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Witten by Bernard Morgan (Engineer) & Adam Apperley (Marketing). Published: February 16, 2024. Updated: March 15, 2024

Understanding Boiler Pump Overrun

Noticing your boiler pump still running after you’ve turned off the heating can cause concern. However, this often indicates a ‘boiler pump overrun.’ This is a standard feature in modern boilers designed to manage residual heat and not a sign you need to replace you boiler.

We’ll quickly go through pump overrun and what it means for your boiler below.

What Is Pump Overrun

Pump overrun is a boiler’s way of cooling down and expelling leftover heat. This function is typical in modern boilers.

It generally takes about five to ten minutes to complete after the boiler stops heating your home or hot water. It ensures the boiler and its components are safeguarded against excessive heat.

Why Pump Overrun Occurs

Boiler pump overrun | BoilerhutThe pump overrun process helps circulate residual hot water through the system, allowing it to cool gradually. This can happen in system, regular or combi boilers.

It prevents boiling water within the heat exchanger, reducing potential damage. Systems with a bypass valve aid in managing the circulation of this residual heat.

Extended Pump Overrun

A pump overrun lasting beyond ten minutes could signify a problem. Common culprits include malfunctioning thermostats or timers, which can disrupt normal operation. Faulty pumps or wiring issues, especially following new boiler installations, can lead to prolonged overrun.

Diagnosing Constant Overrun Conditions

  • Compromised Pump Circuit Board (PCB): The PCB regulates water flow and pressure. A malfunction here can result in incorrect signals about the boiler’s operation. Professional assessment is advised to avoid risking damage.
  • Faulty Thermostat or Timer: These components are integral to heating control. If timers fail to operate or thermostats provide inaccurate readings, they may cause continuous overruns. Signs include erratic temperature control, unusual noises, or the inability to follow heating schedules.
  • Wiring Issues: Incorrect installation can lead to wiring problems. This could make the boiler and pump operate nonstop due to miscommunication about the system’s heat levels.

Resolving Pump Overrun Problems

Addressing issues with your boiler pump overrun may require Gas Safe expertise. It’s crucial, and a legal requirement, to engage a Gas Safe registered engineer for diagnostics and repairs. They can accurately pinpoint the problem, ensuring your heating system returns to normal operation without risking safety or efficiency.


While pump overrun is a standard and necessary feature for managing residual heat in modern boilers, extended operation indicates underlying issues.

Recognising the signs and understanding the potential causes can help homeowners respond appropriately. However, professional intervention is essential for safe and effective resolution, ensuring the longevity and performance of your heating system.

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