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Choosing the Right Size Combi Boiler: Reducing Your Energy Bills

Combi boilers are the most common type of gas boiler in the UK. They don’t take up much space, no hot water tank is needed, and you can have hot water on demand. However, it’s essential to choose the right size boiler.

Boiler Size Calculator – What Size Boiler Do I Need?

Choosing the Right Size Combi Boiler guide and drawing for installers

Choosing the correct size combi boiler for your home is crucial to ensure efficient heating and hot water, and to reduce your energy bills. The key is calculating your heating and hot water requirements accurately.

A combi boiler that’s too large will operate inefficiently and cost more to run. An undersized combi won’t meet your home’s full hot water demands.

This guide explains how to choose the right size combi boiler using a full system boiler size calculator or by estimating your needs. We cover combi boilers specifically, including how they work, as well as types, sizes, and measuring units. Read on for advice on combi boiler sizing for different property sizes and styles.

What Size Combi Boiler should I choose? – Key Points

cavity wall insulation being installed for those qualifying state benefits

The size refers to the boiler’s power output, measured in kW (kilowatts)

·An incorrectly sized boiler wastes energy and costs more to run

·Combi boilers heat water directly when you turn on the tap

·Calculate size based on radiators, hot water use, house size, and insulation

·Oversizing the boiler by too much reduces efficiency significantly

·New boilers are more efficient so often a lower heating output which is adequate

·Speak to a Gas Safe registered engineer for advice on your specific needs

Why it’s important to choose the right size boiler for your home

Heating and hot water account for around 60% of a household’s energy use. Choosing an efficiently sized combi boiler is one of the biggest factors in controlling your energy bills.

The optimal boiler size depends on your home’s heat loss, hot water requirements, and number of radiators. The best way to calculate what size combi boiler you need is to use an online boiler size calculator. You’ll need to input details of your home.

Installing a combi boiler with more output power than necessary is a false economy. Larger models cost more upfront. More importantly, an oversized combi wastes fuel by frequently stopping and starting. This reduces efficiency.

An undersized combi won’t be able to produce enough hot water and heat for your needs. This could leave you with cold showers or parts of your home being chilly.

Getting the size just right means your new combi will operate at peak efficiency. This saves energy and maximises performance.

Calculating your heating and hot water usage

Couple discussing their energy bill for the heating, look worried.

The starting point is working out your property’s heat and hot water demand. As a guide, here are typical domestic combi boiler sizes:

·1 bedroom house or flat – 24kW combi

·2/3 bed house – 28-30kW combi

·3/4 bed house – 32-36kW combi

·4/5 bed house – 38-42kW combi

However, this can vary considerably. A detached 5-bed house may only need a 30kW boiler if it’s modern and well-insulated.

To size your combi accurately, take account of your physical dimensions:

·Level of insulation and draught proofing

·Number and size of radiators

·Hot water usage – baths, showers, taps

·household occupancy

An engineer can assess this in detail during a physical survey. But there are also free online boiler size calculators you can use yourself.

Should I choose an oversized Combi Boiler for my home?

It’s common to think that a more powerful combi boiler is better. Dramatically oversizing your boiler can reduce efficiency.

Larger boilers have higher heat output. This exceeds the demand for radiators and hot water in a typical home. The boiler fires up, heats water very quickly, then shuts down. This on-off cycling happens repeatedly, which wastes fuel.

Moderately oversizing the boiler by around 10% is often recommended. This caters to extremes of weather or heavy use. But oversizing by 30% or more compromises efficiency.

Why is correct boiler sizing important?

Energy efficiency – An oversized boiler cycles on and off, which wastes fuel. Correctly sized combis operate continuously for maximum efficiency.

Running costs – Too large a boiler increases gas or oil consumption. The right-size system boiler uses the minimum fuel to meet your needs.

Boiler lifespan – Repeated heating up and cooling reduces component lifespan. Appropriate sizing maximises longevity.

Carbon emissions – Wasted energy equals higher emissions. Optimal sizing minimises your heating’s environmental impact.

Convenience – A boiler with insufficient output leaves you lacking hot water or warmth. The correctly sized combi maintains constant hot, water supply and heat.

