Main E110 Fault Code: Overheat Safety Thermostat Tripped
The E110 code on a Main (Eco Compact) boiler means the overheat safety thermostat has activated and shut the boiler down. The boiler's internal temperature climbed beyond a safe threshold — either in the primary heat exchanger or the surrounding pipework — and the safety cut-out tripped as a protective measure. This is a deliberate safety response rather than a component failure in itself; the underlying cause still needs to be identified and resolved before the boiler will run reliably again. Main boilers share the Baxi Heating group fault-code system, so this code carries exactly the same meaning across Main, Baxi, and Potterton branded appliances.
General guidance only — not a substitute for professional advice. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
What does the Main E110 fault code mean?
The E110 code on a Main (Eco Compact) boiler means the overheat safety thermostat has activated and shut the boiler down. The boiler's internal temperature climbed beyond a safe threshold — either in the primary heat exchanger or the surrounding pipework — and the safety cut-out tripped as a protective measure. This is a deliberate safety response rather than a component failure in itself; the underlying cause still needs to be identified and resolved before the boiler will run reliably again. Main boilers share the Baxi Heating group fault-code system, so this code carries exactly the same meaning across Main, Baxi, and Potterton branded appliances.
Common causes
- Circulation pump failure or seizure Common
The pump is responsible for keeping water moving around the heating circuit. If it seizes, runs too slowly, or stops altogether, heat builds up rapidly inside the heat exchanger with nowhere to go. This is the single most frequent trigger for an E110 on Main boilers — a seized pump can cause the overheat thermostat to trip within minutes of a call for heat.
- Blocked or scaled heat exchanger Common
Over time, sludge from the heating system or limescale in hard-water areas can coat the internal passages of the heat exchanger, dramatically reducing the rate at which heat transfers into the water. Flow slows, temperatures spike, and the overheat thermostat cuts in. Boilers in hard-water regions or older systems without a magnetic filter are particularly susceptible.
- Low system pressure Common
If pressure drops below around 0.5 bar, there simply isn't enough water in the circuit to absorb and carry away the heat being generated. The boiler effectively overheats the small volume of water present. The pressure gauge on the boiler front panel will confirm whether this is the cause — the normal cold-fill range is 1–1.5 bar.
- Faulty overheat thermostat or temperature sensor Sometimes
The overheat thermostat acts as a switch that opens above roughly 105°C. If the thermostat or an associated NTC temperature sensor has developed a fault, it can trigger the E110 even when actual temperatures are perfectly safe. Repeated E110 faults that clear quickly and recur without obvious cause often point here.
- Frozen or blocked condensate pipe Sometimes
A frozen condensate pipe (common during UK cold snaps) causes combustion gases to back up, which can disrupt heat dissipation and indirectly trigger overheating protection. A blocked flue has a similar effect and is a more serious safety concern requiring immediate professional attention.
- PCB fault Rare
The printed circuit board governs temperature management across the whole boiler. A damaged or failing PCB may misread sensor data or fail to regulate the burner output correctly, causing uncontrolled temperature rises that activate the safety thermostat.
How to fix it
- Check whether other gas appliances in the property are working DIY safe
Before doing anything else, confirm that your gas supply is live. Try a gas hob or check with your supplier. If there is no gas supply, contact your energy provider — this is not a boiler fault.
- Thaw the condensate pipe if the weather is freezing DIY safe
The condensate pipe is the plastic waste pipe (usually 21.5 mm diameter) that exits through an outside wall or runs through an unheated space. If temperatures have been at or below 0°C, pour warm (not boiling) water along its length or apply a warm cloth until you hear gurgling — that means it has cleared. Never use a naked flame.
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the boiler's front panel. Cold-system pressure should sit between 1.0 and 1.5 bar. If it reads below 1.0 bar, use the filling loop (the braided hose with one or two valves beneath the boiler) to top up slowly until the gauge reaches 1.2–1.3 bar, then close the filling loop valves fully. Do not overpressurise beyond 1.5 bar.
- Bleed any radiators that feel cold at the top DIY safe
Trapped air in the system reduces water circulation and can contribute to localised overheating. Use a radiator bleed key to release air from the bleed valve at the top corner of each affected radiator until water appears, then re-check and top up system pressure if it dropped slightly during bleeding.
- Reset the boiler once (twice at most) DIY safe
On most Main Eco Compact models, hold or rotate the selector dial to the reset position and hold for approximately five seconds until the fault code clears. Allow the boiler to fire up and monitor for several minutes. If E110 returns promptly, do not keep resetting — repeated resets without addressing the root cause can stress components further. A maximum of two or three resets is reasonable before calling an engineer.
- Listen and feel around the pump housing DIY safe
With the boiler running (if it will), place your hand carefully near the pump body (usually a cylindrical component with electrical connections on the boiler's underside or side). A working pump is warm and you may feel a faint vibration. A pump that is stone cold and silent, or very hot with no vibration, suggests a problem. Report this observation to your engineer — it is useful diagnostic information, but do not attempt to open or adjust the pump yourself.
- Contact a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
If the E110 returns after the checks above, a Gas Safe engineer needs to investigate. They will typically test the circulation pump for seizure and correct flow rate, inspect and clean or power-flush the heat exchanger if sludge or scale is suspected, check the overheat thermostat and NTC sensors for correct operation and replace any that have failed, inspect the PCB for damage or incorrect output signals, and verify flue integrity. Always ask to see your engineer's Gas Safe ID card before work begins.
Parts you may need
- Circulation pump (Grundfos UPS2 or equivalent) · from £85
- Overheat safety thermostat · from £25
- NTC temperature sensor · from £20
- Magnetic system filter (e.g. Adey MagnaClean 22 mm) · from £75
- PCB (printed circuit board) · from £180
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £180–£380, depending on the underlying cause.
Frequently asked questions
Can I just keep resetting the boiler when the E110 comes back?
Resetting once or twice to see whether the fault clears is fine, but repeatedly resetting without fixing the underlying problem is not a good idea. The overheat thermostat is tripping for a reason, and forcing the boiler to fire repeatedly while the root cause persists can accelerate wear on the heat exchanger, pump, and PCB. If the E110 returns within a short time of resetting, call a Gas Safe engineer rather than resetting again.
How much does it cost to fix an E110 on a Main boiler in the UK?
Most homeowners pay somewhere between £180 and £380 all-in, depending on what is causing the fault. A pump replacement (parts and labour) typically comes to £200–£350. A power flush to clear a sludged heat exchanger usually costs £300–£400 for a typical domestic system. Simple sensor or thermostat replacements are at the lower end. A full PCB replacement is less common but can push costs higher — if your engineer suspects the PCB is at fault, it is worth getting a quote alongside the cost of a new boiler, particularly if the boiler is over ten years old.
Why does my Main boiler show E110 only in very cold weather?
Cold-weather E110 faults are often caused by a frozen condensate pipe. When this pipe freezes, it blocks the route for condensate to drain away, which eventually causes the boiler to lock out. Thawing the pipe with warm water usually clears the fault immediately. If your condensate pipe runs outside or through an unheated garage, ask your engineer about adding pipe lagging or rerouting it internally to prevent future freezing.
Is the E110 fault code dangerous?
The E110 itself is a safety mechanism working correctly — the boiler has detected excessive heat and shut itself down to prevent damage or a hazard. It should not be ignored, but it is not an immediate emergency in the way a gas leak or carbon monoxide alarm would be. Do not attempt to bypass or defeat the overheat thermostat. If you smell gas or your CO detector sounds, leave the property, leave doors open, and call the National Gas Emergency Service on 0800 111 999 immediately.