The most energy efficient boilers

An energy efficient gas boiler combines an A-rated condensing design with the right output, controls and installation for your home.

Worcester Bosch Greenstar 4000, Vaillant ecoTEC plus, Viessmann Vitodens 100-W and Ideal Logic+ Combi2 are models all up there as top performers when it comes to efficiency.

Your potential saving depends mainly on the boiler you are replacing and how much heat you use. If your existing condensing boiler works well, improving its settings and controls may be a better first step than replacing it. This guide focuses on domestic gas boilers.

What do you need help with?

• I’m choosing a new boiler - compare the models.

• I want to know what I could save - see the worked example.

• My boiler still works - find ways to reduce wasted energy.

Which energy efficient boilers should you compare?

Start with the heating rating and hot-water performance, then check how closely the boiler can match your home's heating demand.

This quick comparison covers four specific combi models; system and heat-only boilers are explained under choosing the right system.

iHeat supplies and installs boilers from the brands below.

The comparison uses published manufacturer specifications, rather than independent laboratory testing or measured customer fuel savings.

How to read the figures: the seasonal heating rating is a standardised comparison, not a guaranteed efficiency in your home. Hot-water flow shows how much water the boiler can heat each minute. A 35°C temperature rise means, for example, heating incoming water from 10°C to 45°C.

Quick comparison of four A-rated gas combi boilers

Viessmann's exact-model brochure confirms the A class; a model-specific seasonal percentage is not used here. Minimum heating outputs appear in the detailed reviews with their published water temperatures, because the manufacturers do not all describe those test conditions in the same way.

Sources: Worcester product specifications and ErP data sheet; Vaillant specifications; Viessmann B1KG brochure; Ideal Combi2 specifications. Specifications checked 4 October 2026. Hot-water figures use a 35°C temperature rise; actual delivery also depends on the incoming mains supply.

How much cheaper is a new boiler to run?

The saving depends on the efficiency improvement, how much useful heat you need and your gas unit price. A single “save up to” figure cannot describe every household.

The illustration below assumes 15,000kWh of useful space heat each year, gas at 7p/kWh including VAT, and a replacement achieving 94% seasonal efficiency in use.

These are example assumptions, not a current tariff quote or a prediction that a boiler's label rating will be achieved in your home. Hot water is excluded so the space-heating comparison remains clear.

Illustrative annual space-heating costs and savings

Calculation: annual useful heat ÷ efficiency expressed as a decimal × gas price per kWh. Figures are rounded independently from unrounded calculations.

Standing charges, hot water, boiler electricity, servicing, repairs, installation and finance are excluded. Heat demand and indoor comfort are held constant.

In this example, moving from 70% to 94% cuts gas use for space heating by around 26%. Moving from 90% to 94% cuts it by around 4%.

Neither result supports assigning a fixed saving from the boiler's age alone.

A simple payback calculation also needs the installation price. An illustrative £2,500 replacement divided by £383 annual fuel savings gives about 6.5 years; divided by £50 gives about 50 years.

Should you replace your existing boiler?

Base your decision on efficiency, condition and repair costs, not age alone. A well-maintained condensing boiler may still be worth keeping.

Check these three things first

• Your boiler: find the make and model in your paperwork or on an accessible external label. Never remove the casing.

• Its condition: ask your engineer about efficiency, recurring faults and replacement parts.

• The full cost: compare repair and replacement quotes, including realistic fuel savings and ongoing servicing.

When does replacement make sense?

• Working condensing boiler: check controls, settings and system condition first.

• Recurring faults or an expensive repair: compare the repair cost with a complete replacement.

• Parts no longer available: discuss replacement options with your engineer.

• Poor heating or hot water: have the system assessed before choosing a larger boiler.

Higher bills do not automatically mean your boiler needs replacing. Tariff changes, colder weather and longer heating hours can also increase costs.

Replacing a back boiler?

Compare the complete conversion cost, including the new boiler position, flue, pipework and existing fire arrangement. It involves different work from a straightforward wall-mounted boiler replacement.

How much does an energy efficient boiler cost?

As checked on 4 October 2026, iHeat's published starting prices are £1,945 for a suitable straightforward combi replacement, £2,495 for a system replacement and £2,595 for a heat-only replacement, including installation and VAT.

These are entry prices for eligible packages, not quotations for every model in this guide. Do not assume a system-boiler starting price includes a new cylinder.

See the starting-price scope and installation examples.

What do boiler efficiency ratings actually mean?

Compare the same rating on each boiler. For heating your home, look for seasonal space-heating efficiency on the manufacturer’s ErP product information sheet.

Which figure should you use?

• Seasonal space-heating efficiency: use this to compare heating performance across boilers.

• Water-heating efficiency: a separate rating for producing hot water, relevant when comparing combis.

• Efficiency at a specific test point: performance under particular conditions, not a whole-season rating.

• SEDBUK: an older calculation method. Compare it with the same SEDBUK version, not directly with ErP.

