What E118 means on a Potterton boiler
E118 tells you the sealed heating circuit has dropped below the pressure the boiler needs, so it has stopped itself firing. Potterton's user guide for the Promax Ultra Combi 24, 28, 33 and 40 lists the code as "Primary System Water Pressure Too Low", and the Potterton Assure Combi 25 and 30 user guide (reference 7685721) uses the same words for code 118. Nothing has broken at this point. What matters is how the water got out and whether it will happen again.
The primary circuit is the closed loop of water between the boiler and your radiators, separate from the mains supply to your taps. A sensor watches its pressure, and the Assure guide is plain about the limit: below 0.5 bar the boiler will not work.
How the code looks on the display
The Assure shows 118 next to a symbol. The Promax Ultra code table also lists the bare number, while its repressurising section calls it E118, so you may see either. If you see 117 instead, the Promax Ultra guide lists that as pressure too high.
Which Potterton boilers use E118 and which use E119
Older ranges, including the Promax Combi HE Plus A, the Gold Combi and earlier Promax combis, report the same situation as E119, described as "Water Pressure Switch Not Operated". If your display alternates between "E1" and "19", our Potterton E119 page is the one to follow, because the reset method on those boilers is different.
| Boiler | Low pressure code | What the user guide says about pressure |
|---|---|---|
| Assure Combi 25, 30 | 118 | Recommended 1.5 bar, normal operating 1 to 2 bar, will not work below 0.5 bar |
| Promax Ultra Combi 24, 28, 33, 40 | E118 (118 in the code table) | Normal operating pressure 1 to 2.5 bar |
| Promax Combi HE Plus A, Gold Combi, older Promax | E119 | Covered on our E119 page |
Why a Potterton loses enough pressure to show E118
Both guides treat 118 as low pressure to be topped up, and the Assure guide adds that regular repressurising means "a fault or leak is indicated". Water leaves a sealed system in only a few ways:
Water you let out yourself
Bleeding radiators or draining down for a new valve lets pressure out. If E118 appeared within a day of either job, one top-up should put it right.
A slow weep on the system
Radiator valve spindles, towel rail joints and pipes under floorboards can lose a few drops an hour without an obvious puddle, which over weeks is enough to take a system below 0.5 bar.
The safety valve opening when hot
The Assure guide says the safety pressure valve operates above 3 bar, releasing water through a discharge pipe that often ends outside. Once the system cools, the pressure is lower than before. Pressure climbing that high usually means the expansion vessel has lost its air charge.
A leak inside the boiler, or a sensor reading wrongly
Water under the boiler points to an internal leak. If the gauge reads 1 to 1.5 bar but 118 will not clear, the pressure sensor or its wiring may be giving a wrong reading.
How to top up a Potterton Assure or Promax Ultra
Both guides describe a filling loop of two taps joined by a separate pipe. The Promax Ultra guide says it may be on the boiler or nearby pipework; ask your installer if you cannot find it.
- Turn the heating off and let the system cool. Read the gauge under the boiler or, on the Promax Ultra, the pressure shown on the display.
- If your filling pipe is the removable type, connect it between the two taps and tighten the nuts.
- Open one tap fully, then open the second one slowly while you watch the pressure.
- Close both taps when the reading is between 1 and 1.5 bar. The Assure guide recommends 1.5 bar.
- Undo the pipe (a little water may run out), store it safely and refit any blanking caps.
- The Assure guide says the boiler resets itself once the correct pressure is back. If 118 stays on with the R symbol, press Reset for about one second. The guide warns that holding it for more than two seconds will not reset the boiler. Stop after two or three attempts.
Our Potterton boiler pressure guide goes through the gauge readings and the filling loop in more detail.
Do not fill past 1.5 bar when cold. The pressure rises as the water heats, and above 3 bar the safety valve will release water, which simply starts the cycle again.
When to call a Gas Safe registered 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;
- water is dripping from the discharge pipe outside or from under the boiler;
- the gauge climbs towards 3 bar with the heating on (the Promax Ultra shows E117 at that point);
- the gauge reads normal but 118 will not clear, or a spanner symbol appears, which the Assure guide says cannot be reset by the user.
The engineer will normally check the expansion vessel charge, the safety valve and its discharge pipe, and compare the boiler's reading with a separate gauge. Give them the model name, your hot and cold readings with dates, and any recent plumbing work.
Do not remove the casing to hunt for an internal leak. The expansion vessel, safety valve and pressure sensor sit beside live electrical parts and the gas supply, so they are for a Gas Safe registered engineer only.
Typical repair costs in the UK
| Job | Typical UK cost |
|---|---|
| Topping up through the filling loop yourself | Free |
| Engineer call-out and diagnosis | Typically £60 to £150 |
| Expansion vessel recharge | Typically £80 to £150 |
| Pressure relief valve replacement | Typically £100 to £200 |
| Pressure sensor replacement | Typically £100 to £220 |
| Internal leak or pipe joint repair | Typically £100 to £300 |
Keeping the pressure steady after E118
Check the gauge monthly with the heating cold. The Assure guide advises topping up once it falls below 0.7 bar, so there is no need to wait for the 0.5 bar cut-off. Note each top-up: one every few months is minor, one a week is significant. The same code on current Baxi boilers, which share Potterton's controls, is covered on our Baxi E118 page, and the Potterton fault code hub lists the other codes we explain.