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Last updated: 1st July, 2024


What is Boiler Pump Overrun? - Functions, Issues, and Fixes

What is Boiler Pump Overrun? - Functions, Issues, and Fixes

Key takeaways

  • Pump overrun helps cool your boiler after it switches off.
  • Faulty components can cause the pump to run constantly.
  • Recognising signs early can prevent costly repairs.

Pump overrun in boilers is a feature that helps cool down the system and manage residual heat. It's essential for maintaining the efficiency and longevity of the boiler.

Many homeowners are often puzzled when they encounter the term pump overrun related to their boiler systems. This feature, found in modern boilers, plays a crucial role in maintaining the efficiency and safety of the heating system. Pump overrun refers to the continued operation of the boiler's pump for a short period after the heating is turned off, helping to dissipate any leftover heat.

When the boiler stops heating your home or hot water, the pump keeps running for about five to ten minutes. This process helps prevent the boiler components from overheating, which can extend the lifespan of your heating system. Faulty pressure switches or improperly installed pumps can lead to issues, causing the pump to run constantly, which might need professional attention to fix.

Identifying signs of pump overrun issues can save you from expensive repairs. Symptoms like the pump running longer than usual or not turning off at all indicate potential problems. Knowing these signs can help in timely repairs and maintaining an efficient heating system.

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Understanding Pump Overrun in Boilers

Pump overrun in boilers is a feature that helps cool down the system and manage residual heat. It's essential for maintaining the efficiency and longevity of the boiler.

What Is Pump Overrun?

Pump overrun is a function designed to keep the central heating pump running for a short period after the boiler has been turned off. This heating system feature helps to mitigate residual heat by continuing to circulate water through the system.

This method prevents the boiler components from overheating. Typically, the pump overrun lasts between five to ten minutes. Modern boilers frequently incorporate this function, whereas older models might not.

How does a pump overrun in boiler work?

When the heating or hot water system is turned off, the pump overrun engages to keep the central heating pump operating for a few more minutes. This continued operation helps prevent the heat exchanger and other vital parts from becoming too hot.

During this period, the water keeps circulating, aiding in a gradual cool-down process. This mechanism is particularly useful for maintaining an even temperature across the whole system. It ensures the boiler is protected by moderating excess heat and preventing potential damage.

In essence, pump overrun contributes to the overall efficiency and durability of the boiler, ensuring it remains in good working order. Proper functioning of the pump overrun can prevent common heating issues and extend the life of your heating system.

Signs of Pump Overrun Issues

Pump overrun issues can lead to several noticeable problems with your boiler. Key signs include a lack of hot water or heating, unusual noises from the boiler, water leakage near the unit, and radiators failing to heat up properly.

No Hot Water or Heating

One major sign of pump overrun issues is the absence of hot water or heating. When the pump continues to run after turning off the boiler, it might circulate cold water instead of hot. This means you'll find no hot water coming out of your taps, and your home might remain cold despite the boiler being on.

Frequent occurrences of no hot water can strongly indicate a pump overrun problem. If you notice these symptoms, an engineer should check the pressure switch and other related components to ensure they are functioning correctly.

Strange Noises from the Boiler

Strange noises coming from your boiler can be another sign of pump overrun issues. These noises might include gurgling, banging, or whistling, and they occur due to the continued operation of the pump. The persistent action of the pump can cause air to get trapped in the system, leading to these unsettling sounds.

If you hear any unusual noises, it’s important to get the boiler inspected by a professional. Ignoring these sounds can potentially lead to more serious damage to the boiler in the long run.

Leaking Water Near the Boiler

Discovering water leakage near the boiler is a cause for concern and can be a sign of pump overrun issues. When the pump runs longer than needed, it can create pressure imbalances within the system. This may lead to water escaping from fittings, joints, or the boiler itself.

Leaking water can damage surrounding areas and affect the boiler’s efficiency. If there is any sign of water leaking around the boiler, you should call an engineer straight away to diagnose and fix the problem.

Radiators Not Heating

Radiators that remain cold despite the boiler being on might indicate a pump overrun issue. The pump's extended running time can cause uneven water circulation, resulting in some radiators not receiving enough hot water to heat up properly.

Inconsistent radiator heating can make certain rooms colder than others. It's important to bleed your radiators regularly, but if the problem persists, professional assistance may be required to check for pump overrun and related issues.

