The Main Eco Compact is a popular and reliable boiler, but like all modern systems, it relies on built-in safety components to protect the appliance. One of the most important of these components is the safety overheat thermostat (often referred to as the overheat stat). When this part activates or fails, it can trigger several common fault codes — sometimes without any obvious warning.

This guide explains what the overheat stat does, why it activates, the fault codes it causes, and how a qualified engineer diagnoses and fixes the issue. This article is written for homeowners across Surrey, West Molesey, Kingston, Richmond, Wimbledon, and London who want a clear understanding of why their boiler may be cutting out.

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What Does the Overheat Stat Do?

The safety overheat thermostat continuously monitors the primary water temperature inside the boiler — specifically, the water circulating through the heat exchanger.

If the boiler temperature rises beyond a safe limit, the overheat stat immediately shuts down the burner and forces a lockout. This prevents:

• Internal overheating
• Damage to the heat exchanger
• Dangerous operating conditions

In short, when the stat activates, it is doing exactly what it was designed to do — protecting both the boiler and the property.

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Why Do Boilers Overheat?

A Main Eco Compact boiler will overheat when the heat cannot leave the heat exchanger fast enough. Common causes include:

• Poor system circulation
• Pump failure or weak pump speed
• Air trapped in the system
• Blocked or restricted pipework
• Limescale buildup inside the heat exchanger
• Low system pressure
• Stuck or faulty diverter valve

Because the boiler cannot circulate heat correctly, temperatures rise quickly until the overheat stat steps in.

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Fault Codes Caused by an Overheat Stat Issue

When the overheat stat activates — either because the boiler is genuinely overheating or because the stat itself has failed — the boiler may display one or more of the following Main Eco Compact fault codes. Each code point points to a specific underlying issue that engineers can use to diagnose the root problem.

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E110 – Primary Safety Thermostat Operated (Overheat Lockout)

This is the most common overheat-related fault on the Main Eco Compact.

It means the internal water temperature has exceeded the safe limit.
Typical causes include:

• Poor water circulation
• The pump partially or fully failed
• Low system pressure
• Air trapped in the heat exchanger
• Blocked or scaled heat exchanger
• Stuck diverter valve (on combi models)

When E110 appears, the boiler locks out immediately to prevent heat exchanger damage.

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E130 – Flue / Temperature Safety Trip

While this code often relates to flue temperature issues, it can also occur when internal temperatures rise too quickly, triggering the same overheat circuit.

This is frequently linked to:

• Blocked heat exchanger
• Flue temperature rise due to restricted water flow
• PCB reacting to rapid internal overheating

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E125 – Circulation Fault

Not a direct overheat stat error — but poor circulation almost always leads to overheating.

This code appears when:

• The pump is weak
• Airlock restricting flow
• System sludge is slowing circulation
• Blocked pipes or heat exchanger passages

If left unresolved, E125 can escalate into an E110 lockout.

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E109 – Flow Thermistor / Overheat Detection Issue

This code is commonly triggered when the temperature rises too quickly, often caused by:

• Limescale in the heat exchanger
• Faulty thermistor giving incorrect readings
• Low water content in the system
• Blocked plate heat exchanger (on combis)

This code is sometimes misdiagnosed as a sensor fault, but circulatory issues are very often the cause.

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E133 – Flame Failure (Sometimes Seen After Overheat Events)

This isn’t directly caused by the overheat stat — but an overheat shutdown can cause flame loss or delayed ignition, which then triggers E133 shortly after.

It appears when:

• The boiler overheats and shuts down mid-burn
• Ignition cannot restart due to high temperatures
• The burner chamber gets too hot

If E133 follows an E110 or E125, the real issue is overheating, not a gas supply fault.