Overview
F1 on a Keston S30 system boiler indicates the boiler has detected low system water pressure and has locked out to protect itself. The boiler needs a minimum cold pressure (typically around 1.0–1.5 bar, commonly 1.3 bar as a target) to circulate water through the heating system. If pressure falls below a critical threshold (the manual notes the boiler will not operate if pressure drops below about 0.3 bar) the unit will stop running and display F1. Low pressure can be caused by a simple need to top up the system through the external filling loop, or by a water leak somewhere in the heating system (radiators, pipework, valves, or the boiler itself). Less commonly the pressure sensor or its wiring could be faulty and give a false low reading. If the pressure loss is a one-off and you can safely re-pressurise, this is a DIY task for many homeowners; if the pressure drops again, if you find a leak, or if you are unsure about any step you must call a Gas Safe Registered Engineer (or Registered Gas Installer in IE).
Possible Cause: Low water pressure
Troubleshooting Steps
Safety precautions:
- Do not remove the boiler case or attempt any internal repairs; those must be carried out by a Gas Safe / RGII engineer.
- Isolate electrical power to the boiler before working on pipes close to electrics and avoid spilling water onto electrical components.
- Never tamper with gas components. If you suspect a gas fault, turn off the appliance and contact a Gas Safe engineer.
- Work carefully around hot radiators or pipework; allow the system to cool if recently running.
Initial checks a homeowner can do:
1. Look at the pressure gauge on the boiler. Note the cold pressure reading. The target is usually about 1.0–1.5 bar (aim for ~1.3 bar). If the gauge is in the red or well below 1 bar, that explains F1.
2. Visually inspect visible pipework, radiators, valves, and beneath the boiler for signs of leaks or drips. Check pipe joints, radiator bleed valves and towel rail valves.
3. Check the condensate discharge and any external discharge pipes for signs of recent water (this can indicate a pressure relief valve (PRV) discharge).
4. Confirm no external isolation valves (filling loop valves) are left open or loose and that any external filling loop is installed correctly and not leaking.
If pressure is low and no obvious leak is found: how to re-pressurise the system (homeowner procedure):
1. Locate the filling loop: a braided hose or small pipework under the boiler with one or two taps/valves. Many systems have a removable/fixed filling loop. If you are unsure where it is, consult your manual or installer.
2. Ensure the boiler is powered (some boilers require power to allow filling) and the heating is off. Open the filling loop valves slowly. Watch the pressure gauge while you open the valve(s).
3. Close the filling loop as soon as the gauge reaches about 1.2–1.5 bar (aim for ~1.3 bar). Do not over-pressurise above the boiler manufacturer’s recommended maximum (typically around 1.5 bar cold). Tighten/secure the valves and remove or isolate the filling loop if it is designed to be removable.
4. If you have bled radiators recently or expect air in the system, you may need to bleed radiators after topping up. Bleeding radiators will drop pressure slightly, so top up again if necessary and re-check.
5. Press the boiler RESTART/reset button (on the S30 this is the RESTART button) to clear the fault and allow the boiler to run its ignition sequence. If the boiler does not restart or the F1 returns immediately, switch off and call an engineer.
Diagnostics if pressure keeps dropping or F1 returns:
- Monitor pressure over 24–48 hours. If it falls steadily, you likely have a leak or a faulty component (PRV, internal leak, radiator valve or pipework).
- Re-check for leaks after the system has been pressurised (look for new damp patches, dripping, or rusted areas). Check radiator bleed valves, radiator valves, and under floorboards if accessible.
- Check the PRV discharge pipework for recent discharge or water staining. A leaking PRV or expansion vessel problem can cause pressure loss.
- If the gauge reads normal but the boiler still displays F1, the pressure sensor or its wiring may be faulty. Do not attempt sensor replacement yourself; contact a Gas Safe engineer.
When to call a professional:
- Call a Gas Safe Registered Engineer (or RGII in Ireland) if you find any leak, if the pressure keeps dropping after repressurising, if the boiler will not restart after you have correctly re-pressurised and reset, or if you suspect a faulty pressure sensor/PRV/expansion vessel. Also call a professional if you are unsure about the filling loop, valves, or any step above.
Notes and cautions:
- Do not leave the filling loop valves open once the system is pressurised; this will allow continuous mains water into the heating circuit and can cause damage.
- If the system has been drained for a long period, it should be refilled and bled correctly before use.
- Always follow the boiler manual instructions for repressurising and restarting. If the boiler keeps locking out or shows other fault codes, a qualified engineer must inspect the appliance.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Keston S30 System Gas Boiler.