Hot water demand

A tap with how flowing water and steam from the combi boiler

Most combis can deliver over 10 litres of hot water a minute, with some up to 16 litres. Consider your household’s typical peak hot water demand. Factor in:

·         Number of bathrooms, en-suites, cloakrooms

·         Showers, baths, basin taps

·         Dishwasher, washing machine

·         Number of occupants

·         Time of bathing – mornings and evenings are typically the busiest

Don’t overlook infrequent situations like visiting guests, parties, or the whole family bathing in quick succession. It’s better to slightly oversize the boiler than undersize it.

Number of radiators

The number and size of your radiators determine the necessary boiler output. Count your total number of radiators. Note down the dimensions of each e.g., 600mm x 400mm.

The bigger the radiators, the more powerful the boiler is needed. Modern radiators are smaller as houses are better insulated, so don’t need huge old cast iron ones.

A useful rule of thumb is allowing 1kW of boiler output for each 1m2 (10 square feet) of radiator surface area.

Yound boy looks at the snow outside wearing pjs and looking happy.

Calculating your boiler size – it’s smaller than you think!

Thanks to improved insulation and efficiency, replacement boilers are often smaller than the decades-old model they’re replacing.

A typical 3-bed 1970s semi may have an old 30kW boiler. But a new A-rated 28kW combi is usually ample. The better insulation, draughtproofing, and controls reduce heat loss. New condensing boiler technology also squeezes more heat from less fuel.

So, don’t base your new boiler’s size on the old one. Calculate your actual requirements using a boiler calculator. Get it right and you could install a smaller, less expensive combi and still be warmer!

Property size, layout, and demand

When choosing the right size combi, know that larger properties need more powerful boilers, but the relationship isn’t linear. Double the floor space doesn’t necessarily double the boiler’s size.

Critical factors are the number of bathrooms, overall heat loss, and water demand. For example:

·A well-insulated, modern 3-bed house may need a 24-30kW boiler.

·A poorly insulated 5-bed Victorian house might also suit a 30kW boiler.

·A 3 bed 1970s semi could need a 28kW boiler.

·A new 3-bed detached with 2 bathrooms may require just a 24kW model.

Use your online boiler calculator results to determine the size, not the property type or dimensions. Arrange a heat loss survey if in any doubt.

Impact on boiler efficiency

Oversizing a combi boiler by too much reduces efficiency and wastes gas. This is how:

·The boiler fires up and heats water very rapidly

· It reaches temperature faster than radiators or taps can disperse the heat

·With no immediate demand, the boiler switches off again

·This on-off cycling happens frequently, using fuel less efficiently

·In contrast, a correctly sized combi precisely meets the heat demand. It can operate continuously at a lower intensity for optimum efficiency.

Heat loss

A home’s heat loss is a vital calculation in determining boiler size. Heat escapes through:

·Walls – Solid walls lose more heat than cavity or insulated cavity walls.

·Windows – Single glazing loses far more heat than double or triple glazing.

·Doors – Solid doors lose more heat than insulated ones.

·Roof – Insulation level is key – 100mm is better than 50mm.

·Floors – Suspended timber floors lose heat into the void below.

·Draughts – Gaps around windows, doors, floors, skirting, etc.

The lower the heat loss, the lower the boiler output needed. A modern, well-insulated home needs a far smaller boiler than an unimproved older property.

Type of fuel

The main heating fuel options are:

·         Natural gas – Serves most UK homes, and burns very efficiently.

·         LPG – Bottled gas used where there’s no mains gas supply.

·         Oil – Typically used in rural areas without mains gas.

·         Electric – Direct electric heating or via heat pumps.

For the highest efficiency and lowest costs, gas condensing boilers are recommended. LPG is similarly efficient but more costly per unit.

Oil and electricity are viable, especially with renewable sources like heat pumps and solar power, but the carbon emissions and running costs tend to be higher.

What type of boiler is right for your home?

There are three main boiler types:

·         Combi – Instant hot water, heats water directly when taps are turned on. No hot water tank.

·         Heat only – Heats water stored in a cylinder. No instant hot water.

·         System – Heats water cylinder and provides central heating.

Combis are now by far the most popular choice. They provide convenience and efficiency. Ensure you pick the right combi boiler size using this guide’s advice.

Heat-only boilers require correct cylinder sizing. System boilers need cylinder and boiler sized correctly.

