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Last updated: 18th July, 2024


What Should Combi Boiler Pressure be?

What Should Combi Boiler Pressure be?

Key takeaways

  • Combi boilers should have a pressure of around 1.5 bar when cold.
  • Regular pressure checks can prevent malfunctions and extend boiler life.
  • Maintaining correct pressure helps avoid costly repairs and leaks.

Combi boiler pressure is a crucial factor for maintaining efficiency and safety. For most combi boilers, the ideal pressure range is between 1 and 2 bars.

Understanding the correct pressure for your combi boiler is crucial for ensuring it runs smoothly and efficiently. Combi boilers, popular in the UK for their ability to provide both heating and hot water, should generally maintain a pressure level of around 1.5 bar when the system is cold. This ensures optimal performance and minimises the risk of breakdowns.

The ideal pressure for your combi boiler is typically around 1.5 bar, especially when the system is not operating. If the pressure is too low, the boiler may fail to ignite, while high pressure can lead to leaks and other issues. Regularly checking and maintaining the correct pressure can help prolong the life of your boiler and avoid costly repairs.

UK Homeowners should periodically monitor their boiler’s pressure gauge and know how to adjust it if necessary. By staying informed and proactive, you can ensure your heating system remains reliable throughout the year.

What is a Combi Boiler?

A combi boiler combines heating and hot water in one unit. It saves space and is energy efficient. Homeowners choose it for its convenience and compact design.

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Basics of a Combi Boiler

A combi boiler, short for combination boiler, provides central heating and hot water directly from one unit. It is a gas boiler that takes water directly from the mains supply.

Unlike traditional systems, it does not need a separate hot water storage tank. This makes it a space-saving option for many homes.

The combi boiler heats water instantly when a tap is turned on. This means hot water on demand without waiting. Its compact design makes it easy to fit into small spaces, such as kitchen cupboards.

Advantages of Combi Boilers

Energy Efficiency: Combi boilers are known for their high efficiency. They heat water only when needed, reducing energy wastage and leading to energy savings.

Compact Design: The absence of separate tanks means combi boilers take up less space. This is ideal for smaller properties or homes with limited storage.

Hot Water on Demand: With a combi boiler, there is no waiting time for hot water. It provides a continuous supply, making it convenient for busy households.

Cost-Effective: Installing a combi boiler is typically cheaper than systems requiring hot water storage tanks. Maintenance can also be less expensive due to its simpler setup.

Considerations Before Choosing a Combi Boiler

Property Size and Heating Needs: Combi boilers are best for small to medium-sized homes. Large homes with multiple bathrooms may require a system with a hot water storage tank to meet higher demand.

Water Pressure: The efficiency of a combi boiler depends on mains water pressure. Poor water pressure can affect its performance, providing less optimal heating and hot water.

Installation and Price: Installing a new boiler, particularly a combi boiler, involves upfront costs. The price varies based on the model and installer. It’s essential to consider the total cost, including installation.

Energy Efficiency Ratings: Look for boilers with high energy efficiency ratings to ensure long-term energy savings and lower bills. This is especially crucial in areas with cold climates.

Choosing the right combi boiler requires assessing your specific needs and property characteristics.

What is Boiler Pressure?

Boiler pressure refers to the pressure of hot water in a boiler's heating system. It's measured in bars, with most home boilers operating between 1 and 2 bars.

Boiler pressure ensures the efficient functioning of the heating system. If the pressure is too low, the system may not heat properly. If it's too high, it can damage components.

Key Points to Remember:

  • Normal Pressure Range: 1-2 bars.

  • When Heating Is Off: 1-1.5 bars.

  • When Heating Is On: 1.5-2 bars.

Pressure can change when heating is on or off, so it is important to monitor it regularly. Always refer to the boiler's manual for the recommended pressure level for your specific model.

To check boiler pressure, locate the pressure gauge on the boiler's front. It's a small needle within a semi-circle, often marked with green for normal and red for high and low zones.

What Should Combi Boiler Pressure be?

Combi boiler pressure is a crucial factor for maintaining efficiency and safety. For most combi boilers, the ideal pressure range is between 1 and 2 bars.

When the boiler is off and cold, the pressure should be around 1 to 1.5 barsDuring operation and heating, this can rise to 1.5 to 2 bars.

pressure gauge on the boiler usually shows the current pressure. It often uses green to indicate the normal range and red for low or high pressure zones.

list of key points:

  • 1 to 1.5 bars: When the boiler is off.

