What Does Pump Overrun on Boiler Mean? Causes & Fixes

James Elston
Written by James Elston
Updated on 1st December 2023
Posted on 30th November 2023
Topic: Boiler Advice
What Does “Pump Overrun” Actually Mean

What is Pump Overrun on a Boiler, and What Does It Do?

Can you hear that humming in the cupboard where your boiler lives even when you have switched it off or stopped using your hot water? Here’s what that sometimes annoying noise is…

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Gas boilers play an important role in keeping our homes warm and comfortable, especially during the bitterly colder months. They are responsible for keeping us warm by heating water, which is then distributed throughout the house to provide warmth.

However, many people may not be familiar with the term “pump overrun” and its significance in the efficient operation of a gas boiler. In this enthralling article, we will delve into the concept of pump overrun, how it works, and why it is essential for maintaining optimal performance and energy efficiency.

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Understanding What Boiler Pump Overrun Actually Is

What is Pump Overrun?

Whilst it can be slightly concerning to us common folk, pump overrun is a perfectly normal function where the boiler’s pump continues to operate briefly after the heating or hot water has been used”

Its ultimate purpose is to allow any remaining heat in the system to dissipate and prevent potential damage to the boiler’s components.

During this steady hum of the pump overrun, the central heating pump circulates the hot water through a bypass valve, allowing it to cool down until the heat is dissipated. This helps avoid the risk of water boiling inside the boiler, which could harm its parts or cause a boiler explosion.

Pump overrun typically lasts around 5 to 10 minutes, but the duration can vary depending on the specific system and power used.

The pump overrun function is controlled by either a pump overrun thermostat or a timer. The thermostat keeps the pump running until a specific temperature is reached, while the timer allows the pump to operate for a predetermined period. The need for a bypass circuit depends on the type of heating system you have, such as an S-plan or Y-plan heating system.

Pump Circuit Board (PCB)

It’s important to note that not all boilers require pump overrun, as some larger and more robust boilers can handle the residual heat without this feature being installed.

If you observe that your central heating pump is running continuously for hours or excessively, it could indicate a problem with the system. Potential causes for prolonged pump overrun include a faulty pump overrun, faulty thermostat, or timer, or a damaged Pump Circuit Board (PCB).

If you encounter such issues with your boiler, it is advisable to contact a Gas Safe registered engineer for repairs. Attempting to fix the problem yourself can pose risks to your home and health. Professionals have the expertise to diagnose and safely address the repairs.

How Does Pump Overrun Work?

On most modern replacement boilers in the UK, when the boiler receives a signal to stop heating the water, it activates the pump overrun function. This function operates by keeping the pump running for a pre-determined amount of time, typically between one and five minutes.

During this time, the pump continues circulating the heated water, utilising the remaining heat energy stored in the heat exchanger and distributing it throughout the heating system.

Causes of Faulty Pump Overrun & Effects

Insufficient Pressure Relief:

Sometimes the pump overrun noise can give us a headache (very unlikely in real terms)

A lack of pressure relief valves or improperly adjusted valves can lead to pump overrun. These valves are designed to release excess pressure when the water flow stops, allowing the pump to shut off. If the pressure relief valves are not present or not set correctly, the pump may continue to operate even when it’s not necessary.

Inadequate Pump Sizing:

Pump overrun can occur if the pump is not properly sized for the system it serves. If the pump is too powerful for the demand or if the system has been modified without adjusting the pump capacity accordingly, the pump may continue to run longer than needed to compensate for the imbalance.

Malfunctioning Pressure Switch:

The pressure switch is one of the most important components that control when the pump turns on and off. If the pressure switch is faulty or out of calibration, it may fail to detect the correct pressure levels, leading to a pump overrun. This can be caused by wear and tear, electrical issues, or improper adjustment.

Damaged PCB – Faulty Pump Circuit Board

  • Inspect the damaged PCB for physical damage after ensuring the power supply is turned off.
  • Depending on the extent of the damage, you have two options:
    • Purchase a replacement PCB from the manufacturer.
    • Attempt to repair the pump circuit board yourself.
  • If you choose to repair it yourself, carefully remove the damaged components and find suitable replacements.
  • Install the new components accurately and ensure proper soldering.
  • Test the circuit and address any issues that arise.