Addressing these signs promptly can help maintain your boiler's efficiency and increase its lifespan. Regular maintenance and timely repairs are essential in preventing pump overrun problems from escalating.

Common Causes for Pump Overrun Defects

Pump overrun defects in boilers can result from various issues such as faulty thermostats, damaged PCBs, debris in the system, incorrect timer settings, or bypass valve problems. Addressing these problems can help ensure that your boiler operates efficiently and safely.

Faulty Pump Overrun Thermostat

A faulty thermostat can lead to pump overrun problems in boilers. The thermostat is responsible for regulating the temperature of the water in the system. If it malfunctions, it may not detect when the water has cooled sufficiently, causing the pump to run unnecessarily. Regular maintenance and timely replacement of faulty thermostats can prevent this issue. Testing the thermostat’s accuracy and ensuring it is properly calibrated are crucial steps in maintaining its performance.

Damaged PCB or Wiring Issues

The Printed Circuit Board (PCB) and wiring are central to the boiler's function. If the PCB is damaged or the wiring has issues, it can miscommunicate signals, leading to pump overrun. Common causes of PCB damage include power surges, wear and tear, and moisture exposure. Regular inspections of the PCB and wiring can help detect any issues early. Replacing damaged components promptly is vital to avoid prolonged pump operation that can strain the system and reduce its lifespan.

Debris and Sludge in the System

Debris and sludge buildup inside the boiler can disrupt normal operations and lead to pump overrun. Sludge, which forms from rust and dirt, can clog the system, forcing the pump to work harder and longer. Flushing the boiler system regularly helps remove accumulated debris and sludge. Installing a magnetic filter can also capture particles and prevent them from circulating in the system. Monitoring the system’s cleanliness is essential for preventing pump overrun caused by blockages.

Incorrect Timer Settings

Inaccurate timer settings might cause the pump to run longer than necessary. The timer controls when the boiler turns on and off, as well as the duration for which the pump operates after the boiler shuts down. If set incorrectly, it can lead to prolonged pump activity. Checking and adjusting the timer settings to match the boiler’s operational requirements can resolve this issue. It is beneficial to consult the boiler’s manual or a professional to ensure the settings are accurate.

Problems with Bypass Valve or Flow Rate

The bypass valve regulates the flow of water in the system, ensuring the pump does not overheat due to restricted flow. If the bypass valve is faulty or the flow rate is incorrect, it can cause the pump to run excessively as it tries to maintain the correct pressure. Ensuring the bypass valve is in good working condition and the flow rate aligns with the system’s specifications is crucial. Regular checks and maintenance of the valve and flow rates can help prevent pump overrun defects.

Troubleshooting Pump Overrun

Troubleshooting pump overrun involves inspecting various components, including the pump circuit board, pump overrun thermostat, timer, temperature control, and bypass valve. Addressing issues with these parts can help ensure the boiler runs efficiently.

Inspecting the Pump Circuit Board

The pump circuit board controls the pump’s operations. Checking for faults or damage on this board is crucial. Start by turning off the power to the boiler for safety. Open the boiler’s casing to access the circuit board. Look for any burnt-out components or loose connections.

A multimeter can be used to test electrical continuity. If there are signs of wear or damage, the circuit board might need replacement. Regular maintenance can prevent many issues related to the pump circuit board and enhance energy efficiency.

Checking the Pump Overrun Thermostat

The pump overrun thermostat ensures the pump continues to run until the residual heat dissipates. To check this component, first confirm the boiler is turned off and allow it to cool. Locate the thermostat, which might require referring to the boiler's manual.

Using a thermometer, verify if the thermostat is responding correctly to changes in temperature. If it fails to maintain temperature or shuts off prematurely, it may require adjustment or replacement. Correctly functioning thermostats are vital for maintaining safe operation and energy efficiency.

Evaluating Timer and Temperature Control

The timer and temperature control settings influence when and how long the boiler runs. Malfunctions in these settings can lead to pump overrun. First, examine the timer settings to ensure they align with your heating schedule. Incorrect settings can cause the pump to run unnecessarily.

Next, check the temperature control. Use the manual to adjust settings if they are not within the ideal range for your heating needs. Faulty timers or controls may need replacement to restore proper function and efficiency to the heating system.

Assessing the Bypass Valve

The bypass valve allows water to circulate within the boiler when the pump is off, preventing pressure build-up. Malfunctions in this valve can lead to pump overrun. Start by locating the bypass valve in your boiler system.

