Changing to a combi boiler ensures you get efficient heating and hot water, saving you money and reducing your carbon footprint.

Residential water and space heating accounts for 23% of the UK’s energy demand, and combi gas boilers remain the most common technology in British homes. Their popularity is no accident.

A modern combi condenses wasted heat back into the system, achieving efficiencies above 90%, while freeing up the space once occupied by tanks and cylinders. For homeowners, the change means faster heating, lower gas bills, and a smaller environmental impact all in one upgrade.

The shift also reflects how people live today. Homes are becoming smaller, insulation is improving, and the expectation for instant hot water has become the norm. Installing a combi boiler turns an older, storage-based setup into a compact, high-efficiency system that delivers on both comfort and cost.

With energy prices still high and the government pushing for more sustainable heating, switching to a combi isn’t just convenient but one of the smartest home energy decisions you can make in 2025.

Key Takeaways on Changing to a Combi Boiler:

• Changing to a combi boiler generally takes 1 to 3 days, depending on whether your old tanks and pipework need to be removed or rerouted.
• Modern combi boilers are A-rated and achieve 90–94% efficiency, cutting annual gas bills by roughly £130–£500.
• Combi boilers are best suited for homes with one or two bathrooms, good mains water pressure, and limited space.
• Benefits of combi boilers include instant hot water, compact design, lower maintenance, and compatibility with smart thermostats.
• They’re less suited to homes with multiple bathrooms or low mains pressure, as hot water flow can drop during simultaneous use.
• Always use a Gas Safe-registered engineer to ensure safety, compliance, and valid warranties.

What Is a Combi Boiler and How Does It Work?

A combi boiler (short for combination boiler) is a single, compact unit that provides both central heating and hot water directly from the mains. Unlike traditional systems, it doesn’t rely on a separate cold-water tank in the loft or a hot-water cylinder in the airing cupboard. The boiler heats water on demand as soon as a tap is turned on, so there’s no waiting period and no wasted energy keeping stored water hot.

In practical terms, a combi boiler simplifies your home heating system. It draws cold water straight from the mains, passes it through a heat exchanger, and delivers instant hot water at mains pressure. When the heating is activated, the same boiler diverts heat to your radiators. It’s all managed automatically by internal valves, keeping your home warm and your water supply steady without extra equipment.

Most combis are now condensing boilers, which means they recover heat from exhaust gases that would otherwise escape through the flue. This technology can improve efficiency to over 90%, helping lower both energy bills and carbon emissions.

Key advantages include:

• Instant hot water whenever needed
• Compact design ideal for smaller homes and flats
• Fewer components and lower maintenance
• Reduced heat loss compared to stored systems

Combi boilers are available in various types, such as gas, LPG, oil, and electric, with many new models also hydrogen-ready, ensuring long-term compliance with the UK’s net-zero goals.

Learn all about the different types of boilers, including condensing boilers, LPG boilers, oil boilers, Opentherm boilers, and calculating boiler size in our guide.

When Is It Worth Changing to a Combi Boiler?

Switching to a combi boiler makes the most sense when your current setup is outdated, inefficient, or takes up valuable space. Homes that use a conventional or system boiler with storage tanks often experience slow water heating, inconsistent pressure, or wasted heat from stored hot water. If your system is more than 10 to 15 years old, upgrading can cut running costs and improve daily comfort almost immediately.

The decision also depends on your home’s size, water demand, and mains pressure. A combi performs best in smaller to medium-sized homes with one or two bathrooms, where hot water isn’t being drawn from multiple outlets at once. If you live in a flat or a semi-detached house with limited space, the space savings alone can be significant. Removing loft tanks and cylinders opens storage areas and simplifies maintenance.

Households with low mains water pressure or multiple simultaneous hot water needs may find a combi less suitable. In such cases, a system boiler with a cylinder or a storage combi might be a better alternative. A professional installer can test your pressure and flow rate to confirm what’s viable before you commit to conversion.

Suitability Tip: If your cold tap flow rate is above 10 litres per minute, a combi boiler will typically perform efficiently without noticeable pressure drops.

Suitability at a Glance

Home Feature	Suitable for Combi?	Notes
One bathroom	Yes	Ideal performance
Two bathrooms	Possibly	Check pressure first
Large family home	Not ideal	A system boiler may suit you better
Low mains water pressure	No	Needs boosting or system setup
Small flat or bungalow	Perfect	Compact and efficient

Changing to a combi boiler is also worth considering when you’re planning home renovations or removing an old hot water tank. The efficiency gains, reclaimed space, and lower maintenance costs make it a long-term upgrade that aligns with modern living standards.

