Potterton E110 fault codewhat it means and how to fix it
Safety thermostat operated — the primary water overheated — on every Potterton code display. One reset is permitted (5 s knob or 1–3 s button); a second E110 is a circulation fault for an engineer.
Take care
What was happening before E110?
The right answer depends on what you can see. Pick the one that matches.
Choose the option on the left that matches what your appliance is doing, and the answer for that case appears here.
The system was recently topped up or worked on
Air at the heat exchanger.
Bleed radiators, check pressure, one reset. If it recurs, engineer.
It happens when a hot tap runs
Diverter valve not moving to the hot water circuit, or a scaled plate heat exchanger.
Engineer.
Any time, and more often lately
Pump or sludge.
Engineer; expect a pump or a system clean to be discussed.
What E110 means
Potterton's user guides pair E110 with E130 as the two overheat codes — one for the primary water, one for the flue — and they are the only codes besides E133 that the rotary-switch guides let the owner reset: turn the knob to Reset, hold it at least five seconds, and call the installer if the boiler stays off or the code keeps coming back. The installation manuals separate the pair, assigning E110 to the primary water, and the table row is "Safety Thermostat Operated", a mechanical cut-out clipped to the heat exchanger outlet that opens when the water passes its limit.
On the Assure and Promax Ultra the code is 110 with an R, on the reset list for a one-to-three-second press, and it shares the footnote given to 09, 15 and 384: a recurring 110 sends the engineer to every PCB connection and then to the PCB itself. Section J of the Promax fault-finding chart asks whether the thermostat has operated or failed: allow to cool, check continuity across its terminals, and if it reads more than 1.5 ohm replace it.
But the thermostat is the messenger. Water overheats because it is not moving — a pump that has stopped, a heat exchanger narrowed by sludge, air in the top of the boiler after a refill, or a system with all its valves shut — and Potterton's own chart for E110 after a successful ignition sends the engineer to check pump supply, the three-way valve and primary circulation. A reset that holds means a one-off; one that does not means the circulation problem is still there.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Poor primary circulation | Pump not running or running slowly, or a heat exchanger obstructed by sludge. Section M of the Promax chart: "If pump is running the heat exchanger could be obstructed." | Common |
| Air in the primary circuit | After a top-up or a repair, air collects at the top of the heat exchanger and the water there flashes to steam. The chart's first check for circulation codes is that the automatic air vent cap is open. | Common |
| Safety thermostat itself failed | An open-circuit thermostat reads as operated. The chart's test is continuity across the terminals once cool; over 1.5 ohm and it is replaced (part 7207653 on the Assure). | Occasional |
| Three-way valve stuck or its motor failed | On a combi the burner heats a circuit the valve has not opened. Section K checks for 230 V at the valve motor and replaces the motor (part 7216534). | Occasional |
| PCB | The Assure manual's instruction for a 110 that recurs without a physical cause: PCB connections, then the PCB. | Rare |
What to try yourself
Let the boiler cool for ten minutes. Check the pressure gauge is between 1 and 2 bar and that radiator valves and any thermostatic valves are open — a boiler with nowhere to send heat overheats.
Reset once. Rotary knob: fully anticlockwise to Reset, hold for at least five seconds, release. Assure or Promax Ultra: press Reset for about a second.
If the boiler runs and stays running, mention the event at the next service. If E110 comes back, do not reset again; contact your installer or Baxi Customer Support.
Repeated overheats crack heat exchangers and boil the water in the primary circuit. Two E110s in a day is the limit.
Get a Gas Safe registered engineer to look at your Potterton E110
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What it costs to fix
Typical repair cost
£0–£320
Time on site
15 minutes
Free if one reset holds. Safety thermostat £70–£120 fitted. Pump £180–£300. Diverter valve motor £110–£180. Power flush £350–£600.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Baxi / Potterton / Main safety (overheat) thermostat
7207653£15–£40Authorised Baxi parts distributor
Price checked 28 September 2026
Baxi / Potterton / Main pump assembly
7684006£120–£240Authorised Baxi parts distributor
Price checked 28 September 2026
Baxi / Potterton / Main diverter valve motor
7216534£45–£95Authorised Baxi parts distributor
Price checked 28 September 2026
Baxi / Potterton / Main automatic air vent
7207776£15–£45Authorised Baxi parts distributor
Price checked 28 September 2026
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Related Potterton fault codes
- E125Get it checkedCirculation fault (primary circuit) on every Potterton code display — the water did not warm as expected after lighting, or overshot twice. Pump, air or sludge; an engineer job after one reset.Related: overheat and circulation
- E130Get it checkedFlue NTC operated — the flue gas got too hot — on every Potterton code display. Reset once as the user guide allows; if it returns the heat exchanger or flue needs an engineer.Related: overheat and circulation
- E193Get it checkedThe rotary-switch Pottertons' pre-ignition pump check has failed: the pump ran and the flow never proved. Same causes as E125; no owner reset is offered.Related: overheat and circulation
- E109Get it checkedThe digital-display Pottertons' pre-ignition pump check — Assure Combi, Assure System, Promax Ultra. The flow never proved, so the burner was not lit. Bleed and check pressure; then an engineer.Related: overheat and circulation
- E270Get it checkedCirculation fault (dry fire) on the Potterton Assure and Promax Ultra: the burner lit with too little water passing the heat exchanger. Not resettable by the owner; engineer.Related: overheat and circulation
- E119Get it checkedThe sealed system on a rotary-switch Potterton — Promax, Titanium, Gold, Heatmax — has dropped below half a bar. The window shows E1 and then 19. Fill to 1–1.5 bar and it clears itself.Related: same brand
Own a Potterton boiler? Save it and its codes come first next time.
Related guides
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.