Andrews E48 Fault Code: Causes, Fixes & Repair Costs
What does the Andrews E48 fault code mean?
The E48 fault code on Andrews boilers (which share the Warmflow platform and fault code table) indicates that the boiler has detected water pressure in the sealed heating system has climbed above the safe upper limit. As a safety measure, the boiler shuts itself down and will not restart until the pressure is brought back within the normal operating range. On a healthy system, cold pressure should sit between 1 and 1.5 bar; anything significantly above 2 bar is cause for concern and will typically trigger E48. Note that Andrews combi boilers share their control platform with the Warmflow Agentis series, so this code applies equally across both ranges.
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
- Filling loop left open or not disconnected Common
This is the most frequently cited cause in the Warmflow fault table. When a homeowner tops up system pressure using the filling loop, even leaving the valve fractionally open afterwards can allow mains water — which is typically at 2–4 bar — to push into the system continuously. The result is that pressure climbs well above 2 bar, triggering E48. Always close both filling loop valves firmly and remove any flexible braided connector once topping up is complete.
- Waterlogged or failed expansion vessel Common
Inside your boiler or on the heating circuit there is a small sealed vessel with a rubber diaphragm separating an air cushion from the water side. When water heats up it expands, and the air cushion compresses to absorb that extra volume. If the diaphragm perishes or the air charge leaks away, there is nowhere for the expanding water to go — pressure spikes rapidly when the boiler fires up, triggering E48. A classic tell-tale sign is pressure that looks fine when cold but shoots up as soon as the boiler runs. Water weeping from the pressure relief valve (PRV) discharge pipe is another strong indicator.
- Faulty pressure sensor giving a false reading Sometimes
The boiler relies on a pressure transducer to monitor system pressure. If this sensor develops a fault or drifts out of calibration, it can report a high-pressure condition even when the actual pressure is within the normal range. In this scenario the gauge on the boiler will typically read normally while the E48 code persists — a useful clue that the sensor rather than true over-pressure may be the culprit.
- Pressure relief valve (PRV) opening and refilling cycle Sometimes
If the expansion vessel has lost its charge, the system pressure may rise far enough to lift the PRV at 3 bar, releasing water down the discharge pipe outside. The homeowner then tops up the system — but because there is still no expansion capacity, the cycle repeats. This pattern of the PRV weeping and needing regular top-ups is a strong sign the expansion vessel needs attention rather than repeated refilling.
How to fix it
- Check the boiler pressure gauge DIY safe
Look at the pressure gauge on the front of the boiler. If it reads significantly above 2 bar — for example 2.5 bar or higher — this confirms the system is genuinely over-pressurised rather than a false reading. Note the figure down as it will be useful for an engineer if the fault persists.
- Bleed a radiator to reduce pressure DIY safe
Using a radiator bleed key, slowly open a bleed valve on an upstairs radiator. Have a cloth ready to catch drips. Let a small amount of water out and watch the pressure gauge drop. Aim to bring the reading down to around 1 to 1.5 bar, then close the bleed valve firmly. Do not drain too much water — stop well before the gauge reaches zero.
- Check whether the filling loop is open or still connected DIY safe
Locate the filling loop — usually a short flexible braided hose underneath the boiler connecting two valves. Both valves should be in the closed position (handle perpendicular to the pipework). If the flexible connector is removable, disconnect it and store it safely. This ensures mains pressure cannot continue to push water into the system.
- Reset the boiler DIY safe
Once the pressure is back in the 1 to 1.5 bar range and the filling loop is confirmed closed, press the reset button on the boiler and allow it to restart. Monitor the pressure gauge over the next heating cycle. If pressure climbs again above 2 bar shortly after the boiler fires, this strongly suggests an expansion vessel problem rather than an overfilling issue.
- Check the expansion vessel Schrader valve DIY safe
The expansion vessel has a small valve on it that looks very similar to a car tyre valve. With the boiler off and the system cold, gently depress the pin inside the Schrader valve using a small screwdriver or similar tool. If water squirts out, the diaphragm inside the vessel has likely failed and the vessel will need replacing. If only air comes out and the pressure feels very low or absent, the vessel may simply need recharging with air — a job an engineer can carry out quickly, often during a service visit.
- Call a Gas Safe registered engineer if the fault persists or returns Gas Safe engineer
If E48 returns after following the steps above, or you found water coming from the Schrader valve, the repair requires a qualified professional. A Gas Safe engineer will assess the expansion vessel charge, recharge or replace it as needed, check the pressure sensor and PRV, and confirm the system is operating safely. Do not keep resetting the boiler repeatedly — persistent E48 faults indicate an underlying issue that requires proper diagnosis.
Parts you may need
- Expansion vessel (8–12 litre, suitable for Warmflow/Andrews combi) · from £65
- Expansion vessel diaphragm (service kit) · from £20
- Pressure relief valve (3 bar PRV) · from £18
- Pressure transducer / sensor · from £35
- Filling loop assembly (braided flexible) · from £12
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 fix Andrews E48 myself without calling an engineer?
Partly, yes. A homeowner can safely bleed a radiator to reduce pressure, check and close the filling loop, and reset the boiler — and this resolves E48 in many cases where the loop was accidentally left open. However, if the fault comes back after a reset, or you find water coming from the expansion vessel's Schrader valve, you will need a Gas Safe registered engineer to recharge or replace the expansion vessel and confirm everything is safe. Opening the boiler casing or working on internal components is not something a homeowner should attempt.
Why does my Andrews boiler keep going over pressure every time it heats up?
If pressure climbs every time the boiler fires but looks normal when cold, the expansion vessel is almost certainly the cause. The vessel contains a rubber diaphragm that absorbs the extra volume of water as it heats and expands. When the diaphragm fails or the air cushion leaks away, there is nowhere for the expanding water to go and pressure shoots up. An engineer can often recharge the vessel with air at low cost, though if the diaphragm has split, a replacement vessel is needed — typically £150–£350 all in.
Is high pressure on my boiler dangerous?
A sealed heating system is designed with a pressure relief valve (PRV) set to open at 3 bar, which acts as the last line of defence by releasing water if pressure becomes dangerously high. While E48 is a safety lockout rather than an immediate emergency, repeatedly allowing pressure to reach the PRV opening point puts unnecessary wear on components and can cause water damage from the discharge pipe. You should aim to resolve the underlying cause promptly rather than repeatedly resetting the boiler.
How much does expansion vessel replacement cost in the UK?
Most homeowners pay between £150 and £350 for expansion vessel replacement on an Andrews or Warmflow boiler, covering both the part (roughly £50–£90 for the vessel itself) and the labour. In many cases the vessel does not need replacing at all — an engineer can simply recharge the air pressure through the Schrader valve during a service visit for little or no additional charge. If the boiler is particularly awkward to access or requires significant drain-down time, costs can be higher, but this is the exception rather than the rule.