  • 1.5 to 2 bars: When the boiler is heating.

  • Below 1 bar: Low pressure, may cause inefficiency.

  • Above 2 bars: High pressure, potentially dangerous.

Maintaining the correct pressure is vital for performance and system longevity. Regularly check the pressure gauge and consult the boiler manual for specific guidelines for your model.

How to Check Combi Boiler Pressure

Checking the pressure of a combi boiler is important for its proper functioning.

  1. Locate the Pressure Gauge: The pressure gauge is usually found on the front of the boiler. It is often a circular dial with numbers and coloured zones (green, red).

  2. Observe the Needle Position: The needle should be in the green zone, typically between 1 and 2 bars. If it is in the red, either too high or too low, adjustments are needed.

  3. Turn Off the Boiler: Ensure the boiler is off and cool before making any adjustments. This prevents any accidental burns or other issues.

  4. Use the Filling Loop: If the pressure is low:

    • Find the filling loop, a flexible tube connected to the boiler.

    • Open the valves to let water in until the gauge shows the desired pressure.

    • Close the valves once the correct pressure is reached.

  5. High Pressure Adjustments: If the pressure is too high, bleeding a radiator can release excess pressure. Use a radiator key to carefully open the valve and let out some water.

  6. Recheck the Pressure: After any adjustments, turn the boiler back on and look at the gauge again to ensure it is stable at the desired level.

Maintaining the correct pressure is key to efficient boiler operation and heating performance.

Combi Boiler Pressure Issues

Keeping the pressure of a combi boiler at the right level is essential for it to work properly. Normal pressure levels for a combi boiler are between 1 and 2 bars. When the heating is off, the pressure should be 1 to 1.5 bars. When the heating is on, it should ideally be around 2 bars.

Common Issues

  1. Low Pressure: If the pressure falls below 1 bar, the boiler might not function correctly. Repressurising the system can often fix this issue. Follow the manufacturer's instructions or use a filling loop to add water.

  2. High Pressure: If the pressure goes above 2.75 bars, this could indicate a problem. Overpressure can cause leaks or damage. Allow some water to escape from the system to lower the pressure.

  3. Pressure Fluctuations: Pressure that rises and falls frequently may point to expansion vessel issues or a faulty pressure relief valve. A Gas Safe registered engineer can help diagnose and resolve these problems.

Warranty and Aftercare

  • Boiler Warranty: Manufacturers usually provide warranties that cover pressure-related issues. Check the warranty period and what is included.

  • Aftercare: Regular aftercare and maintenance can prevent pressure issues. Annual services often include a pressure check and adjustments if needed.

Taking care of pressure issues early can save costly repairs and ensure your boiler runs smoothly. Always consult the boiler's manual and consider professional help for persistent problems.

Troubleshooting Common Issues

Low Pressure: If a combi boiler's pressure is too low, it can affect performance and reliability. Check the pressure gauge; it should be between 1 and 1.5 bars. Repressurising is often straightforward. Consult the manufacturer's guide for steps or contact a Gas Safe registered engineer if necessary.

Leaks: Leaks can cause pressure to drop. Common leak points include radiators and the boiler itself. Frequent pressure loss may indicate a more serious issue. Inspect for visible leaks and ensure radiators are properly bled.

No Heat or Hot Water: This problem can be caused by low pressure, faulty components, or a frozen condensate pipe. Ensure the pressure is correct and the condensate pipe is not obstructed. If unsure, professional maintenance is advisable.

Boiler Keeps Turning Off: If the boiler frequently cuts out or makes unusual 'chugging' sounds, it might need servicing. Potential causes include blockages in the flue pipe or dirt on the burners. Regular servicing improves reliability and performance.

Thermostat Issues: A malfunctioning thermostat can lead to inconsistent heating. Check the thermostat batteries and settings. If problems persist, it may need replacement or recalibration by a professional.

Frozen Condensate Pipe: In cold weather, the condensate pipe can freeze, stopping the boiler from working. Thaw the pipe using warm (not boiling) water. Insulating the pipe can help prevent future freezing.

Pilot Light Going Out: A pilot light that won't stay lit might be caused by a faulty thermocouple or gas supply issue. Cleaning the pilot light or replacing the thermocouple often fixes this problem. Seek professional help for gas supply issues.

Using these troubleshooting tips when dealing with combi boiler issues can help maintain performance and reliability. Always refer to the specific manufacturer’s maintenance guide for detailed instructions and safety information.