Inspect the valve for blockages or signs of wear. Use the boiler’s manual as a guide. Proper adjustment is necessary for the valve to function correctly. If the valve is blocked or not set correctly, it should be cleaned or adjusted as per the manual's instructions. This valve plays a key role in maintaining proper pressure and preventing overrun, thereby improving energy efficiency.

How long does a pump overrun last in a Boiler?

Pump overrun is a common feature in modern boilers. It helps to cool down the system by circulating water even after the heating or hot water is turned off.

Usually, pump overrun lasts for 5 to 10 minutes. This duration allows the boiler to safely expel residual heat.

Some larger or older boilers might not need this feature, as they are built to handle excess heat differently.

If the pump overrun exceeds this time regularly, it could point to issues like a faulty pressure switch or other component problems. Regular maintenance can help identify and fix such problems to ensure optimal boiler performance.

How to Fix Pump Overrun Boiler: Professional Repairs and Maintenance

Fixing a pump overrun boiler usually involves regular maintenance, professional repairs, or considering a replacement if needed. The key is to address the issue early to prevent more serious problems.

Annual Boiler Service for Prevention

An annual boiler service is crucial for preventing pump overrun issues. A gas safe registered engineer will inspect the boiler and its components. They will check for wear and tear, ensuring everything is working correctly. Regular servicing can identify faults before they become severe, like a worn-out pump or a faulty pressure switch.

During the service, the engineer will clean, lubricate, and test the pump overrun function. This helps maintain the boiler's efficiency and extends its lifespan. Keeping up with annual services can save homeowners from unexpected repair costs.

Options for Boiler Repair or Replacement

When a boiler experiences pump overrun issues, consulting a professional for boiler repair is often the first step. A heating engineer can diagnose the problem, whether it’s a mechanical fault or an electronic issue. They may replace or adjust components like the pressure switch or the pump itself.

If the boiler is old, repairs might not be cost-effective. In such cases, considering a replacement boiler is a better option. Modern boilers are more efficient and come with improved pump overrun features, ensuring better performance and reliability.

Fixed price repair services provided by many companies can give homeowners peace of mind, knowing the costs upfront without unexpected charges.

How Much Does a New Pump Overrun Cost?

The cost of a new pump overrun can vary based on the boiler model and the complexity of the installation. On average, a replacement pump costs between £100 and £300, excluding labour. The fixed price for installation by a gas safe registered engineer might add another £100 to £200.

Investing in a high-quality pump can ensure better performance and longevity. It’s recommended to get quotes from multiple professionals to compare prices and services. Proper installation by a qualified engineer is essential to avoid future issues and ensure safety.

Improving Boiler and Heating Efficiency

Maintaining and enhancing boiler and heating system efficiency is essential for energy savings and long-term performance. Key strategies involve installing magnetic system filters, upgrading to modern boilers, and following best practices during installation.

Role of Magnetic System Filters

Magnetic system filters play a crucial role in maintaining boiler efficiency. They remove metallic debris from the heating system, preventing blockages and corrosion. This not only helps in reducing wear and tear on the boiler but also ensures smooth and uninterrupted operation.

When these filters are correctly installed, they prolong the lifespan of the boiler and help maintain optimal performance. Regular cleaning of these filters is necessary to keep them functional. This ensures that the boiler runs efficiently and can heat your home effectively.

Benefits of Upgrading to a New Boiler

Upgrading to a new boiler can significantly improve energy efficiency. Modern boilers are designed to be more efficient than older models, often with higher energy ratings. They consume less fuel, reducing energy bills and carbon footprint.

A new boiler comes with advanced controls and better heat exchangers. These features ensure better heat distribution and less energy loss. Additionally, newer models are more compact and quieter, making them a better fit for most homes.

Investing in a new boiler also decreases the likelihood of frequent breakdowns and expensive repairs. With improved technology, modern boilers can adapt to varying heating demands more efficiently.

iHeat Boiler Service

At iHeat all of our subcontracted engineers are Gas Safe registered and can service your boiler swiftly and safely. Prices of an expert boiler service with iHeat start from £99.These figures might fluctuate regionally and can also depend on the specific type of boiler you own, such as a combi, gas, or oil boiler. 