Mains water pressure

Combi boilers require sufficient mains water pressure – 1 bar minimum, but 1.5-2 bars ideal.

Low pressure causes weak showers. Check pressure and consider a booster pump if fitting a combi boiler.

Fixed price boilers, fitted fast

Once you’ve worked through choosing the right size

boiler and decided on the make and model, it’s time to arrange professional installation. Many plumbers offer fixed-price combi boiler changes. This includes all work for one all-in fee, avoiding surprise extras. A combi can usually be swapped on the same day. With minimal disruption, you can be enjoying reliable heating and water as soon as possible.

How to get the best price on a new (correct size!) boiler

·         Use online boiler sizing calculators to determine your needs

·         Research which makes and models best suit your requirements

·         Compare prices from national and local installers

·         Don’t assume bigger is better – correctly sized small combis cost less

·         Arrange a heat loss survey if your needs are complex or unusual

·         Ask what controls are included – room thermostats and TRVs optimise efficiency

·         Get fixed price quotes for supply, installation, warranty, controls, etc.

What size combi boiler by house type

Here is a very general guide to typical combi boiler sizes for broad property types:

·Studio flat – Typically 24kW

·1 bed flat – 24-28kW

·2 bed flat – 28-30kW

· 2-bed house – 28-30kW

· 3-bed house – 30-32kW

· 4-bed house – 32-36kW

· 5-bed house – 36-40kW

· 6-bed house – 38-42kW

But don’t rely on this! Use an online boiler calculator for your specific property’s needs.

How is Combi Boiler size measured?

Combi boiler power output is measured in kilowatts (kW). The higher the kW, the more powerful the boiler.

Typical domestic combi boiler sizes are:

·         24kW – Suitable for smaller 1-2 bed house/flat

·         28kW – Ideal for 2-3 bed house

·         30kW – Meets demands of most 3-4 bed houses

·         35kW – For larger 4-5 bed detached houses

Combis over 40kW are unusual for domestic settings but are sometimes needed in big properties.

Ensure you choose the boiler size your home needs, no bigger or smaller.

Conventional Boilers

Conventional boilers, also known as heat-only or regular boilers, work best with a hot water cylinder. Here are typical cylinder sizes by property boiler type below:

·1-2 bed house – 90-110 litre cylinder

· 3-bed house – 110-150 litres

· 4-bed house – 150-180 litres

· 5-bed house – 180-210 litres

Increase cylinder and boiler size if there’s more than one bathroom. Base it on your water demand.

Conventional boilers often suit homes with multiple bathrooms better than combis.

Sizing a combi boiler

Follow this process when choosing the right combi boiler:

·         Estimate boiler size needed from house type

·         Use an online boiler calculator for your specific property

·         Allow extra capacity for extremes of weather or demand

·         Choose a boiler with a sufficient heat output for heating and hot water

·         Select the smallest combination that meets your requirements

Radiator with a cat laid on top looking happy and warm

What size combi boiler do you need for your home?

Most households require a combi boiler in the range of 24-38kW. Use an online boiler calculator to determine your specific requirements. Oversize by 10% maximum – installing a combi far bigger than needed wastes energy and money.

Sizing a heat-only boiler

Unlike combination boilers, heat-only boilers work with a hot water cylinder to provide heating and hot water. Correctly size your cylinder and boiler:


Match cylinder capacity to your property’s water demand in litres. Common sizes:

·         Up to 2 bed – 90-120 litres

·         3 bed – 120-150 litres

·         4 bed – 150-180 litres

·         5 bed – 180+ litres


Choose a boiler with 1kW output for every 10 litres of cylinder capacity, e.g.:

· 100-litre cylinder – 10kW boiler

· 150-litre cylinder – 15kW boiler

· 200-litre cylinder – 20kW boiler

This ensures your new boiler that can heat the cylinder fully.

Further, help and advice

Sizing a boiler correctly requires accurate calculations and expert knowledge. If in any doubt, speak to qualified heating engineers. They can assess your home and suggest the perfect, efficient boiler-size solution. Most will provide free quotes with no obligation. They can also explain grant funding, incentives, and payment options.

Sizing for your hot water cylinder

If opting for a conventional or system boiler, your water cylinder’s size is critical.

