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Last updated: 23rd April, 2024


Underfloor Heating with a Combi Boiler

Underfloor Heating with a Combi Boiler

Key takeaways

  • Underfloor heating works efficiently with combi boilers, offering a seamless heat distribution.
  • Suitable combi boilers and professional installation are paramount for system compatibility.
  • Regular maintenance and suited controls are vital for optimal performance and energy efficiency.

Integrating underfloor heating with existing combi boilers can be a practical option for many. However, not all boilers are suitable for such systems.

Underfloor heating systems have become increasingly popular as a comfortable and efficient method of heating homes, and when paired with a combi boiler, they provide a streamlined solution to heating needs. This system offers a more even distribution of heat compared to traditional radiators, as it warms the entire floor surface, which then radiates heat upwards into the living space. 

Combi boilers, known for providing hot water and central heating without the need for a separate water tank, can work in conjunction with underfloor heating, provided the setup includes essential components such as a two-port valve to manage the flow of hot water.

Integrating underfloor heating with existing combi boilers can be a practical option for many, particularly when extending a property or looking to maintain a compact heating setup. However, not all boilers are suitable for such systems. It is important to consult a qualified heating engineer to ensure that the boiler has the necessary capacity and controls. 

The installation process involves careful planning to ensure the underfloor heating operates efficiently, with appropriate controls for operation and temperature regulation to maximise the potential savings in energy costs.

Despite the appeal, understanding the technicalities, costs, and maintenance requirements is crucial before undertaking the installation. While combi boilers and underfloor heating can form a harmonious pair, the efficiency of the system depends on several factors including the quality of insulation, the type of flooring, and the layout of the property. 

Maintaining the system will also play a critical role in its longevity and effectiveness, with troubleshooting undertaken by professionals to ensure the continued performance and efficiency of the heating system.

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Understanding Underfloor Heating

Underfloor heating offers a blend of luxury and efficiency, creating a consistent ambient temperature free from cold spots. It is installed beneath a floor to provide heat across a large surface area.

Types of Underfloor Heating

Underfloor heating systems come primarily in two types: 'wet' and 'dry' systems. Wet systems circulate warm water through pipes under the floor, making them hydronic, whereas dry systems use electrical wires for generating heat. These are also known as electric underfloor heating systems.

Benefits of Underfloor Heating

Underfloor heating provides a high level of comfort by evenly distributing heat across a room. It is efficient, potentially reducing energy usage as it operates at a lower temperature while maintaining the desired ambient temperature. The gradual heat distribution can also add a sense of luxury to a property.

Components of Underfloor Heating

A typical underfloor heating system includes pipes or electrical wires, a thermostat, and temperature sensors. For a wet system, a network of pipes is embedded under the floor, while a dry system consists of a series of electrical wires. The thermostat and sensors enable precise control over the heat output.

How Underfloor Heating Works

Underfloor heating works by emitting a consistent level of heat from the floor upwards, avoiding cold spots commonly found with traditional radiators. For wet systems, warm water circulates through pipes laid out in a grid pattern beneath the surface. Dry systems heat the floor through electric wires, both responding to the thermostat's regulation.

Underfloor Heating with Combi Boilers - Explored

Exploring underfloor heating in the context of combi boilers reveals the intricacies of integration, the benefits of such systems, and the considerations needed for optimal performance.

What is a Combi Boiler

A combi (combination) boiler is a space-saving, high-efficiency heating unit that provides both hot water and central heating from a single compact source without the need for a separate hot water cylinder. These boilers heat water directly from the mains on demand, ensuring that hot water is available instantaneously.

Benefits of Combi Boilers

Combi boilers offer a variety of advantages:

  • Space-saving: Due to the absence of a hot water cylinder or cold water tank.

  • Energy efficient: Condensing boilers capture additional heat from exhaust gases, enhancing efficiency.

  • Cost-effective: Reducing energy consumption results in lower heating bills.

  • Convenience: Provides hot water on demand without waiting times.

Compatibility of Combi Boilers and Underfloor Heating

Combination boilers can be compatible with underfloor heating (UFH) systems, provided that the boiler's output is sufficient to meet the demand. It is essential to assess:

  • Boiler capacity: Ensuring the combi boiler can handle the additional load of the underfloor heating.

  • Flow rate: The boiler must maintain an appropriate flow rate to provide consistent heat across all zones.

  • Two-port valve: Often a necessary addition to separate the heating circuits of radiators and underfloor heating.

Necessary Central Heating Augmentation for Underfloor Heating with Combi Boiler

Central heating systems may require specific augmentations when integrating underfloor heating with a combi boiler:

  • Zones: Establishing distinct heating zones, controlled by thermostats, to manage the heat distribution effectively.