Selecting the Right Combi Boiler for Your Home

Choosing the right combi boiler for your home involves understanding power output, efficiency, sizing, and energy efficiency ratings to ensure optimal performance and cost-effectiveness.

Assessing Power Output and Efficiency

When selecting a combi boiler, the power output, measured in kilowatts (kW), is crucial. It determines how well the boiler can meet your heating and hot water needs. For smaller homes with one bathroom, a combi boiler with a power output of 24-27 kW is usually sufficient.

For larger properties with multiple bathrooms, you might need a boiler with a higher power output, such as 28-34 kW. Higher output boilers ensure that the central heating system can handle the increased demand for heat and hot water. Efficient boilers not only perform better, but they also help reduce energy bills and are more environmentally friendly.

Combi Boiler Sizing

Proper sizing of a combi boiler is essential for optimal performance. An undersized boiler will struggle to meet your heating needs, while an oversized one can lead to inefficiency and higher operating costs.

The size of your property and the number of radiators and bathrooms it has will influence the size of the required boiler. For example, a compact combi boiler is ideal for smaller homes or flats, where space is at a premium. It’s important to assess the central heating output and hot water demand to choose the right boiler size.

Energy Efficiency and Ratings

Energy efficiency should be a key consideration when selecting a combi boiler. Look for boilers with high energy efficiency ratings, such as those with an A rating. These boilers are designed to convert more fuel into heat, reducing energy wastage and saving money on utility bills.

Boilers with good efficiency ratings are more cost-effective and environmentally friendly. Some models even offer advanced features like smart controls, enabling users to optimise energy use further. Installing an energy-efficient boiler can lead to significant energy savings over time.

Choosing the right combi boiler involves a careful assessment of power output, sizing, and energy efficiency to ensure that it meets your home’s specific needs effectively.

Installation and Initial Setup

Getting a combi boiler installed requires careful planning and professional help. First, a qualified boiler installer evaluates the property to select the right boiler size. Professional engineers play a crucial role in ensuring all components are fitted correctly.

Steps in Installation

  1. Assessment: An initial assessment checks the current heating system and determines the best option for your home.

  2. Removal: If there is an old boiler, it needs to be safely removed.

  3. Positioning: The new boiler is positioned, typically in a convenient and accessible location.

  4. Connecting: Piping and electrical connections are made to integrate the boiler with the existing system.

  5. Flue Installation: Fit a balanced or fan flue to vent gases safely.

Quick Installation Tips

  • Consult Experts: Always consult professional engineers to ensure safe and efficient installation.

  • Certified Installers: Use Gas Safe-registered boiler installers.

  • Boiler Location: Choose a location that is easy to access for maintenance.

Important Points

Make sure to verify that all vents and pipes are sealed properly. After the setup, the installer will usually fill and pressurise the system. Monitoring the pressure and adjusting it as necessary is crucial in the initial few days.

Reminder: Always follow the manufacturer's guidelines for boiler pressure and initial setup.

Use the table below for quick reference:




Evaluate current system, choose suitable boiler


Safely remove old boiler


Place the new boiler in a convenient location


Connect piping and electrical systems

Flue Installation

Fit balanced or fan flue for safe venting

Careful installation and setup ensure the boiler operates efficiently, providing reliable heating and hot water.

Maintenance and Care

Proper maintenance of a combi boiler is crucial for its efficiency and longevity. Regular checks and servicing by qualified engineers can prevent issues before they become major problems.

Scheduled Servicing

A combi boiler should have a service at least once a year. This annual check-up ensures that the boiler is working safely and efficiently. Many manufacturers require yearly servicing to keep the warranty valid.

Pressure Checks

Maintaining the correct pressure is essential. Regularly check the pressure gauge; it should typically be between 1.0 and 1.5 bar when the system is cold. If the pressure drops frequently, consult an engineer.

Bleeding Radiators

Trapped air in the radiators can affect boiler performance. Bleeding radiators can help maintain the system's efficiency. This simple task can often be done without specialist help.

Vents and Fans

Ensure that vents and fans are clear of obstructions. Blocked vents can lead to overheating and reduced efficiency. Regularly clean the area around the boiler to keep it free from dust and debris.

Pipe Insulation

Insulating exposed pipes can prevent heat loss and protect against freezing in winter. This can help maintain pressure and efficiency, particularly in colder months.