Cost of Changing to a Combi Boiler in 2025 (Detailed Breakdown)

The cost of changing to a combi boiler in the UK typically ranges from £2,000 to £4,500, depending on the home type, the existing system, and the installation complexity. The Energy Saving Trust estimates that the average cost for a gas boiler replacement in Great Britain is around £3,700, a helpful benchmark for understanding where your quote sits within the national average.

This figure includes the boiler, labour, pipework adjustments, and certification by a Gas Safe engineer. The total may rise if you’re upgrading from a traditional setup with tanks or relocating your boiler to a new position in the property.

Average Cost by Conversion Type

Conversion Type	Typical Cost Range	Average Labour Time	What’s Involved
Regular → Combi	£2,500–£4,000	2–3 days	Tank and cylinder removal, re-piping, and new boiler installation
System → Combi	£2,000–£3,500	1–2 days	Cylinder removal and flue reconfiguration
Back Boiler → Combi	£3,000–£4,500	Up to 3 days	Full decommission, flue relocation, and pipe upgrades
Combi → Combi	£1,800–£2,800	1 day	Straight replacement, minimal disruption

Cost by Home Size and Boiler Output

Home Type	Recommended Boiler Size (kW)	Typical Installation Cost
1–2 bed flat	24–27 kW	£2,000–£2,600
3–4 bed semi-detached	28–33 kW	£2,500–£3,500
5+ bed detached	35–40 kW	£3,500–£4,500

Converting from a conventional system to a combi can fall slightly above or below average costs, depending on the scope of work.

Optional Extras That Affect the Final Price

• Smart thermostat installation: £100–£300
• Magnetic filter or scale reducer: £100–£150
• Flue relocation: £150–£300
• System flush or power flush: £100–£200
• Extended manufacturer warranty: £100–£200

Regional Variations and Labour Rates

Installation costs vary by region. Homeowners in London and the South East often pay 10–15% more than those in the northern areas due to higher labour rates. Always choose a Gas Safe registered engineer to ensure the installation meets UK safety and compliance standards.

Check out our guide on options available if you need a new boiler but can’t afford it, as well as boilers on finance and government boiler grants here.

The Process of Changing To A Combi Boiler

Changing from a conventional or system boiler to a combi involves more than a simple swap. It’s a complete system conversion that replaces stored hot water tanks and reroutes pipework so the new boiler can heat water directly from the mains. The process usually takes one to three days, depending on your home’s layout and the complexity of your existing system.

1. Initial Assessment and Quotation

A Gas Safe engineer begins by assessing your current setup. They’ll check the location of your old boiler, the condition of the flue, and the pressure of your mains water supply. This assessment determines whether a combi is suitable and if any upgrades to pipework or radiators are needed.

At this stage, you’ll receive a detailed quote covering the boiler type, labour, materials, and optional extras such as filters or smart thermostats.

2. Removal of Old Equipment

If you have a regular or system boiler, the engineer will remove the hot-water cylinder (usually in an airing cupboard) and the cold-water tank in the loft. This step frees up space and simplifies the overall heating system. Old pipework connected to these tanks is either capped or rerouted.

3. Preparing the Pipework

The heating and hot water circuits are adapted so the combi can supply water directly from the mains. This may involve running new copper pipes and upgrading gas supply lines if the existing ones are too narrow to meet modern safety and efficiency standards.

4. Installing the New Combi Boiler

The new boiler is often mounted in the same place as the old unit, though some homeowners choose a kitchen or utility room for convenience. The engineer connects it to the water, gas, and heating circuits, installs the flue for ventilation, and fits a condensate pipe to safely discharge wastewater.

5. System Flush and Pressure Testing

Before the system is sealed, the engineer performs a chemical or power flush to remove sludge and debris. This helps protect the new boiler from corrosion and improves overall efficiency. Once flushed, the entire system is refilled, pressurised, and tested for leaks.

6. Wiring, Controls, and Smart Setup

The installer connects the boiler to your thermostat or smart control system. Modern combis often pair with smart thermostats like Hive or Google Nest, giving you precise control over temperature schedules and energy use.

