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Last updated: 10th May, 2024


What is Boiler Modulation?

What is Boiler Modulation?

Key takeaways

  • Boiler modulation optimises efficiency in heating systems by precisely controlling heat output.
  • Different types of modulating boilers cater to specific requirements and contribute to environmental sustainability.
  • Modulating boilers work to reduce energy waste, component wear and provide long-term cost savings.

A modulating boiler not only adjusts its output to meet the varying heat demands of your home but also works to minimise energy wastage and the wear and tear of components.

Boiler modulation is a vital aspect of modern, efficient heating systems, enabling the fine-tuning of heat output to match the specific demands of your living space. 

While most UK homeowners are familiar with the presence of a boiler in their homes, understanding how these devices work and interact with your heating system is essential to maximise their efficiency and your comfort.

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A modulating boiler not only adjusts its output to meet the varying heat demands of your home but also works to minimise energy wastage and the wear and tear of components. 

These boilers come in different types and offer various benefits, including reduced fuel consumption, lower emissions, and increased longevity, making them an environmentally responsible choice for any home.

Understanding Boiler Modulation;

Boiler Modulation Basics

Boiler modulation is a vital feature in modern, energy-efficient boilers that you ought to understand. Essentially, it enables boilers to adjust their output according to the fluctuations in heating loads. This adjustment is critical because it ensures the boiler is only generating the required amount of heat, thus achieving optimal efficiency and minimising energy wastage.

In simple terms, it is the boiler's ability to vary its capacity depending on the heating demands inside a property. When the demand for heat is low, the boiler operates at a minimum output. Conversely, when more heat is needed, the boiler ramps up to a higher output.

Output and Modulation Ratio

The modulation ratio is an important concept when discussing boiler modulation. It refers to the boiler's minimum output in relation to its maximum output, represented as a fraction. For example, a boiler with a maximum output of 30kW and a minimum output of 5kW would have a modulation ratio of 1 to 6. This range represents an average modulation ratio for a modern boiler.

A wide modulation ratio is generally considered more favourable, as it allows the boiler to cater to a broader range of heating requirements. If a boiler has a narrow range, it may frequently switch on and off, causing energy loss and reducing efficiency.

How Modulating Boilers Work

Combustion Process

A modulating boiler is designed to provide the exact level of heat needed in real-time. Unlike the traditional, non-modulating boilers that operate at a fixed output and cycle repeatedly, modulating boilers can adjust their flame output. This results in improved energy efficiency and reduced fuel consumption. In the combustion process, modulating boilers adjust the flame size by varying the amount of fuel being burned and mixing it with the right quantity of air. With a finely tuned operation, these boilers can achieve peak efficiency and lower emissions.

Role of Components

The primary components of a modulating boiler play a crucial role in ensuring optimum efficiency. Amidst these components, the burner, gas valve, and fan work synergistically to modulate the combustion process. The fan controls the flow of combustion air, the gas valve regulates the supply of fuel, and the burner creates the flame. By synchronising their operations, the boiler achieves a balance between heat demand, fuel consumption and flame output.

Use of Sensors

Sensors play a significant part in modulating boilers. They detect variations in temperature, pressure, and other relevant parameters, helping the boiler to adapt its output accordingly. For instance, a temperature sensor monitors the temperature of water or circulated air within the heating system. When the temperature reaches the desired level, the sensor communicates this information to the boiler's control system. The control system then adjusts the flame output, and if necessary, the fan speed, to maintain the target temperature. In this way, sensors ensure that the modulating boiler operates efficiently and adjusts its performance to changing heat demands.

Types of Modulating Boilers

In recent years, modulating boilers have gained significant popularity due to their energy efficiency and ability to adapt to varying heating demands. There are three primary types of modulating boilers: Combi Boilers, System Boilers, and Heat Only Boilers. Each of these types offers unique features and advantages, suitable for different applications.

Combi Boilers

Combination or combi boilers are an increasingly popular choice for homeowners due to their space-saving design and efficient performance. These boilers provide both central heating and hot water in one compact unit, eliminating the need for separate storage tanks. 

By modulating their output in response to real-time heating demands, combi boilers can maintain a consistent temperature, reduce fuel consumption, and decrease emissions. Furthermore, some of the best combi boilers on the market have a wide modulation ratio allowing them to perform even more efficiently.