Warranty and Aftercare

Most combi boilers come with a warranty. Regular maintenance helps ensure that this warranty remains valid. Familiarise yourself with the terms of the warranty and stick to recommended maintenance schedules for the best aftercare.

Top Combi Boiler Brands and Models

This section examines the leading combi boiler brands in the UK for 2024, highlighting their top models, innovative features, and what sets them apart in the market.

Reviewing Market-Leading Combi Boilers UK 2024

Worcester Bosch is hailed as a top brand for reliability and efficiency. Their Greenstar 8000 Style and Greenstar 4000 models are well-reviewed for their excellent warranties and customer satisfaction.

Viessmann offers the Vitodens 100-W and Vitodens 050-W, celebrated for innovative technology and high efficiency.

Vaillant's ecoTec Plus remains a favourite for homes of all sizes due to its dependable performance.

Lastly, Baxi Platinum Compact is noted for its balance between cost and features.

Features of the Best Combi Boilers

The best combi boilers often come with stainless steel heat exchangers, which are durable and efficient.

Worcester Bosch models excel with their smart controls that allow users to manage heating remotely.

Viessmann's Vitodens range includes advanced technology for superior efficiency and reliability.

Both Worcester Bosch and Viessmann have hydrogen-ready features, ensuring future compatibility as the UK moves toward greener energy sources.

Innovation and Technology in Modern Combi Boilers

Modern combi boilers incorporate cutting-edge technology to enhance performance and user experience.

Smart controls allow seamless integration with home automation systems, providing users with greater control over their heating.

Hydrogen-ready boilers like those from Worcester Bosch and Viessmann are designed to reduce carbon emissions and support future energy transitions.

Advanced stainless steel heat exchangers offer longevity and improve heat transfer efficiency, making them a key innovation in top models such as the Worcester Bosch Greenstar and Viessmann Vitodens series.

Budgeting for Your New Combi Boiler

Budgeting for a combi boiler involves balancing cost, quality, and long-term operation expenses. Understanding both initial pricing and ongoing costs is key to making an informed purchase.

Balancing Cost and Quality

When purchasing a combi boiler, it’s crucial to consider both price and quality. A cheaper boiler might save money upfront but could lead to higher maintenance costs and energy bills.

Choosing a high-quality and energy-efficient boiler, while more expensive initially, often provides better value for money over time. Look for models with excellent warranties and good customer reviews, as these typically signal durability and efficiency.

Exploring Affordable Combi Boiler Options

Finding budget-friendly combi boilers without sacrificing too much on quality is possible. Some reputable brands offer efficient models at lower prices. Worcester BoschVaillant, and Ideal Heating are noted for their value-for-money products.

When evaluating affordability, check if the boiler qualifies for any energy efficiency grants or discounts. Models with good fuel economy can offset their initial cost through energy savings over their lifespan.

Cost of Ownership and Operation

Owning a combi boiler involves more than the purchase price. Consider the cost of installation, which varies based on the complexity of the work and your location. Look into the warranty period, as a longer warranty can reduce maintenance expenses.

Energy-efficient models help lower energy bills, contributing to a more cost-effective operation. Routine maintenance is essential to keep the boiler running efficiently and extend its lifespan, which means lower long-term costs and better value.

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 cost 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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Frequently Asked Questions

What is the ideal pressure for a combi boiler?

The ideal pressure for a combi boiler is generally between 1.0 and 1.5 bars when the system is cold. This range ensures the boiler runs efficiently and safely.

Is 2 too high for boiler pressure?

Yes, a pressure reading of 2 bars or more is considered too high for a combi boiler. High pressure can stress the system and might cause leaks or damage.

How do I turn the pressure up on a combi boiler?

To increase the pressure, locate the filling loop, usually found below the boiler. Open the valves to allow water into the system until you reach the desired pressure, then close the valves.

What happens if my combi boiler pressure is too low?

If the pressure is too low, the boiler may not function properly. You might notice that the system struggles to heat water or radiators may not get warm.

How do you fix a low water pressure on a combi boiler?

To fix low water pressure, you typically need to use the filling loop. Open the valves to let more water into the system until the pressure is back within the recommended range.

Can a combi boiler lose pressure without a leak?

Yes, a combi boiler can lose pressure without an obvious leak. This can be due to gradual escape of air from the system or minor faults in the expansion vessel.

Is it normal for a combi boiler to lose pressure over time?

It is normal for a combi boiler to lose a small amount of pressure over time. Regular maintenance checks can help mitigate significant pressure drops.

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.