Johnson & Starley F40 Fault Code: Causes, Fixes & Repair Costs
What does the Johnson & Starley F40 fault code mean?
F40 on a Johnson & Starley unit reports the opposite problem to F37: the sealed heating circuit's pressure is too high — typically approaching or beyond the 2.5–3 bar region where the pressure-relief valve begins to lift. On the water-carrying models (WarmCair D/DW and Aquair units), over-pressure usually has one of three stories behind it: the system was over-filled at the last top-up, the expansion vessel has lost its charge so normal thermal expansion sends pressure soaring on every heat-up, or the filling loop is letting mains water seep in continuously. Bleeding a radiator to shed the excess is homeowner territory; a pressure that repeatedly climbs on its own is a component fault needing an engineer.
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
- Over-filled at the last top-up Common
Filling with the system hot, or simply overshooting the gauge, leaves the circuit over-pressurised. Shedding the excess restores normality — a one-off with no further significance.
- Flat expansion vessel Common
With no air cushion, water's thermal expansion has nowhere to go and pressure spikes every heat-up — often paired with the relief valve dripping outside. The definitive cause of repeated F40s.
- Passing filling loop or heat-exchanger crossover Sometimes
A filling loop left cracked open (or with a failed check valve) feeds mains pressure in continuously; on some configurations a failing heat exchanger can do the same from the hot-water side.
How to fix it
- Check the filling loop is fully closed and disconnected DIY safe
Both filling-loop valves should be firmly shut; best practice is the braided hose removed between uses. A seeping loop is the simplest explanation for creeping pressure.
- Bleed a radiator to drop the pressure into the green band DIY safe
With the system cool, bleed a radiator until the gauge reads about 1.0–1.5 bar. Have a cloth ready — the water may be grubby.
- Have a Gas Safe registered engineer complete the documented checks Gas Safe engineer
If pressure climbs again by itself, the engineer will test the expansion vessel charge and re-pressurise or replace it, check the filling loop's check valve, and inspect the pressure-relief valve — which may also need replacing if it has been passing for a while, as seats erode once they weep.
Parts you may need
- Expansion vessel · from £80
- Pressure relief valve · from £35
- Filling loop check valve · 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 £0–£320, depending on the underlying cause.
Frequently asked questions
What does F40 mean on my Johnson & Starley?
The sealed circuit's water pressure is too high. Check the filling loop is fully closed, then bleed a radiator with the system cool until the gauge shows 1.0–1.5 bar. If pressure climbs again on its own, the expansion vessel or filling-loop check valve has failed — an engineer's job.
Why does the pressure shoot up when the heating runs?
That pattern is the signature of a flat expansion vessel: water expands as it heats, and with no air cushion to absorb it, pressure spikes — often lifting the relief valve. Re-charging or replacing the vessel fixes it.
Is there water dripping from a pipe outside?
That is the pressure-relief valve discharging excess — confirmation the system is genuinely over-pressure. Once a relief valve has passed water for a while its seat erodes, so mention the drip to the engineer; the valve often needs replacing along with the underlying fix.