System Boilers

Designed for larger homes with multiple bathrooms or high hot water demands, system boilers work in conjunction with a separate hot water storage cylinder. They efficiently supply central heating while maintaining a steady supply of hot water to the property. System boilers benefit from modulating technology as well, adjusting their output to match heating requirements. 

This makes them a more energy-efficient option compared to traditional boilers. Modulating condensing system boilers are particularly effective at reducing energy consumption and emissions, making them an eco-friendly choice for homeowners.

Heat Only Boilers

Heat only boilers, also known as regular or conventional boilers, primarily provide central heating and require a hot water storage cylinder and a cold-water storage tank. Often found in older homes, they are typically larger than combi and system boilers. 

Although heat only boilers are not as compact or modern as their counterparts, some models feature modulating capabilities, contributing to improved efficiency and reduced fuel consumption. Upgrading an older heat only boiler to a modulating condensing boiler can result in significant energy savings and lower emissions.

In conclusion, modulating boilers provide efficient and adaptable heating solutions for various environments. Combi boilers are an excellent option for smaller properties seeking a space-saving solution, while system boilers are suitable for larger homes with high hot water demands. Heat only boilers are less popular in new installations but can be improved through modulating technology. With the advancement of technology, modulating boilers are set to become even more efficient, making them a prevalent choice among homeowners looking for sustainable, cost-effective heating solutions.

Practical Impact of Modulating Boilers

Energy Efficiency

Modulating boilers are known for their high energy efficiency. Unlike traditional boilers that operate at fixed heat outputs, modulating boilers adjust their heat output based on the current demand for heat. Consequently, they use fuel more efficiently and provide a consistent temperature throughout the home.

For instance, if a boiler has a maximum output of 30kW and a minimum output of 7.5kW, the modulation ratio is less than 4:1. This ratio indicates the variation in heat output, allowing the boiler to operate at different rates based on the needs of the home, thus ensuring minimal fuel waste.

Heat and Water Output

A significant advantage offered by modulating boilers is their ability to provide the exact amount of heat and hot water required by a household. Since they can adjust their power in real-time, homeowners have precise control over their heat output and water temperature.

Furthermore, modulating boilers reduces the phenomenon of short cycling. In traditional boilers, the on and off cycling of the heating system creates inefficiencies in the output of heat. These on and off cycles lead to not only energy waste but also additional wear and tear on the boiler. By adjusting to the heat demand in real-time, modulating boilers avoid this problem, providing a consistent and efficient heat supply.

In summary, modulating boilers offer significant energy efficiency benefits while maintaining excellent heat and water output. By adjusting to the heat demand in real-time, these boilers ensure optimal fuel use and a comfortable temperature for homeowners.

Modulating Boilers and Heating Systems

Modulating boilers are an essential part of modern, energy-efficient heating systems. These boilers can adjust their power output in real-time to provide the exact level of heat needed, making them highly efficient in terms of fuel usage.

Relation to Heating Load

A boiler's modulation ratio is its minimum output related to its maximum output, stated as a fraction. For instance, a boiler with a 30kW maximum output and a 5kW minimum output would have a modulation ratio of 1 to 6. The wider the modulation ratio, the better the boiler can adapt to changes in heating demand.

Boiler modulation allows the boiler to operate at lower firing rates when less heat is required and at higher firing rates when more heat is needed. This directly corresponds to the heating load, as the boiler's output is adjusted to the exact heating demand, resulting in improved energy efficiency and reduced fuel consumption.

Impact on Heating Controls

The modulating function of a boiler can positively impact heating controls since it helps maintain a consistent and comfortable indoor temperature. This is because the modulation helps to avoid large temperature fluctuations and short cycling, leading to smoother operation and better comfort levels.

In UK heating systems, heating controls often play an essential role in managing energy consumption and ensuring the correct heating levels are maintained. When integrated with a modulating boiler, these controls can enhance the overall efficiency and performance of the system.

By dynamically adjusting the output of the boiler, the heating controls can help maintain optimal room temperatures and avoid overheating or inefficient operation. As a result, modulating boilers can contribute to reduced energy bills and a more comfortable living environment.

Environmental Implications of Boiler Modulation

Emissions and Insulation

Boiler modulation plays a significant role in reducing emissions and improving insulation in heating systems. By adjusting the operation level according to demand, modulating boilers can achieve more efficient combustion, which results in lower carbon dioxide and nitrogen oxide emissions. As a consequence, modulating boilers contribute considerably to a cleaner environment and better air quality.

