Keston E24 Fault Code: Causes, Fixes & Repair Costs
What does the Keston E24 fault code mean?
The E24 code appears on Keston boilers — most notably the C36 — when the system water pressure has dropped enough to require topping up on four or more occasions within a single 24-hour period. Rather than flagging a one-off pressure dip, the boiler's control board is telling you it has noticed a recurring pattern of pressure loss, which strongly suggests water is escaping from somewhere in the system. When E24 triggers, the boiler locks out and will not restart until the Reset button is pressed. Simply re-pressurising and resetting may bring the boiler back temporarily, but the code will return unless the underlying cause — almost always a leak — is properly identified and fixed.
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
- Leak in the heating pipework or at radiator joints Common
A slow but persistent drip anywhere along the heating circuit — at a radiator valve, a compression fitting, or a length of pipework — will gradually bleed pressure from the system. Because each individual drop is small, the leak can be hard to spot visually, yet it is easily the most common reason the boiler accumulates four pressure-loss events in 24 hours.
- Leaking pressure relief valve (PRV) Common
The PRV is designed to release water if system pressure climbs dangerously high, but a worn or sticking valve can weep continuously even at normal operating pressure. Water often drips quietly out through the discharge pipe to an external drain, making this cause easy to miss during a quick visual check.
- Internal boiler component leak Sometimes
Seals around the pump, heat exchanger connections, or other internal pipework joints can deteriorate over time, particularly in older or heavily scaled boilers. Water may pool inside the boiler casing rather than appearing on the floor, so damage can progress unnoticed until the engineer opens the unit up.
- Faulty water pressure sensor Sometimes
If the sensor that monitors system pressure is giving inaccurate readings, the boiler may record phantom pressure-loss events that did not actually occur. This is less common than a genuine leak but worth investigating if a thorough inspection finds no trace of water anywhere.
- Waterlogged or failed expansion vessel Sometimes
The expansion vessel absorbs the pressure changes that occur as the system heats and cools. If the vessel's internal diaphragm has failed, pressure can swing wildly during normal operation, causing the boiler to log repeated low-pressure events even though no water has actually left the system.
How to fix it
- Check for visible signs of water around the boiler, pipework, and radiators DIY safe
Walk around every radiator, look under the boiler, and inspect any exposed pipework for damp patches, discolouration, or active drips. Pay close attention to radiator valve connections and any compression or push-fit joints you can see. If you spot a leak, do not top up the pressure or reset the boiler — contact a Gas Safe engineer straight away to avoid water damage spreading to floors, ceilings, or electrical components.
- Top up system pressure using the filling loop — but only if no leak is visible DIY safe
Locate the external filling loop, which is usually a flexible braided hose with two valves situated beneath the boiler. Connect it if it is the removable type, then open both valves slowly and watch the pressure gauge rise to between 1.0 and 1.5 bar. Once you reach that range, close both valves firmly, disconnect and store the loop. If pressure climbs above 2 bar, stop immediately and call an engineer. Do not top up more than once — if pressure drops again soon after, a leak is almost certainly present.
- Reset the boiler once — and monitor closely DIY safe
Press and hold the Reset button as described in your Keston boiler's user guide. The boiler should attempt to restart. Watch the pressure gauge over the next few hours. If the pressure holds steady and the boiler runs normally, continue to monitor daily. If the E24 code returns, or if pressure falls again within hours, stop resetting and arrange for a Gas Safe engineer to inspect the system.
- Call a Gas Safe registered engineer to locate and repair the leak Gas Safe engineer
A qualified engineer will carry out a pressure test across the entire heating circuit to pinpoint where water is escaping. They will inspect the pressure relief valve, pump seals, internal boiler components, and all external pipework and radiator joints. They will also check the expansion vessel charge pressure and assess the water pressure sensor. Any faulty parts — seals, valves, sensor, or vessel — will be replaced before the system is re-pressurised, tested at temperature, and signed off as safe.
Parts you may need
- Water pressure sensor · from £25
- Pressure relief valve (PRV) · from £20
- Expansion vessel · from £55
- Pump seal kit · from £18
- Filling loop assembly · 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 £120–£350, depending on the underlying cause.
Frequently asked questions
Can I just keep topping up the pressure to clear the E24 code?
No — and it is important not to. The E24 code fires specifically because the boiler has already detected pressure loss four times in one day, which means re-pressurising is only masking a persistent problem. Every time you top up without fixing the root cause, more water is leaking out somewhere, potentially damaging floorboards, plaster, and nearby electrical components. Top up once to restore heating temporarily if you have no choice, but arrange for a Gas Safe engineer to investigate as soon as possible.
How much does it typically cost to fix a Keston E24 fault?
For the most common scenarios — a leaking radiator valve, a dripping PRV, or a faulty pressure sensor — most homeowners pay between £120 and £350 once parts and labour are included. A straightforward seal or joint repair with a single call-out is often at the lower end, while a more involved leak trace with several components replaced sits towards the upper end. If the investigation reveals a failed heat exchanger or a major internal component, costs can rise above £500, at which point your engineer may advise weighing up a replacement boiler instead.
Is E24 different from E23 on a Keston boiler?
Yes. E23 is a water filling error that points specifically at the pressure sensor itself — the boiler flags a problem with the sensor reading rather than confirmed pressure loss. E24, by contrast, means the boiler has actually recorded multiple pressure-loss events over 24 hours, making a real leak the primary suspect. The two codes can sometimes overlap if a faulty sensor generates false pressure readings, but E24 should always prompt a physical inspection for leaks first.
My boiler pressure looks fine now — why has E24 appeared?
The E24 code is based on the boiler's internal log rather than a single snapshot of current pressure. Even if you topped up the pressure earlier in the day and the gauge reads correctly right now, the boiler has already counted four separate drops and triggered the lockout. The pressure appearing normal at this moment simply means water may have been lost gradually across several short episodes. The code will keep returning until a Gas Safe engineer finds and fixes the source of those repeated drops.