7. Testing and Handover

Finally, the engineer runs multiple safety and performance checks to confirm everything is operating correctly. They’ll issue a Gas Safe certificate, register the boiler warranty with the manufacturer, and explain how to use your new controls effectively.

Typical Timeline

• Day 1: Old system removal and preparation
• Day 2: Boiler installation and pipework
• Day 3: Testing, commissioning, and cleanup

A straightforward combi-to-combi replacement can be completed in a single day. A whole system or a regular conversion may take up to three days if extensive repiping or flue relocation is required.

Key Benefits of Changing to a Combi Boiler

Upgrading to a modern combi boiler brings both immediate and long-term advantages. The system’s compact, efficient design means you can enjoy reliable heating and hot water without the energy waste or space requirements of older models. Below are the key reasons most UK homeowners now choose to make the switch.

1. Higher Energy Efficiency

Combi boilers are designed to extract as much heat as possible from the fuel they use. Most models are A-rated for efficiency, operating at 90–94% efficiency compared to 70% or less for older boilers. This translates into significant savings on gas bills and a measurable reduction in household carbon emissions. Combis heat water only when needed, meaning there’s no wasted energy maintaining a full tank of stored hot water.

Energy Saving Example: Replacing a 70%-efficient G-rated boiler with an A-rated combi can save from £130 to £500 per year, based on current Energy Saving Trust data and typical gas prices.

2. Instant Hot Water on Demand

Unlike system or regular boilers, combis draw water directly from the mains. When a tap is turned on, the boiler fires up instantly to provide continuous hot water. This means no waiting for a tank to reheat and no running out of hot water, even after long showers.

3. Space Saving Design

Because they eliminate the need for both a cold-water storage tank and a separate hot-water cylinder, combi boilers are ideal for smaller homes, flats, or properties without loft space. The entire system fits neatly into a kitchen cupboard or utility area, freeing up valuable storage room elsewhere in the house.

4. Reduced Installation and Maintenance Costs

With fewer components, installation is usually quicker and cheaper compared to systems that require multiple tanks and cylinders. The simplified setup also means less maintenance over time, and many modern combis come with extended warranties of up to 10 or 12 years for peace of mind.

5. Smart Controls and Modern Compatibility

Most new combi boilers are compatible with smart thermostats such as Hive, Nest, or Tado, allowing homeowners to monitor usage and control heating remotely. These smart systems help fine-tune energy consumption, reducing unnecessary heating and further improving efficiency.

6. Environmentally Friendly Performance

Modern combi boilers produce lower carbon emissions than older systems, and many are hydrogen-ready, meaning they can operate with a blend of natural gas and hydrogen as the UK transitions toward low-carbon heating.

Quick Recap Box

• Efficiency rating: Up to 94%
• Potential annual savings: £350–£500
• Space saved: Loft tank and cylinder removed
• Typical lifespan: 10–15 years
• Hydrogen-ready and smart control compatible

Drawbacks and When a Combi Isn’t the Best Choice

While combi boilers are the most popular choice for modern homes, they aren’t ideal for every household. Understanding the limitations before you commit to conversion helps ensure that your system matches your home’s size, pressure, and water demands.

1. Limited Hot Water Flow for Multiple Outlets

A combi boiler heats water on demand from a single mains feed. This means that if two or more taps or showers are used at the same time, the water flow rate can drop noticeably.

Homes with two or more bathrooms may experience lower pressure or fluctuating temperatures when multiple outlets are in use simultaneously. In these cases, a system boiler with a storage cylinder is usually the better fit.

2. Dependence on Mains Water Pressure

Because a combi doesn’t rely on stored water, it depends entirely on the pressure of your incoming mains supply. If the flow rate is below 10 litres per minute, the boiler may struggle to deliver hot water effectively. Areas with low mains pressure might need a booster pump or an alternative boiler type.

3. More Complex Installation for Older Properties

Older homes built with regular or back boiler systems may require extensive repiping to accommodate a combi setup. Removing loft tanks, re-routing pipes, or installing a new flue can add time and cost to the project. In rare cases, the home’s structure may limit where the boiler can be installed.

4. No Back-Up Hot Water Supply

Traditional systems store hot water in a cylinder, meaning you can still access hot water even if the boiler breaks down temporarily. A combi boiler, on the other hand, has no stored reserve, so any fault could mean a short period without hot water until repairs are made.

5. Energy Demand for Large Households

For large families or homes with frequent, simultaneous hot water use, a combi may not keep up with demand. In such cases, a storage combi or system boiler can offer higher flow rates without sacrificing efficiency.