  • Two-port valve: This valve is essential for directing the flow between radiators and the underfloor heating, preventing concurrent operation.

Selecting the Right Boiler for Underfloor Heating

When choosing a combi boiler for underfloor heating, consider:

  • Boiler size: Match the boiler's kW output with the size and requirements of the underfloor heating system.

  • Heat pump option: A heat pump could serve as an alternative or supplement, offering potential energy savings, albeit usually with a higher initial cost.

  • Expert advice: Seeking professional guidance ensures the boiler selection aligns with the property's heating demands.

Each aspect, from boiler size to the inclusion of a two-port valve, plays a pivotal role in the harmonious operation of the underfloor heating system and the combi boiler, creating an efficient and cost-effective home heating solution.

What type of boilers are not suited to underfloor heating?

Not all boilers are compatible with underfloor heating systems. Regular boilers, also known as conventional or heat-only boilers, are less suited to underfloor heating. These boilers necessitate a cold water tank and a hot water cylinder, complicating integration. Regular boilers are better matched with radiator-based systems rather than underfloor heating because they typically operate at higher temperatures.

Older combi boilers can also pose challenges. These models might struggle with the low-temperature water flow that underfloor heating requires. They are designed for higher-temperature outputs, necessary for traditional radiators, and may not efficiently modulate to the lower temperatures needed for underfloor systems.

Here is a summary of boiler types that may not be ideal for underfloor heating:

  • Regular boilers (conventional/heat-only): Require additional components such as tanks and cylinders.

  • Old combi boilers: May not efficiently support low-temperature heating.

Underfloor heating systems perform optimally with a low-temperature water flow, usually between 35℃ and 55℃. Boilers that cannot consistently supply such temperatures, like some older combi and regular boilers, can lead to inefficient operation and increased running costs. It is crucial to consult with a certified heating engineer before selecting a boiler, to ensure it is compatible with the intended underfloor heating.

Installation Process

The installation of underfloor heating systems in conjunction with combi boilers involves a series of methodical steps. They require careful planning and execution to ensure compatibility with the existing boiler and optimal heating performance.

Initial Considerations

Before laying any pipes or insulation, one must assess the suitability of the existing combi boiler. Boiler requirements vary depending on the size of the area to be heated. A professional installer typically evaluates the power output of the boiler, measured in kilowatts, to ensure it can handle the demand of the additional underfloor heating system. Essential to this assessment is checking that the flow can be separated between the radiators and the underfloor heating if necessary, often involving the addition of a two-port valve.

Laying the Heating System

Installing the underfloor heating consists of several specific phases:

  1. Insulating the Floor: Proper insulation beneath the pipes or heating cables is crucial to direct heat upwards efficiently. This step prevents heat loss and improves the system’s overall efficiency.

  2. Positioning Pipes or Electrical Cables: The laying pattern must be meticulously planned to provide even heat distribution across the floor. For water-based systems, the pipes carry hot water throughout the floor area, whereas electrical cables are used for electric systems.

  3. Flooring: The choice of flooring material can greatly influence the heating system's performance. Tiles and stone are ideal for conducting heat, while thicker materials such as carpet may require additional considerations due to their insulative properties.

Integrating with a Combi Boiler

The integration process involves the precise calibrating of the combi boiler to work in tandem with the newly laid underfloor heating. Key aspects include ensuring that the boiler requirements align with the additional load from the underfloor system and that the boiler can accurately control the water temperature flowing through the underfloor pipes. A suitably qualified heating engineer must handle the integration, as it often requires fine-tuning and potential system upgrades such as installing a two-port valve to manage the different heating zones effectively.

Operation and Control

Managing underfloor heating with a combi boiler hinges on precise control systems and effective heat distribution strategies. These elements work in tandem to deliver a balanced indoor temperature and optimise energy efficiency.

Thermostat and Temperature Control

Underfloor heating systems integrate with thermostats to gauge ambient room temperature and adjust the flow temperature accordingly. The combi boiler's output is modulated to provide water at a temperature that suits the underfloor heating requirements, which is typically lower than that for radiators.

For example:

  • Optimal flow temperature for underfloor heating: 30°C to 45°C

  • Standard radiator heating temperature: 70°C to 80°C

This is managed by a blending valve which mixes hot water from the boiler with cooler water returning from the pipes. Scheduled maintenance of these components ensures sustained efficiency and reduces the likelihood of overheating or damage to the flooring.

Zoning and Heat Distribution

Zoning plays a pivotal role in heat distribution for underfloor heating systems. Each zone corresponds to a specific area or room and can be controlled independently.