Choose a tank capacity to suit your home’s peak water demand. Here are common cylinder sizes:

·         1 bathroom – 90-120 litres

·         2 bathrooms – 120-150 litres

·         3 bathrooms – 150-180 litres

·         4 bathrooms – 180+ litres

Increase for high-demand homes with multiple showers. Reduce for properties with very low water needs.

Your boiler must be capable of heating the entire cylinder contents. Allow around 1kW of boiler power per 10 litres of cylinder capacity.

How to calculate what size system or regular boiler your home needs

System and regular (conventional) boilers both heat a hot water cylinder rather than instant water like a combi. Follow this process:

·         Estimate cylinder size needed based on bathrooms and occupancy

·         Choose the actual cylinder capacity appropriate for your maximum water demand

·         Allow 1kW of boiler power for every 10 litres of cylinder capacity

·         Add estimated boiler size for your radiators’ requirements

·         Select the boiler with the total kW output calculated

·         Alternatively, arrange a heating engineer’s survey for expert advice

This ensures your boiler can provide sufficient heat and sufficient heated water, as well as fully meet your central heating system requirements.

Combi boilers according to brand and budget

There is an extensive range of makes and models of combi boilers to suit all requirements and budgets. Here are typical power outputs by brand:

·         Worcester Bosch – 24kW to 42kW combis

·         Vaillant – 24kW to 38kW combis

·         Baxi – 24kW to 42kW combis

·         Ideal – 24kW to 42kW combis

·         Potterton – 24kW to 38kW combis

·         Glow-worm – 24kW to 40kW combis

Higher cost does not necessarily mean better or more suitable. Get quotes comparing branded boilers matching your calculated size requirements.

Combi boilers

Combi boilers are the most popular choice for UK homes. They provide central heating and instant hot water, heat water directly when you turn on the tap – no cylinder or water tank – and are suitable for most homes but ensure sufficient mains water pressure. Choosing the right size combi

saves money and maximises efficiency.

What happens if a boiler is oversized?

It’s often thought that bigger is better when it comes to boilers. But significantly oversizing your new combi boiler can be counterproductive.

If a boiler has more output capacity than your home can use, the following occurs:

·         The boiler quickly heats water to the set temperature

·         It reaches this temperature faster than radiators can emit heat

·         With no immediate demand, the boiler then switches off again

·         This on-off cycling happens frequently, using fuel less efficiently

The boiler is continually stopping and starting which compromises efficiency. This uses more gas or oil than necessary, costing you more.

In contrast, a properly sized boiler precisely meets your home’s demands. It can maintain a continuous, steady optimal operation rather than cycling on and off.

Radiator balancing

For maximum boiler efficiency, ensure all radiators emit heat effectively. Balance radiators across each room by:

·         Identifying poorly heating radiators and checking for obstructions

·         Adjusting lock shield valves to regulate each radiator

·         Fitting thermostatic radiator valves (TRVs) throughout

·         Upgrading old radiators – replace with newer smaller models

·         Bleeding radiators to avoid trapped air impeding water flow rate water flow

This optimises heat emission. Your boiler only needs to heat water to the temperature required rather than overcompensating for imbalanced radiators.

Guide to boiler efficiencies

All boilers must meet minimum efficiency standards. Choose A-rated models for maximum performance.

Standard boilers – Now phased out, older non-condensing boilers with efficiency of 60-80%. Offer basic heating and hot water only.

Combi and system boilers – Condensing technology improves efficiency to around 90-95%. Recovered heat from flue gases heats water.

Biomass boilers – Burn wood pellets, logs, or chips. Offer efficiency of 60-90%. Lower greenhouse gas emissions than fossil fuels.

Gas condensing boilers – Achieve over 90% efficiency by recovering waste heat. Compact and suitable for most homes. Low carbon option when replacing old conventional boiler used.

Oil condensing boilers – Similar technology to gas models. Must have condensing capability to achieve over 90% efficiency.

LPG (liquefied petroleum gas) condensing boilers – Use liquid petroleum gas supplied in tanks. Condensing models offer efficiency comparable to natural- gas boilers.

Choosing the right size combi boiler means reliable heating and water for years to come.

Disclosure: This post contains affiliate links, including Amazon Associate links, through which I earn from qualifying purchases at no extra cost to you. I only recommend products I genuinely believe in. Visit my Privacy Policy page for more information.