Keston F1 Fault Code: Causes, Fixes & Repair Costs
What does the Keston F1 fault code mean?
The F1 code on a Keston S30 system boiler means the boiler has detected that the water pressure in your central heating system has dropped too low — typically below around 0.5 bar. A pressure sensor inside the boiler monitors system pressure continuously, and when it falls beneath the safe threshold, the boiler shuts itself down to prevent the heat exchanger running dry and being damaged. Once the fault is cleared and the pressure is restored, the boiler must be manually reset before it will fire up again. Note that other Keston models (such as the C40) use a different code — E44 — for the same underlying low-pressure condition; if you own a C40 and see E44, the diagnosis and fixes described here apply equally.
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
- Pressure naturally dropped over time or after bleeding radiators Common
The most common reason for F1. Heating systems lose small amounts of pressure gradually through normal use, and bleeding radiators releases water as well as air, which drops pressure quickly. If you have recently bled radiators and didn't top the system up afterwards, this is almost certainly the cause.
- Leak somewhere in the heating system Common
If the pressure keeps falling shortly after you top it up, water is escaping somewhere — through a joint, a radiator valve, the boiler itself, or a pipe hidden under the floor or behind a wall. Repeated top-ups without tracing the leak will accelerate internal corrosion and sludge build-up.
- Pressure relief valve (PRV) passing or weeping Sometimes
The PRV is a safety valve designed to release water if pressure gets too high. Over time these valves can start to weep or pass water even at normal pressures, causing a slow but steady pressure loss. You may notice a damp patch or dripping near the discharge pipe outside.
- Flat or waterlogged expansion vessel Sometimes
The expansion vessel absorbs the extra volume of water as it heats up. If the vessel loses its nitrogen charge or becomes waterlogged, system pressure becomes erratic — often spiking high when hot and falling low when cold, eventually triggering F1.
- Faulty pressure sensor giving a false reading Rare
If the pressure gauge on the boiler looks fine (around 1–1.5 bar) but the F1 code still appears, the pressure sensor or transducer inside the boiler may be defective and sending an incorrect signal to the PCB.
- Trapped air or sludge affecting pressure readings Rare
Significant air pockets or heavy sludge contamination in the system can interfere with how pressure is distributed and sensed, occasionally causing the boiler to register low pressure even when the gauge appears acceptable.
How to fix it
- Check the system pressure gauge DIY safe
Look at the pressure gauge on your Keston S30 — it is usually on the front or underside of the boiler. A healthy reading when the system is cold is between 1.0 and 1.5 bar. If it is reading below 1 bar (or the needle is in the red zone), low pressure is confirmed as the cause of F1.
- Repressurise the system using the filling loop DIY safe
Switch the boiler off and allow the system to cool for at least 20 minutes. Locate the filling loop — typically a short braided or silver flexible hose with two valves, found beneath the boiler. Open the first valve fully, then slowly crack open the second valve and listen for water entering the system. Watch the gauge closely and stop when it reaches approximately 1.2 bar. Close both valves completely and make sure the loop is not left open.
- Bleed any radiators that feel cold at the top DIY safe
If you hear gurgling or banging from radiators, air may be trapped. Use a radiator bleed key to open each bleed valve slightly until water — not air — comes out, then close it. After bleeding, recheck the pressure gauge and top up again via the filling loop if it has dropped below 1 bar.
- Reset the boiler DIY safe
Once the pressure gauge shows a steady 1.0–1.5 bar, press the reset button on the boiler (typically marked with a flame or reset symbol). Hold it for a few seconds if needed. The boiler should attempt to restart. Do not reset more than two or three times in succession — repeated resets without a fix can cause further faults.
- Monitor pressure over the next 24–48 hours DIY safe
Keep an eye on the gauge over the following day or two. A small drop (e.g. from 1.2 to 1.0 bar) can be normal as air works its way out. If the pressure falls sharply back below 1 bar, or if F1 returns within a short time, stop topping up and move to the next step — a leak or component fault is almost certainly present.
- Check for visible leaks around the boiler and radiators DIY safe
Look under and around the boiler, along exposed pipework, at radiator valves, and at the pipe leading from the PRV to outside the property. Damp patches, limescale staining, or dripping water are signs of a leak. Do not attempt to repair any leaking components yourself — note the location and report it to your engineer.
- Call a Gas Safe registered engineer with Keston experience Gas Safe engineer
If pressure continues to drop after topping up, if you cannot find a visible leak, or if F1 returns repeatedly, you need a professional. A Gas Safe engineer will pressure-test the system to trace hidden leaks, check and recharge or replace the expansion vessel, test the PRV, and inspect the pressure sensor and PCB. Keston boilers have a twin-flue design and modulating pump that not every engineer is familiar with — it is worth asking whether they have worked on Keston appliances before booking.
Parts you may need
- Pressure sensor / transducer · from £25
- Pressure relief valve (PRV) · from £20
- Expansion vessel · from £60
- Filling loop assembly · from £15
- 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 £80–£350, depending on the underlying cause.
Frequently asked questions
Can I just keep topping up the pressure when F1 appears on my Keston S30?
Topping up once is fine as a first step. However, if the pressure drops again within a few days and F1 returns, you should stop filling and call an engineer. Every time you add fresh water to the system you are also introducing dissolved oxygen, which accelerates internal corrosion and sludge build-up. Repeatedly masking the symptom without fixing the root cause — whether that is a weeping PRV, a failing expansion vessel, or a hidden pipe leak — will lead to bigger and more expensive problems over time.
My Keston S30 pressure gauge looks normal but F1 is still showing — why?
If the gauge is reading a healthy 1.0–1.5 bar but the boiler is still displaying F1, the most likely explanation is a faulty pressure sensor inside the boiler. The sensor is a separate component from the gauge and can fail independently, sending an incorrect low-pressure signal to the PCB even when actual system pressure is fine. A Gas Safe engineer can test the sensor and replace it — typically a modest repair costing around £120–£200 including parts and labour.
Is my Keston boiler still under warranty, and does that affect what I should do?
Keston extended their standard warranty to five years for combi and system boilers purchased from 1st January 2018 onwards. If your S30 is still within the warranty period, contact Keston directly before booking an independent engineer, as using an unregistered or non-approved engineer could void the warranty. Outside of warranty, any Gas Safe registered engineer can carry out the repair — but because Keston boilers are more complex than average, choosing one with prior Keston experience is advisable.
How much does it cost to fix F1 on a Keston boiler?
The cost depends on the underlying cause. If topping up the pressure yourself resolves it permanently, the fix is free. An engineer call-out to repressurise and investigate typically costs £80–£150. Replacing a pressure sensor runs to around £120–£200 all in, and a new pressure relief valve or expansion vessel is usually £150–£350 including labour. If a hidden pipe leak needs tracing and repairing, costs can rise to £300–£500 depending on where the leak is. In rare cases where the PCB is implicated, that part alone can cost £250–£400 plus labour — but PCB failure is an uncommon cause of F1 specifically.