What the E118 code means
E118 is the low pressure code on newer Baxi boilers, including the Baxi 100 and 600 series and many EcoBlue models. The manuals describe it as "primary system water pressure too low". Older Baxi ranges such as the Duo-tec and Platinum report low pressure as E119 instead, so the code you see depends on the age of your boiler rather than on a different fault.
The primary system is the sealed loop of water that the boiler heats and pumps round your radiators. A pressure sensor checks that there is enough water in it before the burner is allowed to fire. Baxi user guides give a normal working range of about 1 to 2 bar and say the boiler will stop below 0.5 bar. Stopping protects the pump and heat exchanger from running short of water.
On many newer Baxi displays the fault shows as the number 118 next to a symbol, rather than with a letter E in front. An R symbol means the code can be reset by the user once the cause is dealt with. A spanner symbol means it needs an engineer.
Common causes of E118
Gradual loss over months
Sealed systems slowly lose a little pressure over time. If nobody has checked the gauge for a year or more, a slow drift below the limit may be all that has happened.
Radiators bled recently
Letting air out of radiators also lets out some pressure. Bleeding several in one session often takes the system below the minimum.
Leaks
Weeping radiator valves, towel rails and pipe joints can lose enough water to trigger E118 within days or weeks. Look for damp carpets, stains on ceilings and green or white crust around fittings. Our guide to a boiler leaking water covers leaks at the boiler itself.
Relief valve and expansion vessel
If the expansion vessel has lost its charge, pressure rises too far when hot and the relief valve lets water out. Once the system cools, the pressure is lower than before. Repeated over weeks, this pattern ends in E118.
Faulty pressure sensor
If the gauge shows normal pressure but E118 remains, the sensor or its wiring may be at fault.
| Cause | Typical sign | Who fixes it |
|---|---|---|
| Slow pressure loss | No other symptoms, gauge low | You, via the filling loop |
| Radiators recently bled | Pressure fell straight afterwards | You, via the filling loop |
| Leak on pipes or radiators | Damp patches, pressure keeps dropping | Plumber or Gas Safe engineer |
| Expansion vessel or relief valve | High when hot, drips from outside pipe | Gas Safe engineer |
| Faulty pressure sensor | Gauge normal, E118 will not clear | Gas Safe engineer |
Safe checks you can do yourself
- Turn the heating off and let the system cool, then read the pressure gauge under or on the front of the boiler.
- Find the filling loop. Many Baxi boilers have one built in under the boiler, while others use a separate braided hose with a valve at each end. Your user guide shows which you have.
- Open the filling loop slowly and watch the gauge. Close it as soon as the reading reaches 1 to 1.5 bar.
- Check the valves are fully closed. A loop left slightly open will push the pressure too high and can cause E117 on models that use it.
- If the code does not clear by itself, press reset for about one second as the manual describes. Try no more than two or three times.
- Check radiators, visible pipes and the area under the boiler for signs of water.
- Write down the pressure and check it daily for a week.
New to topping up? Our step-by-step guide on how to repressurise a boiler walks through it with pictures of the common filling loop types.
Do not overfill. If the cold pressure is above 1.5 bar it can climb too high once the heating runs.
When to call a Gas Safe engineer
One top-up after bleeding radiators is routine. Book a Gas Safe registered engineer if you need to top up again within a few weeks, if water drips from the copper pipe outside, if you find water under the boiler, or if the gauge reads normal but E118 will not clear. The expansion vessel, relief valve and pressure sensor are all inside the boiler, so they must not be touched by a homeowner. A leak on a radiator or pipe away from the boiler can normally be fixed by a plumber.
When you book, tell the engineer how often you have been topping up and by how much. Losing half a bar over a month suggests a very slow weep, while losing a whole bar in a day suggests a larger leak or the relief valve opening.
Never remove the boiler casing to look for an internal leak. Water near live electrical parts and gas components is a job for a qualified engineer only.
Typical repair costs in the UK
These are typical ranges, not quotes, and vary by area and access.
| Job | Typical UK cost |
|---|---|
| Topping up pressure yourself | Free |
| Engineer call-out and diagnosis | Typically £80 to £150 |
| Radiator valve or joint leak repair | Typically £80 to £200 |
| Expansion vessel recharge | Typically £80 to £150 |
| Pressure sensor replacement | Typically £100 to £200 |
| Pressure relief valve replacement | Typically £120 to £220 |
How to stop E118 coming back
Check the gauge once a month and before winter starts. Top up after bleeding radiators rather than waiting for the boiler to lock out. Deal with small drips early, as a slow leak is the most common reason for repeated E118. An annual service lets an engineer check the expansion vessel and relief valve. Our boiler service cost guide explains what to expect.