Why can one boiler have two different percentages?

Worcester Bosch lists these figures for the Greenstar 4000:

• 94%: seasonal space-heating efficiency.

• 98.9%: useful efficiency at a specified low-temperature, part-load test point.

Both are valid measurements. The higher figure does not mean the boiler achieves that efficiency throughout the year. Worcester Bosch data sheet

What helps a condensing boiler run efficiently?

Condensing boilers recover heat from exhaust gases. Cooler water returning from the radiators helps this process.

Getting the most from your boiler therefore also depends on:

• Radiator sizing: enough heat output to keep rooms comfortable at suitable water temperatures.

• Heating controls: matching heating to when and where you need it.

• Temperature settings: allowing efficient operation while meeting your home’s needs.

The boiler’s rating is only part of the picture. Your heating system and settings matter too. Energy Saving Trust guidance

How to choose an efficient boiler for your home

Choose the right system type

What should you check beyond efficiency?

You do not need a combi to get condensing technology. Modern system and regular boilers can also recover heat from exhaust gases. Boiler-type guidance

Before choosing, check:

• Heating output: ask your installer to calculate your home’s heat loss. Maximum output should cover cold-weather demand; a low minimum output helps avoid frequent stopping and starting when less heat is needed.

• Hot-water performance: compare combi flow rates at the same temperature rise and check your mains supply can support them. A higher hot-water output does not mean your home needs more heating power.

• Compatible controls: ask whether the quoted system supports weather or load compensation. These adjust heating to outdoor conditions or room demand; app access alone does not provide either feature. Heating-controls guidance

Detailed boiler reviews: output, controls and noise

Compare outputs at the same test temperatures. 50/30°C means water leaves the boiler at 50°C and returns at 30°C.

Compare noise figures only when measured using the same method. Noise affects comfort, not efficiency.

Jump to: Worcester Bosch · Vaillant · Viessmann · Ideal.

Worcester Bosch Greenstar 4000

The useful distinction between the 25kW and 30kW Greenstar 4000 combis is their hot-water performance.

Choosing the 30kW version therefore gives more hot-water capacity without increasing the minimum heating output shown below.

Worcester Bosch Greenstar 4000. Check the exact model and output on your quotation.

Figures apply to the standard natural-gas combis, not separately listed Low NOx versions. Overall height includes the lower enclosure; side-panel height is 710mm. Dimensions exclude required installation and service clearances. Sources: Greenstar 4000 brochure and installation manual, technical data.

Controls and electricity use: the range is compatible with Bosch EasyControl.

Ask which controller and connectivity accessories are included. For the 25kW model, the ErP fiche declares auxiliary electricity consumption of 39W at full load, 13W at part load and 2W in standby.

These are separate operating conditions, not figures to add together or multiply by every hour of the year. Control compatibility; 25kW ErP fiche.

Noise and suitability: both combis share the brochure's 45dB(A) sound-power figure. That is a declared appliance measurement, not a promise of 45dB at your kitchen table.

A location near a bedroom still needs thought about wall construction and pipe vibration. Compare the two models on hot-water demand and installed price rather than assuming the larger number means better heating efficiency.

Vaillant ecoTEC plus

The ecoTEC plus 826 is worth considering where the home needs relatively little heat but still requires combi hot water.

The 832 increases both heating capacity and hot-water performance. Its slightly higher seasonal rating does not, by itself, make it the more economical fit.

Vaillant ecoTEC plus. Check the exact model and output on your quotation.

Casing dimensions only. Vaillant states its combis have a flow regulator set for a 42°C temperature rise; the 35°C figures are published comparison values, not an assurance of actual tap flow under every condition. Source: Vaillant ecoTEC plus specifications.

Why the 826 can suit a smaller heating load: at the stated 50/30°C conditions, a 3kW demand sits within its output range but below the 832's minimum. That gives the 826 scope to run continuously at that load, provided the rest of the system permits it. This is an illustrative comparison, not a prediction of annual savings. The 826 must still cover the property's cold-weather heat loss and the household's hot-water needs.

What IoniDETECT adds: Vaillant describes this as combustion monitoring that supports adjustment of the gas-burning process and a wider modulation range. It does not replace a heat-loss calculation or correct an unsuitable control setup.

Choose the controls as part of the package. Vaillant's sensoCOMFORT supports weather compensation, adjusting the system's operation to outdoor conditions. App connectivity is a separate consideration: ask whether a myVAILLANT connect gateway and the appropriate controller are included.

A connected boiler is not automatically configured for efficient low-temperature operation. sensoCOMFORT information.

Noise trade-off: the declared difference is only 1dB. That does not justify saying one will sound dramatically quieter in your home. Position, installation and operating load can influence what you actually hear; noise is a comfort consideration rather than a measure of gas efficiency.

Viessmann Vitodens 100-W

The current UK brochure includes the B1KG generation of the Vitodens 100-W combi. The example below is the B1KG-30, which provides 30kW continuous hot-water output but a lower maximum output for space heating.