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iHeat New Boiler Cost

New boilers can be a daunting purchase for many people as they’re an appliance with a lot of responsibility, providing heat for you and your family is something you want to get right. Boilers aren’t exactly a quickly disposable item either, potentially lasting you a decade.

Effectively, new boiler cost can be split into two segments: the first is the actual boiler itself (unit price), and the second is the cost of the boiler being installed (set up) in your property by an expert engineer. 

Here at iHeat, we remove all of this undue stress and make the decision making process of upgrading to a new heating system, as easy as possible.

New boiler costs can vary depending on a number of factors including their brand, model, fuel, output, warranty, labour and boiler installation type. Typically a new boiler will cost between £1,845 and £3,500, below is a list of average boiler installations offered by iHeat (guide only).

Installation Type

Price (inc VAT)

Combi to combi swap


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System to combi conversion


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New boiler install


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Back boiler to a combi


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System to system


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Installation Best Practices for Efficiency

Proper installation is vital for achieving the highest efficiency from your boiler. Ensuring that the boiler is fitted in the best location, with adequate ventilation, can make a significant difference.

Qualified professionals should handle the installation to ensure compliance with safety standards and efficiency guidelines. Correctly sized boilers tailored to the home’s heating needs prevent unnecessary energy consumption.

Moreover, regular maintenance post-installation is essential for keeping the system running at peak efficiency. This includes annual servicing, checking for leaks, and ensuring that all components are free of obstructions. Proper installation and maintenance practices collectively enhance overall system efficiency and longevity.

Understanding Heating System Configurations

Different heating system configurations can impact the efficiency and performance of your boiler. Let's explore some common systems, focusing on Y-Plan and S-Plan setups, as well as the distinctions between combi boilers and conventional systems.

Y-Plan and S-Plan Systems

Y-Plan and S-Plan systems are widely used in central heating setups. A Y-Plan system uses a three-port valve that directs hot water either to the heating circuit, the hot water cylinder, or both. This makes it a flexible and relatively simple system to install.

S-Plan systems, on the other hand, utilise two separate two-port valves: one for the central heating and another for the hot water cylinder. This setup allows for more precise control over heating and hot water, achieving better energy efficiency and comfort.

Y-Plan systems are often easier to service due to fewer components, while S-Plan systems offer enhanced control and efficiency. Choice between these depends on individual needs and existing system compatibility.

Combi Boilers vs. Conventional Systems

Combi boilers, or combination boilers, heat water directly from the mains. They do not require a separate hot water cylinder or cold water storage tank. This makes combi boilers ideal for homes with limited space. They provide hot water on demand, which ensures high efficiency and convenience.

Conventional systems, also known as regular boilers, require both a cylinder and a storage tank. These systems are more suitable for larger homes with higher hot water demands. They can simultaneously supply multiple outlets without a drop in water pressure.

Choosing between a combi boiler and a conventional system depends on factors like space availability, hot water demand, and installation costs. Combi boilers are often preferred for smaller homes, while conventional systems are better for larger properties with significant hot water usage.

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Frequently Asked Questions

How to fix pump overrun Ideal boiler?

First, check the pressure switch. If it’s faulty, replace or recalibrate it. Ensure all electrical connections are secure. If the issue persists, consult a professional for a thorough inspection.

What does pump overrun mean on my boiler?

Pump overrun is a feature where the pump continues to run for a few minutes after the boiler has been turned off. This helps cool down the boiler by dissipating residual heat.

What is the pump overrun phase?

During the pump overrun phase, the pump circulates water to distribute leftover heat. This phase usually lasts between five to ten minutes after the boiler stops heating your home or hot water.

Where is the pump overrun thermostat?

The pump overrun thermostat is typically located within the boiler control panel. It monitors the temperature and controls the pump's operation to ensure proper cooling.

Why is the pump running when the heating is off?

The pump runs after the heating is off to cool down the boiler by circulating water. This is normal and prevents overheating. If it runs continuously, it could indicate a problem with the thermostat or pump relay.

Why is the pump gone on my central heating?

If the pump is not running at all, it may be due to a power failure, a broken pump, or an issue with the wiring. Check for faulty electrical connections and ensure the pump has power.

Where is the boiler pump located?

The boiler pump is generally located near the boiler unit itself. It’s usually mounted on the pipework that distributes hot water or heating through the home.

Stephen Day profile photo
Article by
Stephen Day | Co-founder
Gas Safe registered and FGAS certified engineer with over 20 years experience in the heating and cooling industry.