is confirmed, the engineer may be able to dry and clean the board if the fault is caught early. If the board has failed, a replacement PCB will be required. On a boiler over eight to ten years old, the engineer should advise you to weigh the PCB replacement cost against the cost of a new boiler installation, as these can be comparable.
- Call a Gas Safe registered engineer if the fault has not been resolved Gas Safe engineer
If the reset and basic checks above have not resolved the F27, contact a Gas Safe registered engineer. You can verify any engineer's registration at gassaferegister.co.uk. Do not continue to run or reset a boiler that is repeatedly locking out on F27, as this indicates an underlying combustion or electrical fault that needs professional diagnosis.
Parts you may need
- Ignition electrode assembly · from £35
- Ignition lead / HT lead · from £25
- Ionisation / flame sensor probe · from £30
- Condensate trap / sump assembly · from £20
- Gas valve · from £160
- PCB (printed circuit board) · from £280
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £150–£400, depending on the underlying cause.
Frequently asked questions
Can I fix the Heatline F27 fault myself?
The safe DIY steps are limited to resetting the boiler once or twice, confirming your gas supply is on, and checking for visible condensate drips beneath the boiler casing. Everything beyond that — inspecting the electrodes, cleaning the flame sensor, testing or replacing the gas valve, or any work involving the PCB — legally requires a Gas Safe registered engineer. Removing the boiler casing and working on gas or high-voltage components without Gas Safe registration is illegal in the UK.
How much does it cost to repair an F27 fault on a Heatline boiler?
Most F27 repairs fall between £150 and £400 including parts and labour once a call-out has been made. Simple jobs such as replacing an ignition electrode or cleaning the flame sensor sit at the lower end of that range. A gas valve replacement typically costs £200–£350 all-in. PCB replacement is more expensive — usually £350–£500 including parts and labour — and if your boiler is over eight to ten years old, it is worth comparing that figure against the cost of a new boiler installation before committing to the repair.
Why does my Heatline boiler keep coming back with F27 even after a reset?
If the F27 returns within minutes of a reset, or keeps recurring over days or weeks, it is a strong sign that there is an underlying component fault rather than a one-off ignition glitch. The most common repeating causes are moisture on the PCB from a leaking condensate trap, a deteriorating ignition electrode, or a gas valve that is drifting out of calibration. Repeated resets without fixing the root cause will not resolve the problem and may mask a worsening fault — you should call a Gas Safe engineer rather than continuing to reset.
Is the Heatline F27 the same fault code as on a Vaillant boiler?
Yes. Heatline is part of the Vaillant Group, and the Capriz, Advance, and Kalder models share design heritage and fault code logic with the Vaillant ecoTEC range. The F27 code means exactly the same thing on both brands — an incorrect or missing flame signal causing a lockout — and the diagnostic and repair approach is identical. An engineer experienced with Vaillant ecoTEC boilers will be equally capable of working on a Heatline carrying this fault.