Key Consideration Box

• Best for: Homes with 1–2 bathrooms and good mains pressure
• Avoid for: Properties with multiple bathrooms or low pressure
• Check before converting: Flow rate, pipe condition, and boiler size

Comparison Overview

Feature	Combi Boiler	System Boiler	Regular Boiler
Space Requirement	Minimal	Moderate	High
Hot Water Storage	None	Cylinder	Cylinder + Cold Tank
Water Pressure	Mains dependent	Strong	Moderate
Installation Complexity	Medium	Low	High
Ideal For	Small to medium homes	Medium to large homes	Older systems

For many households, a combi boiler remains the most practical and efficient choice, but confirming mains pressure, bathroom count, and hot water habits with a qualified engineer ensures that your investment pays off in comfort and reliability.

Factors That Affect the Cost of Changing To A Combi Boiler

Several key factors determine how much it will cost to replace your existing boiler with a combi. Even when two households have similar-sized properties, small technical differences, such as boiler location, flue routing, or pipe condition, can shift the final quote by hundreds of pounds.

Understanding these cost variables helps you make an informed decision and avoid hidden expenses.

1. Type and Complexity of Conversion

The most significant influence on cost is the type of boiler system you’re converting from. A straightforward combi-to-combi swap is the cheapest option, while converting from a regular or back boiler system is more complex. Removing storage tanks, re-piping, and modifying the flue can add both time and labour costs. Complex installations often fall toward the upper end of the £4,500 range.

2. Boiler Brand and Model

Boiler prices vary widely between manufacturers. Premium models from Worcester Bosch, Vaillant, or Viessmann typically cost £1,200–£1,800 for the unit alone, while mid-range brands such as Ideal or Baxi often start around £800. The higher price usually comes with longer warranties, better energy efficiency, and quieter operation, essential factors if your boiler is installed near living spaces.

3. Boiler Output and Property Size

Larger homes require higher-output boilers to maintain a steady heating and hot water supply. A 24 kW combi is sufficient for a small flat, but a 35–40 kW unit is more appropriate for a large detached home. As output increases, both boiler costs and gas consumption rise, affecting long-term operating costs.

4. Pipework and Flue Requirements

If your existing pipework is outdated or too narrow, it may need to be upgraded to comply with modern safety and performance standards. Relocating the flue or vent to meet current Building Regulations can also increase costs, especially when drilling through walls or routing through upper floors is required.

5. Smart Controls and Accessories

Integrating your boiler with smart thermostats such as Hive, Nest, or Tado adds convenience and can help you save additional energy each year. Accessories like magnetic filters, scale reducers, or condensate pumps improve performance but also add £100–£300 to the total installation cost.

6. Regional Labour Rates

Labour costs differ across the UK. Installations in London and the South East tend to be around 10–15% more expensive due to higher overheads, while rates in Northern England, Scotland, and Wales are often lower. Checking multiple local quotes helps ensure fair pricing.

7. Warranty and Aftercare

Choosing a boiler with a 10–12-year warranty provides long-term peace of mind but can slightly increase upfront costs. Many manufacturers offer extended coverage if the system is installed by an approved engineer and serviced annually, which can be a worthwhile investment for homeowners planning to stay in the property for several years.

Tip: Always confirm that your installer is Gas Safe registered. It’s not only a legal requirement but also protects your warranty and ensures that all safety checks are appropriately completed.

How to Prepare for Changing To A Combi Boiler

Preparing your home before a combi boiler conversion makes the installation smoother, faster, and often cheaper. A few practical steps can help avoid delays, ensure safety, and reduce labour time on the day your engineer arrives.

1. Clear Access for the Engineer

Ensure the engineer has easy access to the current boiler, airing cupboard, and loft tanks (if they’re being removed). Move any stored items or obstructions near these areas. If your new boiler is going into a kitchen or utility room cupboard, clear the surrounding space so pipework and controls are easy to reach.

2. Confirm Gas and Water Pressure

Your installer will check the mains water pressure and gas flow rate before fitting the new unit. If the pressure is below 1 bar or the flow rate is low, your engineer might recommend a booster pump or an alternative system. Knowing this in advance helps avoid last-minute design changes.

3. Discuss Boiler Location and Flue Options

Modern combi boilers can be wall-mounted in a kitchen, utility room, or airing cupboard, but the flue must be installed in accordance with Building Regulations. Discuss with your engineer where the new boiler will sit and how the flue will vent safely outside. If the boiler is being relocated, extra pipework or brickwork may be required, so plan accordingly.

