Keston F7 Fault Code: Causes, Fixes & Repair Costs
What does the Keston F7 fault code mean?
The F7 fault code on Keston boilers signals that the mains electrical supply reaching the boiler has dropped below the acceptable threshold — typically under 190V, against the required 230V ~ 50Hz supply. When voltage falls this low, the boiler's controls, pump, fan, and ignition circuitry cannot operate reliably, so the appliance locks out and displays F7 as a protective measure. Crucially, this fault originates in your home's electrical supply rather than inside the boiler itself, making it one of the few boiler fault codes where a qualified electrician — not a Gas Safe engineer — is usually the first professional you need to call. Keston is now part of the Ideal Heating group, and the F7 code carries the same meaning across both brands.
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
- Unstable or low mains supply from the grid Common
A dip in the incoming mains voltage — whether from a local power cut, a surge event, or a wider network issue — can push supply below the 190V threshold the boiler needs. If other appliances in the property are also behaving oddly (lights dimming, devices restarting), this is a strong indicator that the fault lies with the incoming supply rather than anything in your home's wiring or the boiler itself.
- Fault or overload in the property's consumer unit Common
A tripped breaker or a faulty RCD in the consumer unit can reduce or interrupt the voltage on the circuit feeding the boiler. Checking whether any switches in the fuse board have tripped is a straightforward homeowner check. If the circuit looks normal but the fault persists, a qualified electrician should test actual voltage at the boiler's spur.
- Faulty or degraded wiring to the boiler Sometimes
Loose connections, corroded terminals, or undersized cable on the dedicated boiler spur can cause a voltage drop under load. This is more likely in older properties or if recent electrical work has been carried out nearby. The boiler requires a 3A fused, properly earthed supply, so any deviation from this can trigger F7.
- PCB or internal electrical component fault Rare
In rare cases, a voltage problem that went unaddressed for some time may have damaged the boiler's PCB or internal wiring. Here the F7 code can persist even once the mains supply has been restored to normal levels. This scenario requires a Gas Safe engineer to diagnose, as it involves work inside the appliance.
How to fix it
- Check whether other appliances in your home are affected DIY safe
Look around the property for signs of a broader electrical issue — flickering or dimmed lights, other appliances losing power, or switches tripping on the consumer unit. If you see any of these, the problem is likely upstream of your boiler and needs a qualified electrician, not a boiler engineer.
- Check your consumer unit for tripped breakers or RCDs DIY safe
Open your fuse board and look for any switches that are in the middle or off position. If a breaker has tripped, reset it. If it trips again immediately or repeatedly, do not keep resetting it — call a qualified electrician, as something on that circuit is drawing too much current or there is a fault to earth.
- Switch the boiler off at the mains isolator and attempt a reset DIY safe
Turn the boiler off at its dedicated mains switch and leave it off for two to three minutes. Then switch it back on and follow the reset procedure in your user guide (typically holding the reset button for a few seconds). If the F7 clears and does not return, monitor the boiler over the next day or two. If it reappears, move to the next steps rather than repeatedly resetting.
- Call a qualified electrician to test the mains supply Gas Safe engineer
If the fault returns after a reset, a qualified electrician should attend and measure the actual voltage being delivered to the boiler's spur under load. They can identify whether the issue is the incoming supply, the consumer unit, or the wiring to the boiler, and carry out any remedial work required. Do not attempt to inspect wiring inside the boiler or at the terminals yourself.
- Contact a Gas Safe registered engineer if the fault persists after the electrical supply is confirmed as normal Gas Safe engineer
Once an electrician has verified that the boiler is receiving a correct and stable 230V supply and the F7 code still appears, the fault may lie inside the boiler — most likely the PCB or internal wiring. A Gas Safe registered engineer with Keston experience should attend to carry out a full electrical and component check on the appliance. Always ask engineers whether they have worked on Keston boilers, as their twin-flue, modulating-pump design is not universally familiar.
Parts you may need
- Boiler PCB (printed circuit board) · from £220
- Boiler mains terminal block / wiring harness · from £45
- Fused spur isolator switch (3A) · from £15
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 £80–£300, depending on the underlying cause.
Frequently asked questions
Is the Keston F7 fault dangerous?
The boiler locking out is itself a safety measure, so leaving it off while you investigate is the right approach. The underlying electrical issue — such as a faulty wiring connection or a supply problem — should be treated with care. Do not probe wiring yourself. Call a qualified electrician promptly, particularly if you notice burning smells, scorch marks around sockets, or other appliances behaving erratically, as these could indicate a more serious electrical hazard.
Can I fix the Keston F7 fault myself?
The only homeowner-safe steps are checking other appliances for signs of a wider power problem, inspecting your consumer unit for tripped breakers, and performing a single boiler reset. All electrical testing and any work inside the boiler must be carried out by qualified professionals. Attempting to check wiring inside the appliance or at the boiler terminals yourself is unsafe and could also void any remaining warranty.
How much will it cost to fix the F7 fault on my Keston boiler?
Most people pay between £80 and £200 for an electrician to inspect and rectify the mains supply issue, with the lower end covering a simple connection repair and the higher end reflecting more complex wiring or consumer unit work. If a Gas Safe engineer then needs to attend to check the boiler, expect an additional callout fee of around £60–£100. In the less common scenario where the PCB has been damaged by the voltage problem, a replacement board can add a further £180–£300 to the bill. Prices in London and the South East tend to run around 15–25% above these figures.
Why should I ask for an engineer with Keston experience?
Keston boilers use a twin-flue arrangement and a modulating pump, which sets them apart from mainstream combi boilers that most engineers work on daily. Not every Gas Safe engineer will be confident diagnosing faults on them. Asking upfront whether the engineer has Keston or high-efficiency condensing system boiler experience can save you from a wasted callout and ensure the diagnosis is accurate.