What E10 means on an Alpha boiler
E10 is Alpha's low pressure fault: the boiler has detected that there is not enough water pressure in the sealed heating circuit and has shut down. The error table in Alpha's E-Tec 28 and 33 installation and servicing instructions lists code 10 as "primary system pressure low", with the note that the primary pressure switch has operated. The same entry appears in the E-Tec Plus 28 and 33 installation and servicing instructions. The tables print the number on its own, while the display shows it as E10.
The pressure switch is there to stop the boiler firing without enough water around the heat exchanger. So E10 is the boiler protecting itself, not proof that something has broken. What matters is whether the pressure dropped once for an obvious reason, or whether it keeps falling.
Which Alpha boilers show E10, and the A and B lights
The code is confirmed in the manuals for the E-Tec 28, E-Tec 33, E-Tec Plus 28 and E-Tec Plus 33. Older Alpha boilers without a digital display report the same problem differently. Alpha's FAQ explains that if the A and B lights on the front of the boiler flash alternately, the system pressure is too low. On those boilers the gauge will usually read below 1 bar, which Alpha describes as outside the green area. The fix is the same top-up described below, and once the pressure is right the lights should stop flashing.
Our Alpha fault codes page lists the other codes from the same manuals if your display shows something different.
What causes E10
Alpha's manual gives four things for an engineer to check. Each one shows up in a slightly different way, which helps you work out how serious your E10 is.
| Cause listed by Alpha | What you might notice | Who deals with it |
|---|---|---|
| A leak in the system | Pressure falls again within days, damp patches, staining or crusty deposits around radiator valves or pipe joints | Gas Safe engineer or heating engineer |
| Expansion vessel pressure low (Alpha gives 1 bar as the check figure) | Gauge climbs sharply when the heating is on, then falls back low when it cools | Gas Safe engineer |
| Expansion relief valve has operated | Water dripping from the small pipe that runs from the boiler to outside, often after the pressure ran high | Gas Safe engineer |
| Faulty primary pressure switch | Gauge sits in the green area but E10 will not clear | Gas Safe engineer |
A one-off drop is often explained by something you did recently. Bleeding radiators lets out water as well as air, and so does any work on the heating such as replacing a radiator. If you have done either in the last few days, a single top-up is usually all you need.
How to top up the pressure
This is the one E10 job that is meant for the homeowner. Do it with the heating off and the radiators cool, so the gauge gives an honest reading.
- Find the pressure gauge on the front or underside of the boiler and note where the needle sits against the green area.
- Find the filling loop under the boiler. Alpha's FAQ describes opening the tap or taps a quarter of a turn to let water in.
- Watch the gauge as it rises and close the taps as soon as the needle is back in the green area. Make sure they are fully shut.
- If the lights or the code are still showing, add a little more, but do not go beyond the green area.
- If you overfill it, Alpha's advice is to bleed a little water from a radiator to bring the pressure down.
- Press reset if the boiler does not restart on its own, and let it try to light.
Our guide to fixing low boiler pressure goes through the top-up in more detail if you have not used a filling loop before.
Write the pressure reading down after topping up, then check it at the same time each day for a week. That record is the quickest way for an engineer to tell a slow leak from an expansion vessel problem.
When E10 keeps coming back
Topping up now and again is normal. Needing to top up every week is not. Book a Gas Safe registered engineer if any of these apply:
- The pressure falls back below the green area within a few days of a top-up.
- You can see water coming from the pipe outside, or from under the boiler.
- The needle swings high when the heating is running and drops low when it is off.
- The gauge looks right but E10 stays on the display after two or three resets.
Do not take the casing off to look for a leak, and do not try to adjust the expansion vessel valve or the relief valve. Checking the vessel charge, replacing the relief valve and testing the pressure switch are all jobs for a Gas Safe registered engineer.
Typical repair costs in the UK
If a top-up fixes it, E10 costs nothing. Where a part or a leak is to blame, these are the typical ranges. Your final bill depends on the area and on what the engineer finds.
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £60 to £150 |
| Finding and repairing a leak or pipe joint | Typically £100 to £300 |
| Expansion vessel recharge | Typically £80 to £150 |
| Expansion vessel replacement | Typically £180 to £400 |
| Pressure relief valve replacement | Typically £100 to £200 |
| Pressure switch replacement | Typically £100 to £220 |
What to tell the engineer
Give the exact model (E-Tec or E-Tec Plus, 28 or 33), how often you have topped up and by how much, and your daily pressure readings. Mention whether the outside pipe has been dripping and whether the pressure climbs when the heating runs. If the pressure is dropping fast, our guide on a boiler that keeps losing pressure covers the places leaks most often hide.