4. Schedule Around Downtime

During installation, the heating and hot water supply will be switched off. For most conversions, this downtime lasts between one and three days, depending on complexity. Plan showers, cooking, and laundry around the installation period, especially during colder months.

5. Choose Controls and Accessories

Decide whether you’ll install a smart thermostat, filter, or scale reducer before work begins. These additions are easiest to fit during installation rather than after, and they improve long-term efficiency and performance.

6. Prepare for a System Flush

Expect your engineer to perform a system flush before the new boiler is connected. This process clears sludge and debris from old radiators and pipes, helping to protect your new combi and extend its lifespan. Make sure the engineer includes this step in the quote.

Customer Preparation Checklist

• Clear access to the boiler, loft, and airing cupboard
• Confirm water pressure and flow rate
• Discuss boiler location and flue position
• Plan for 1–3 days without hot water
• Approve thermostat, filter, and accessories
• Ensure a system flush is included in your quote

Taking these steps ensures a smooth, compliant installation and reduces the likelihood of follow-up work or additional charges.

Final Thoughts: Is Changing to a Combi Boiler the Right Choice?

Switching to a combi boiler is one of the most effective ways to modernise your home’s heating system and reduce energy waste. It combines efficiency, convenience, and space savings in a single upgrade, offering a strong balance between upfront investment and long-term value. For most small- to medium-sized UK homes, a combi boiler delivers reliable heating, lower running costs, and a smaller carbon footprint than traditional systems.

However, the right choice depends on your home’s layout, water pressure, and daily hot water demand. Households with multiple bathrooms or low mains pressure may be better served by a system boiler or storage combi, both of which retain efficiency while supporting higher demand.

Ultimately, the key to a successful conversion lies in professional assessment and quality installation. A Gas Safe-registered engineer can confirm whether your property is suitable, calculate the correct boiler size, and ensure the system is installed safely and efficiently.

FAQs About Changing to a Combi Boiler

Is It Worth Converting to a Combi Boiler?

Yes. For most small- to medium-sized UK homes, converting to a combi boiler is both cost-effective and energy-efficient. You’ll remove bulky tanks, gain instant hot water, and reduce energy waste. The long-term savings on gas bills (up to £500 per year) and the space gained often make it worthwhile.

Is a Combi Boiler Better Than a Water Tank?

For many households, yes. A combi boiler provides hot water on demand directly from the mains, eliminating waiting time and storage losses. Systems with water tanks can handle higher simultaneous demand but lose efficiency due to heat loss from stored water.

How to Replace a Back Boiler With a Combi Boiler?

Replacing a back boiler requires a complete conversion. The engineer will decommission the old unit, often located behind a fireplace, and install the new combi in a more accessible location, such as the kitchen or utility room. New pipework and a flue will be fitted, and a system flush performed. This process takes around 3 days.

How Much to Remove the Water Tank and Fit a Combi Boiler?

According to the Energy Saving Trust, the average cost of a gas boiler replacement in Great Britain is around £3,700. The final price depends on the complexity of the installation and whether pipework or flue relocation is required.

What Are the Disadvantages of a Combi Boiler?

Combi boilers rely entirely on mains water pressure, so areas with low pressure may experience weaker flow. They also struggle to supply hot water to multiple bathrooms simultaneously, making them less suitable for large households. In addition, because combis don’t store hot water, a fault can leave you temporarily without a supply until repairs are completed.

Can You Change a Water Tank to a Combi Boiler?

Yes. A qualified Gas Safe engineer can safely remove your cold and hot water tanks and reconfigure the pipework to install a combi boiler. The process takes 1–3 days, depending on your current system and includes testing, system flushing, and Gas Safe certification. It’s one of the most common upgrades in UK homes.

Should I Get Rid of My Water Tank?

If you have reliable mains pressure and a moderate hot water demand, removing your tank and switching to a combi boiler can save space and energy. However, if your home has several bathrooms or low pressure, keeping a system boiler with a hot water cylinder might be a better choice. A professional assessment can help you confirm which option offers the best balance of comfort, cost, and performance.

Sources and References

• Building Services Engineering Research & Technology – Space heating operation of combination boilers in the UK: The case for addressing real-world boiler performance
• Energy Saving Trust – Boiler types explained