Overview
E24 on a Keston C36 combi means the boiler has detected low water pressure on four separate occasions within the previous 24 hours. The boiler monitors system pressure and will log this as a fault when repeated pressure loss occurs because repeated drops usually indicate an ongoing problem rather than a one-off event. Low system pressure prevents correct circulation of water through the heat exchanger and can cause the boiler to lock out to protect itself and the heating system. Severity is moderate to high. A single small pressure drop can often be resolved by topping up the system, but repeated losses can lead to regular lockouts, poor heating performance, or in the worst case damage to internal components (for example corrosion or freeze damage if temperatures are low). You can carry out simple checks and basic actions yourself (visual leak search, bleeding radiators, and repressurising the system). However, when pressure continues to drop, or if you suspect an internal leak, faulty safety valve, faulty expansion vessel or a control/pressure-sensor fault, you should call a qualified gas engineer (Gas Safe registered) because repairs involve gas and sealed-water-system components and may require part replacement or system drainage and re-commissioning.
Possible Cause: Water pressure lost 4 times in 24 hours – possible leak, check system and boiler
Troubleshooting Steps
Safety precautions:
- If you find active water leaking from pipes or the boiler, switch the boiler off at the electrical supply, turn off the mains cold water feed to the boiler (and the gas supply at the meter if you suspect any damage), and isolate the heating system if possible. Mop up water to avoid electrical hazards and damage.
- Do not attempt internal electrical or gas appliance repairs yourself. Only a Gas Safe qualified engineer should open the boiler or work on gas or sealed-system components.
Initial homeowner checks you can safely do:
1. Check the pressure gauge on the boiler when the system is cold. Normal range for most combi boilers is about 1.0–1.5 bar cold; heating on often increases this. Note the reading.
2. Look for visible leaks: check under and around the boiler, pipework, radiator valves, between radiator and pipes, and the PRV (pressure relief valve) discharge pipe (often a small pipe discharging outside or into a condensate/drain). Also check under sinks, in lofts and airing cupboards for damp patches.
3. Check radiators for recent bleeding: if someone bled radiators without repressurising the system, pressure will drop.
4. Check the condensate/drain and any visible discharge pipework for signs of recent water (this can show PRV operation or leaks).
If pressure is low and there are no obvious leaks, safe diagnostic and basic fix steps:
1. Re-pressurise the system using the boiler filling loop (consult your boiler manual to locate the filling loop and the correct procedure for your model). Typical procedure: with the boiler off, open the filling loop isolation valves slowly until the pressure rises to about 1.0–1.5 bar, then close the valves. If your filling loop is a removable braided hose, some installers recommend removing it after use; follow your manual.
2. Do not exceed 2.0–2.5 bar when repressurising. Over-pressurising can cause the PRV to operate or damage the system.
3. Once the system is back to pressure, switch the boiler on and run the heating. Check the pressure gauge again after the system warms and after cooling to see if the pressure holds.
4. If pressure holds, monitor for several days. If the E24 was a one-off because the system had been bled or recently worked on, the problem may be resolved.
If pressure drops again or will not hold:
1. Re-check all pipework, joints, radiator valves (including lockshield valves) and radiators for damp or water staining—small leaks can be hard to spot but will usually show evidence over time.
2. Check the boiler drain/PRV discharge outlet for fresh water; a leaking PRV will drip when the pressure exceeds its set point and can indicate a faulty PRV or debris in the valve.
3. If pressure falls slowly only when the heating is on, suspect an expansion vessel problem (loss of air charge) or an internal leak in the boiler/heat exchanger. These require an engineer to test the expansion vessel charge and inspect internal components.
4. If pressure falls quickly or you find water on the floor and cannot stop it, isolate the boiler water supply and electrical power, and call an emergency plumber or engineer.
5. If there are no visible leaks and the system still loses pressure, the fault may be a defective pressure sensor/switch or a control box logging error. A qualified engineer can test the pressure switch, replace faulty sensors, check wiring and, if necessary, replace the control PCB.
When to call a professional:
- Call a Gas Safe engineer if you suspect an internal boiler leak, the PRV is discharging, the expansion vessel needs checking or replacing, the pressure switch/sensor or control PCB may be faulty, or if you are unsure. Any repair that involves opening the boiler, replacing sealed components, or working on gas parts must be done by a registered engineer.
- If you cannot find a visible leak but the pressure keeps dropping after you have properly repressurised and monitored the system, book a professional inspection. Tell the engineer you have had repeated low-pressure events (E24) so they can prioritise leak detection, PRV and expansion vessel checks.
Notes and warnings:
- E24 specifically indicates repeated pressure loss (4 times in 24 hours) rather than a single low-pressure occurrence; that pattern usually points to an ongoing leak or failing component rather than a simple one-off topping-up need.
- Do not attempt to repair or replace the PRV, expansion vessel, pressure switch or internal boiler components yourself unless you are a qualified Gas Safe engineer. Incorrect repairs can be dangerous and invalidate warranties.
- After any professional repair, ensure the engineer fully commissions the system and shows you how to check and repressurise safely if required.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Keston C36 Combi Gas Boiler.