Think you have a faulty pump? Read our guide here to central heating pump costs for replacements in the UK.

Boiler Pump Overrun Solutions and Prevention

Now that we’ve identified some common causes of pump overrun, let’s explore practical solutions to tackle this issue and prevent it from recurring:

Check and Adjust Pressure Relief Valves:

Ensure that pressure relief valves are present and properly adjusted to release excess pressure when the hot water flow stops. Regularly inspect and maintain these valves to ensure their effectiveness.

Proper Pump Sizing and System Design:

Consult a professional to ensure that your pump is correctly sized for the demand and that your plumbing system is designed to accommodate any modifications or expansions. A well-matched pump and system will minimise the chances of pump overrun.

Regular Maintenance

Regular Maintenance and Calibration:

Schedule regular maintenance for your pump system, including checking and calibrating the pressure switch. Keeping the pressure switch in optimal condition will ensure accurate detection of pressure levels, reducing the likelihood of pump overrun.

Check Valve Maintenance:

Regularly inspect and clean check valves to prevent debris buildup that could hinder their proper functioning. Replace faulty check valves promptly to avoid pump overrun caused by water flowing back into the system.

The Importance of Pump Overrun in a Boiler

Energy Efficiency

One of the key reasons why pump overrun is important is its impact on energy efficiency. By allowing the pump to run for a short period after the boiler stops firing, pump overrun ensures that all the heat produced is effectively utilised and not wasted.

This means that more of the energy used to heat the water is transferred to the living spaces, reducing energy consumption and lowering utility bills.

Preventing Boiler Cycling

Boiler cycling refers to the frequent turning on and off of the boiler as it tries to maintain a desired temperature. This constant cycling can lead to increased wear and tear on the boiler components, reducing its lifespan and potentially causing breakdowns.

Pump overrun helps prevent boiler cycling by circulating the remaining heat and maintaining a more stable temperature in the heating system. This reduces the frequency of boiler cycling, prolonging its life and improving overall reliability.

Common Types of Pump Overrun Controls

Time Delay Controls

Time delay controls are a common type of pump overrun control. They operate by activating the pump to run for a specific duration after the boiler switches off. The duration can be adjusted according to the specific requirements of the heating system.

Temperature Differential Controls

Temperature differential controls use sensors to monitor the temperature of the water in the heating system. Once the desired temperature is reached, the pump is allowed to run for a set period, ensuring efficient heat distribution. These controls are particularly useful in systems with variable flow rates.

Smart Controls

With advancements in technology, smart controls have emerged as an option for managing pump overruns. These controls utilise sensors and algorithms to optimise the pump overrun period based on factors such as heat demand and outdoor temperature. Smart controls offer greater precision and adaptability, resulting in improved energy efficiency.

Benefits of Pump Overrun

Reduced Energy Consumption

One of the significant benefits of pump overrun is its contribution to reduced energy consumption. By maximising the utilisation of heat energy, pump overrun minimises wastage and ensures that the heat produced by the boiler is effectively delivered to the living spaces. This leads to lower energy bills and a more environmentally friendly heating system.

Extended Boiler Lifespan

Boilers are a significant investment, and it’s important to ensure their longevity. Pump overrun plays a role in extending the lifespan of a boiler by reducing the frequency of boiler cycling and minimising stress on the components. By maintaining a more stable temperature and preventing abrupt temperature fluctuations, pump overrun helps to keep the boiler in good condition for longer.

Enhanced Comfort and Convenience

Pump overrun contributes to enhanced comfort and convenience in a heating system. By circulating the remaining heat, it ensures a more even distribution of warmth throughout the living spaces. This helps eliminate cold spots and provides consistent heating, creating a more comfortable environment for occupants.

Pump Overrun and Radiators

Ensuring Optimal Heat Distribution

Pump overrun is particularly important when it comes to maintaining optimal heat distribution in a heating system with radiators. Radiators rely on the circulation of hot water to release heat into the rooms. Pump overrun allows the heated water to reach all the radiators, ensuring even heat distribution and preventing any areas from being significantly colder than others.