Insulation is equally important when considering environmental implications. Efficient boiler modulation reduces temperature fluctuations within the heating system, lessening heat loss through the building envelope. By maintaining consistent temperature levels, the insulating properties of building materials can be utilised more effectively, minimising energy waste and emissions even further.

Energy Bills and Savings

In addition to being environmentally friendly, boiler modulation can lead to substantial energy savings. As the output of modulating boilers is precisely matched to the heating demand, they consume a lower amount of fuel than traditional on/off boilers. This efficient fuel consumption results in reduced energy bills for households and businesses alike.

Here are some ways users can benefit from boiler modulation:

  • Reduced energy consumption: a modulating boiler with a 1 to 6 ratio, for instance, can adapt its output to use only the required amount of energy, preventing excessive fuel consumption.

  • Lower energy bills: as energy consumption decreases, so do the energy costs. Over time, these savings can cover the initial investment in a modulating boiler.

  • Extended equipment lifespan: modulating boilers experience less wear and tear due to their ability to operate at different rates, prolonging their lifespan.

  • Improved energy efficiency: boiler modulation contributes to higher thermal efficiencies, helping households and businesses achieve better energy performance ratings.

Considerations for Boiler Modulation

Manufacturers and Models

A variety of boiler manufacturers offer modulation capabilities, with some being well-known for their high-quality and efficient products. When selecting a modulating boiler, it is essential to research different manufacturers and models to ensure that the unit will meet the specific requirements of a home or business.

Modulation ratios can vary between models, with some offering a more significant range of adjustment than others. For instance, a boiler with a 30kW maximum output and a 5kW minimum output would yield a ratio of 1 to 6. Examining these specifications is crucial for selecting the most energy-efficient and cost-effective modulating boiler.

Wear and Tear

Boiler modulation has several benefits, including improved energy efficiency and reduced wear and tear. Since modulating boilers operate at reduced firing rates, they can more accurately match the heat output to demand. Consequently, it results in less strain on the boiler and a significant decrease in unnecessary cycling.

However, it is essential to note that any mechanical system will experience some degree of wear and tear over time. To minimise this, regular maintenance and inspections are crucial. Additionally, high-quality boilers from reputable manufacturers, like Viessmann, are more likely to have less wear and tear due to robust design and premium materials.

In conclusion, when considering boiler modulation, it is essential to investigate various manufacturers, models, and modulation ratios. Maintaining the unit regularly will also ensure that it operates efficiently and minimises wear and tear.

New Boiler Benefits and 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.

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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Some location factors that can influence the price of your new boiler

You will be given a specific quote from the boiler installation company based on:

  • The type of home you have (i.e. terraced, semi-detached, etc.)

  • How many bedrooms you have

  • How many bathrooms you have

  • Your postcode and specific location

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Frequently Asked Questions

How does a modulating boiler work?

A modulating boiler adjusts its flame intensity according to the heating demand, making it more efficient than a non-modulating boiler. Instead of constantly turning on and off, the boiler lowers its output when a lower heat output is required, saving energy and increasing the longevity of the boiler.

What causes a boiler to modulate?

The boiler's control system measures the temperature difference between the flow temperature and the required setpoint. Based on this information, the boiler modulates its flame intensity to adjust the heat output accordingly. This ensures efficient and necessary heat production, avoiding energy waste.

How do I know if my boiler is modulating?

Check your boiler's specifications or contact the manufacturer to verify if it has a modulating feature. Modulating boilers will typically mention their modulation ratio, which represents the boiler's lowest output relative to its highest output.

Is boiler modulation worth it?

Yes, boiler modulation is worth it because it enhances the efficiency, performance, and longevity of the boiler. By adjusting heat output according to the demand, modulating boilers consume less energy, leading to reduced fuel consumption and lower energy bills.

Can you oversize a modulating boiler?

Oversizing a modulating boiler is not recommended, as it can lead to short cycling, wasted energy, and reduced system efficiency. It is crucial to select a boiler with the appropriate capacity for your needs, taking into account factors like building size, insulation, and climate conditions.

Are combi boilers modulating?

Yes, modern combi boilers are typically modulating boilers. They continuously adjust their output based on heating and hot water demands, ensuring efficient energy usage and maintaining a comfortable environment in your 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.