What the E110 code means
The Promax, Titanium, Gold and Assure manuals all list E110 as "safety thermostat operated". The safety thermostat is an overheat cut-out that works separately from the boiler's normal temperature control. If the water in the boiler passes a set limit, it switches the burner off and the boiler locks out.
The older Promax manual groups E110 with E130 (flue sensor operated) as its overheat faults, and gives them a slightly different reset from other codes, covered below. Because Potterton is part of the Baxi group, the fault works in much the same way as Baxi E110.
On Promax, Titanium and Gold boilers the code shows as "E1" alternating with "10". On an Assure it shows as 110 with a symbol beside it.
Why the boiler overheats
The burner puts heat into the water, and the circulating water carries it away to the radiators or the hot water. If the water slows down or stops, heat builds up in the heat exchanger within seconds. That is why nearly every E110 investigation starts with circulation. The Gold Combi HE manual, for example, labels the related code E125 as a "pump fault", which shows how closely the two are linked.
Common causes of E110
| Cause | Typical sign | Who fixes it |
|---|---|---|
| Low system pressure | Gauge below 1 bar when cold | You, via the filling loop |
| Air in radiators | Cold at the top, gurgling | You, by bleeding radiators |
| Pump sticking or worn | Humming, no heat reaching radiators | Gas Safe engineer |
| Sludge or blocked heat exchanger | Cold spots, kettling noises from the boiler | Gas Safe engineer |
| Diverter valve sticking (combis) | Overheats mainly on heating or on hot water | Gas Safe engineer |
| Faulty safety thermostat or sensor | Circulation fine but E110 repeats | Gas Safe engineer |
Circulation problems
Low pressure, air and a tired pump are the most common triggers. On a Promax or Titanium that has run for many years, the pump is a frequent suspect.
Sludge and scale
Magnetite sludge in the heating circuit restricts flow. A kettling or rumbling sound from the boiler is a common clue that deposits are building up in the heat exchanger.
Diverter valve on combis
On a combi, the diverter valve sends hot water either to the radiators or to the hot water side. If it sticks part way, flow can drop enough to cause overheating.
Faulty components
Occasionally the safety thermostat or a temperature sensor fails and trips when the water is not actually too hot.
Safe checks you can do yourself
- Switch the heating and hot water off at the controls and give the boiler 15 to 20 minutes to cool.
- Check the pressure gauge. If it is below 1 bar when cool, top up to 1 to 1.5 bar using the filling loop. Our guide on how to repressurise a boiler explains the steps.
- Bleed any radiators that are cold at the top, with the heating off, then check the pressure again.
- Make sure several radiators are turned on.
- Reset the boiler. On a Promax, Titanium or Gold, the manual says to turn the selector to the reset position and hold it for at least five seconds for E110. On an Assure, press Reset for about one second. Try no more than two or three times.
Do not remove the casing, try to free the pump or touch the safety thermostat. These parts sit inside the boiler next to live wiring and gas components, so only a Gas Safe registered engineer should work on them.
When to call a Gas Safe engineer
If E110 returns after two or three resets, or you hear humming, kettling or banging from the boiler, book a Gas Safe registered engineer. They will usually check the pump, vent trapped air, test the safety thermostat and sensors, check the diverter valve on a combi and look for sludge. Mention if a circulation fault such as E125 has also appeared, as this helps narrow things down.
Heating or hot water?
It is worth noting whether E110 appears when the radiators are running, when you run a hot tap, or both. On a combi, overheating only on hot water can point towards the diverter valve or a scaled plate heat exchanger, while overheating only on heating points more towards the radiator circuit, the pump or sludge. That simple observation can save the engineer time. Also mention how long the boiler runs before the code appears, whether it is seconds after lighting or after half an hour of normal use.
Typical repair costs in the UK
Typical ranges only. Prices vary with the model, the parts and your area.
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £80 to £150 |
| Safety thermostat replacement | Typically £100 to £180 |
| Pump replacement | Typically £250 to £450 fitted |
| Diverter valve repair or replacement | Typically £150 to £350 |
| Powerflush | Typically £400 to £700 |
On an older Promax or Titanium, ask for a combined quote if more than one part is worn, and an honest view on whether a repair is still worthwhile.
How to stop E110 coming back
Keep the cold pressure between 1 and 1.5 bar and check it monthly. Run the heating briefly once a month over the summer so the pump and diverter valve do not stick. A magnetic filter and inhibitor help keep sludge under control on older systems. Book a yearly service so an engineer can spot a tired pump early. Our boiler service cost guide explains what is included, and our Potterton fault codes page lists the other codes we cover.