  • Zone Control: Individual thermostats for each zone.

  • Heat Distribution: Utilisation of manifold controls to regulate water flow.

A two-port valve may be used when the system is installed alongside standard radiators to separate the underfloor heating from the rest of the system. This separation is crucial; it prevents overloading the combi boiler and ensures that heat distribution is evenly spread across each desired area, maximising comfort and system efficiency.

Efficiency and Costs

Integrating a combination boiler with underfloor heating can have a substantial impact on both energy efficiency and the financial aspects of running a home's heating system.

Energy Efficiency

Underfloor heating systems paired with combi boilers can contribute to a home’s overall energy efficiency. These boilers adjust the air supply to maintain an optimal gas/air mixture, which can reduce fuel consumption. The innate design of underfloor heating provides an even distribution of warmth, often resulting in a lower thermostat setting and thus lower energy bills. To maximise efficiency, it's pivotal to ensure that the boiler's capacity aligns with the space's demands.

Running and Maintenance Costs

Running and maintenance costs for underfloor heating systems with combi boilers can be onerously affected by multiple factors. A 24kw combi boiler, for instance, generally suffices for one room, while larger spaces may require up to a 40kw model. Higher-capacity boilers may lead to an increase in price. Regular maintenance is key to ensuring the longevity and efficiency of the system, potentially aiding in keeping running costs stable.

Mitigating Heat Loss

Minimising heat loss is imperative to maintain efficiency. Proper insulation is the linchpin in this endeavour; it helps to contain the heat generated, directly influencing energy consumption and costs. With well-insulated premises, underfloor heating systems require less energy to maintain a consistent temperature, which sequentially can lead to a reduction in energy bills. Ensuring the property is adequately insulated will greatly support the overall efficiency of the heating system.

Troubleshooting and Maintenance

When managing underfloor heating systems paired with combi boilers, regular checks and a methodical approach to identifying issues ensure the consistent warmth and efficiency of your home.

Common Underfloor Heating Issues

Problems with underfloor heating often manifest as inconsistent heating or complete system failure. A frequent issue is the system entering fault mode, which may happen if the boiler attempts to heat both the radiators and underfloor heating simultaneously, leading to an overheat. This scenario necessitates checking for a two-port valve that allows separate operation of the systems. Additionally, the presence of leaks can compromise the performance, calling for immediate attention to prevent damage to the structure and the system itself.

Maintaining Your System

Effective maintenance of underfloor heating involves periodically inspecting for physical damage, ensuring the correct pressure levels, and verifying the proper function of all valves and actuators. It is also vital that the system is flushed to remove any debris that could cause blockages. Where modifications are needed, a professional installer should be consulted to ensure the reliability and safety of the system, especially when dealing with gas boilers, electric boilers, oil boilers, or LPG boilers.

Boiler Health and Efficiency

The health of a combi boiler is paramount to the smooth operation of an underfloor heating system. Regular servicing by a professional can prevent the system from going into fault mode or overheating. Moreover, it's essential to ensure that the boiler is appropriately sized for the heating demand and that any modifications are compatible with existing setups. Proper upkeep includes timely checks and adequate maintenance to extend the boiler's lifespan and maintain its efficiency.

Alternatives to Combi Boiler Underfloor Heating

While combi boilers can be paired with underfloor heating systems, alternatives exist that might better suit certain properties and their specific heating needs. These include using heat pumps or electric boilers, especially in systems where underfloor heating is a desired feature.

Heat Pumps and Underfloor Heating

Heat pumps offer an energy-efficient alternative to combi boilers for underfloor heating. They use natural heat from the air, ground, or water and are particularly compatible with the lower temperatures that underfloor heating systems require.

  • Air source heat pumps (ASHPs) extract heat from the outside air.

  • Ground source heat pumps (GSHPs) harness heat from the ground via a network of water pipes buried underground.

Heat pumps can provide both heating and hot water, and they work exceptionally well with large surface area systems like underfloor heating. However, they may entail higher initial installation costs.

Electric Boilers and Dry Systems

Electric underfloor heating systems, also known as dry systems, can serve as another alternative. These systems do not use water for heat transfer but instead rely on a network of electric cables.

  • Ideal for smaller areas or individual rooms.

  • Less complex to install compared to wet systems.

  • Can be laid under different types of flooring.

While electric boilers are less common than gas-fired boilers, they can be an effective alternative for homes that have no gas supply or prefer to use electricity. Electric boilers can be used with both wet and dry underfloor heating, but it's worth noting that electric heating systems can be more expensive to run compared to gas systems, depending on the electricity rates and the insulation quality of the property.

Do I Need A New Boiler for Underfloor Heating?