Minimising Cold Spots

Cold spots can be a common issue in heating systems, especially in larger properties or those with complex layouts.

Pump overrun helps address this problem by constantly continuing the circulation of heated water, reaching all areas of the system and minimising the occurrence of cold spots. This ensures that every room receives adequate heat and eliminates discomfort caused by uneven heating.

Troubleshooting Pump Overrun Issues

Pump Overrun Not Working

If the pump overrun function is not working as intended, it can result in inefficient heat distribution and energy wastage. Possible causes of a non-functioning pump overrun include faulty controls, wiring issues, or sensor malfunctions. In such cases, it is advisable to contact a qualified Gas Safe engineer to diagnose and rectify the problem.

Continuous Pump Operation

Sometimes, the pump may continue to run continuously without stopping, even after the boiler has shut down. This can be caused by a faulty pump overrun control or an issue with the boiler’s control system.

Continuous or a noisy central heating pump in operation can lead to unnecessary energy consumption and increased wear on the pump. Professional assistance should be sought to investigate and resolve this issue.

Inconsistent Heating

If you notice inconsistent heating or areas of your home that are not receiving sufficient warmth, it could indicate a pump overrun problem. Insufficient pump overrun time or an improperly functioning control system may be to blame. An experienced Gas Safe registered engineer can assess the situation and determine the best course of action to rectify the issue.

Maintenance and Care

Regular Boiler Servicing

To ensure the optimal performance of a boiler and its associated pump overrun function, getting an annual boiler service is essential. Professional and complete boiler servicing involves thorough inspection, cleaning, and adjustment of components to maintain efficiency and reliability. It is recommended to schedule annual boiler servicing to identify and address any potential issues promptly.

Checking Pump Overrun Functionality

As a homeowner, you can periodically check the functionality of the pump overrun feature. Start by turning on the heating system and observing whether the pump continues to run for a short period after the boiler switches off. If you notice any irregularities or the pump does not run at all, it is advisable to contact a qualified heating engineer for further investigation.


Pump overrun is an integral aspect of boiler operation that ensures the efficient distribution of heat and maximises energy efficiency. By allowing the pump to continue circulating heated water after the boiler stops firing, the pump overrun prevents wastage of residual heat and minimises boiler cycling.

This results in reduced energy consumption, extended boiler lifespan, and enhanced comfort for occupants.

Regular boiler maintenance and care, along with periodic checks of pump overrun functionality, are essential for optimal performance. Remember to consult a professional if you encounter any issues with pump overrun to ensure the continued efficiency of your heating system.


How does pump overrun save energy?

Most new boilers in the UK have this feature as the pump overrun saves energy by utilising the remaining heat produced by the boiler and distributing it throughout the heating system, reducing wastage and maximising efficiency.

Can the pump overrun be adjusted?

Yes, the duration of the pump overrun can be adjusted to suit the specific requirements of the heating system. This can be done through the controls or settings of the boiler.

What causes pump overrun to malfunction?

Pump overrun malfunctions can be caused by faulty controls, wiring issues, sensor malfunctions, or problems with the boiler’s control system. Consulting a professional is recommended for proper diagnosis and repair.

How often should a boiler be serviced?

Boilers should be serviced annually to ensure their optimal performance, efficiency, and safety. Regular servicing helps identify and address any potential issues before they escalate.

Can pump overrun be retrofitted to an existing heating system?

Yes, in many cases, pump overrun functionality can be added to an existing heating system through the installation of compatible controls or by upgrading to a more advanced heating control system. It is best to consult a heating engineer to determine the feasibility and best options for retrofitting pump overrun

Written by
James Elston
James Elston Director Of Boiler Central
Posted on: 30th November 2023
Topic: Boiler Advice

James Elston is our resident boiler replacement and heating expert here at Boiler Central. With over 20 years experience in the boiler installation industry, James ensures that he knows everything there is about our Gas Safe boiler installations, energy saving and home heating solutions. This can be from simply procuring the latest best combi boilers, to reviewing and ensuring that Boiler Central maintains the highest standards across our boiler installation company.

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