Do not substitute B1KF, system-boiler or heat-only specifications simply because the casing carries the same 100-W name.

Viessmann Vitodens 100-W. Check the exact model and output on your quotation.

Sources: Viessmann UK B1KG brochure and Viessmann B1HG/B1KG installation manual, hosted by a distributor. Dimensions exclude installation clearances.

These load-specific sound measurements must not be ranked directly against a single ErP sound-power declaration.

Efficiency rating: the UK B1KG brochure confirms an A rating for both space heating and water heating. This comparison uses those confirmed classes; it does not assign the range's advertised maximum percentage to this individual model.

Why the output range matters: the published minimum is close to the Greenstar 4000's at 50/30°C.

That does not establish equal annual consumption, but it is a more meaningful starting point than comparing one boiler's low-temperature output with another's high-temperature figure. A small heating demand can still fall below either appliance's minimum.

What the technology contributes: the stainless-steel heat exchanger transfers heat to the heating water, while Lambda Pro controls combustion. ViCare provides access to operating information and heating schedules.

Ask which compatible room control and weather-compensation arrangement the quote includes: Wi-Fi provides connectivity, while the control strategy determines how heat is requested.

Noise needs context: the two declared figures show why a boiler can be relatively unobtrusive at part load but more noticeable when heating hot water.

Neither describes the sound level at a particular distance in your room. Do not use the 31.9dB(A) figure alone to claim this is the quietest boiler in the comparison.

Ideal Logic+ Combi2

The Logic+ Combi2's three sizes share a 94% seasonal space-heating rating, but their minimum heating outputs differ. A larger hot-water model may therefore be less well matched to a small heating load, even though its efficiency label looks identical.

Ideal Logic+ Combi2. Check the exact model and output on your quotation.

Source: Ideal Logic+ Combi2 specification sheet, Rev C. Figures apply to the Logic+ Combi2 generation. Ideal specifies mean water temperatures; these are not the same as flow temperatures.

What the numbers mean: moving from the 24 to the 35 increases hot-water delivery, while maximum heating output stays the same at each listed temperature condition. Minimum heating output rises.

Select the smallest hot-water capacity that genuinely meets demand, rather than assuming a bigger model creates a more efficient heating system.

For example, a 4kW heating demand is below all three listed minimum outputs at a mean water temperature of 40°C.

That does not mean the boilers cannot heat the home; it means continuous firing at that particular demand is unavailable. Controls and the heating system's ability to absorb heat determine how operation cycles.

Controls can improve how the system responds. Ideal's Logic2 controls guide describes load compensation through compatible Halo controls and weather compensation with Halo Wi-Fi or an appropriate outdoor-sensor arrangement.

Confirm the exact controller and enabled functions in the quote. Compensation cannot reduce the burner below its physical minimum output. Ideal controls guide.

Hot-water comfort has a trade-off: Combi2 includes intelligent pre-heat. Pre-heating can shorten the wait for hot water, but keeping components warm uses energy. Ask how the setting suits your usage pattern and follow the user guide. Do not assume every comfort feature minimises fuel consumption.

Noise: the Combi2 30 and 35 share the listed 46dB(A) sound-power value. That does not make the larger version equally well suited to a small heating load: acoustic performance and minimum burner output answer different questions.

How to save energy with your existing boiler

You may be able to reduce wasted heat before buying a replacement:

• Review the heating schedule. Match heating periods to occupancy while maintaining suitable warmth.

• Use room and radiator controls. Avoid overheating rooms and ask for help if the controls conflict or behave unexpectedly.

• Keep radiators unobstructed. Furniture and long curtains can restrict heat reaching the room.

• Address heat loss. Consider appropriate insulation and draught-proofing without blocking required ventilation.

• Have uneven heating investigated. System balancing, water condition and control faults may need professional attention.

• Follow the service schedule. Servicing supports safe operation and can identify faults; it is not a guarantee of a particular percentage saving.

Can lowering the boiler flow temperature help?

On a suitable condensing system, lower heating flow temperatures can improve efficiency by reducing the return temperature. They can also make rooms warm up more slowly, so settings must still meet the home's needs.

Radiator-water temperature, room temperature and stored hot-water temperature are different settings.

For a combi without a cylinder, follow the manual's instructions for the user-accessible heating control. If the boiler also heats a cylinder, or you are unsure how the controls interact, ask the installer before changing temperatures: stored-water hygiene requirements must be maintained.

Do not remove the casing or alter installer settings. Energy Saving Trust flow-temperature guidance.

What should you do next?

If your boiler is working well, start with its controls and condition. Identify the exact model, review your gas consumption in kWh and ask whether the system can operate more efficiently.

If replacement makes sense, compare complete installation packages. Ask each installer to explain the chosen heating output, hot-water capacity and control setup.

Use the boiler installation cost guide to compare the work included. If moving away from gas is part of your plan, also assess whether a heat pump could suit the property.