When considering underfloor heating, homeowners often inquire about the compatibility of their current boiler with this system.

Best Boiler Brands: Vaillant, Viessmann, Alpha, Ideal, Worcester Bosch

Selecting the right boiler brand is pivotal for ensuring efficiency, reliability, and longevity in home heating. Among the myriad of options, brands like Vaillant, Viessmann, Alpha, Ideal, and Worcester Bosch distinguish themselves with innovative technology and customer satisfaction. Below is an in-depth comparison of these brands, focusing on their standout models and unique industry positions.

Comparative Analysis of Standout Models


Standout Model

Energy Efficiency

Unique Features

Environmental Impact


ecoTEC Plus


Eco-friendly, quiet operation

Reduced carbon emissions


Vitodens 100-W


Compact design, smart compatibility

Low emissions technology


E-Tec Plus

High efficiency

Compact, cost-effective

Energy-saving operation


Logic Plus


User-friendly, easy installation

Efficient energy use

Worcester Bosch

Greenstar i


Robust build, smart controls

Eco-conscious design


Vaillant's eco-friendly ethos shines in their ecoTEC range, particularly the ecoTEC Plus and ecoTEC Exclusive models. These boast top-tier energy efficiency and reduced carbon footprints. They're known for quiet operation and user-friendly controls, making them a harmonious blend of performance and environmental responsibility.


Viessmann's Vitodens series, especially the 100-W and 200-W, are hallmarks of high-efficiency and technology. These models feature compact designs and stainless steel Inox-Radial heat exchangers for durability. Advanced features like smart thermostat compatibility offer enhanced control and user convenience.


Alpha Heating Innovation's E-Tec range, particularly the E-Tec Plus, offers a fine balance between affordability and quality. Notable for their impressive energy efficiency and compact size, these boilers are suitable for various home types, making them an ideal choice for straightforward, efficient heating solutions.


Ideal Boilers, especially the Logic Plus range, are lauded for their reliability and value. These boilers stand out with their efficiency, user-friendly interface, and compactness, favoured for their ease of installation and maintenance. They come with comprehensive warranties, offering peace of mind and reliability.

Worcester Bosch

A leader in the market, Worcester Bosch is synonymous with quality and reliability. The Greenstar range, including models like Greenstar i, Si, and CDi, is known for its efficiency and robust build quality. These boilers blend innovative features such as smart thermostat compatibility with user-friendly controls, making them a top choice in many homes.

New Boiler Costs

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.

Boiler costs 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 want to remove all of this undue stress and make the decision making process of upgrading to a new central heating system, as easy as possible.

Boiler costs can vary depending on a number of factors including their brand, model, fuel, output, warranty, labour and 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


Get a quote

System to combi conversion


Get a quote

New boiler install


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Back boiler to a combi


Get a quote

System to system


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Frequently Asked Questions

What size combi boiler do I need for underfloor heating?

The appropriate size for a combi boiler hinges on several factors including the total area of underfloor heating and the heat demand of the property. A qualified heating engineer should perform heat loss calculations to determine the boiler's capacity required, typically measured in kilowatts (kW).

What is the downside of underfloor heating?

Underfloor heating may present a slower heat-up time compared to traditional radiators and could be costly and disruptive to install in existing properties. Additionally, it may require careful consideration of floor heights and floor coverings to ensure optimal operation and efficiency.

What temperature should a combi boiler be at for underfloor heating?

The recommended temperature for a combi boiler powering underfloor heating is usually lower than that for radiators, generally around 40-50°C. This helps maintain efficiency and provides a consistent level of warmth. The precise temperature will depend on the property's specific requirements.

What type of boiler is best for underfloor heating?

While combi boilers can be used for underfloor heating, system boilers are often considered more suitable, especially for larger systems that include hot water storage. System boilers maintain a constant supply of hot water, which can be beneficial for expansive underfloor heating installations.

What is the most economical way to run underfloor heating?

The most economical operation of underfloor heating entails proper insulation, employing a low temperature over a longer period, and using programmable thermostats to control the heat based on actual usage patterns. Opting for high-efficiency boilers can further reduce running costs.

What is the cheapest way to run underfloor heating?

To run underfloor heating cost-effectively, maintain a consistent low temperature rather than frequent temperature fluctuation. Insulating the property adequately and employing smart controls for zoning and scheduling can also minimise the operational costs.

Is it cheaper to leave underfloor heating on constantly?

It might not always be cheaper to leave underfloor heating running at all times; however, due to its nature of slow heat-up times, it can be more cost-efficient to keep it on at a lower temperature continuously, rather than reheating from cold each time. Adjusting the temperature for when the space is in use can yield savings.

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.