Replacing a combi boiler usually comes down to four names: Worcester Bosch, Vaillant, Viessmann and Ideal. The models most homeowners are quoted are the Greenstar 4000, the ecoTEC plus, the Vitodens 100-W and the Logic+ Combi2, and all four are A-rated condensing boilers.
As a heating engineer, I look past the energy label to the things that decide how a boiler treats you over ten years or more: the size range, hot water flow, the heat exchanger, the warranty conditions and how easy it is to get serviced. This guide compares them on exactly that.
This page contains affiliate links. If you get a quote through them, I may earn a commission at no extra cost to you. The boilers were chosen and reviewed independently. How this site earns money.
The four boilers in short
- All four are A-rated condensing combis, so the efficiency gap between them is small. Correct sizing and set-up matter more than the badge.
- Worcester Bosch Greenstar 4000: the safe all-rounder, with strong UK support and up to 10 years’ guarantee through an accredited installer.
- Vaillant ecoTEC plus: stainless steel heat exchanger and the widest range of sizes, up to about 16 litres a minute of hot water.
- Viessmann Vitodens 100-W: stainless steel heat exchanger and 7 years’ warranty as standard, just by registering it.
- Ideal Logic+ Combi2: the slimmest and lightest, usually the cheapest, with a 7-year warranty and an aluminium heat exchanger.
The four boilers at a glance
| Worcester Bosch Greenstar 4000 | Vaillant ecoTEC plus | Viessmann Vitodens 100-W | Ideal Logic+ Combi2 | |
|---|---|---|---|---|
| Combi sizes | 25 and 30 kW | About 26 to 40 kW | 26, 30 and 35 kW | 24, 30 and 35 kW |
| Hot water flow (35°C rise) | 10.5 to 12.3 l/min | About 10.5 to 15.9 l/min | About 10.6 to 14.3 l/min | 9.9 to 14.5 l/min |
| ErP rating | A | A | A | A |
| Heat exchanger | Not published by maker | Stainless steel | Stainless steel | Aluminium |
| Size (H x W x D) | 724 x 400 x 310 mm | 720 x 440 x 338 mm (smaller models) | 700 x 400 x 350 mm | 700 x 395 x 278 mm |
| Warranty | Up to 10 years, accredited installer | Up to 10 years, Vaillant Advance installer | 7 years if registered within 30 days | 7 years |
| Best for | Peace of mind and support | Higher hot water demand | Long cover without an accredited installer | Tight cupboards and budgets |
Figures from manufacturer and merchant spec sheets, October 2026. Exact outputs and flow rates vary by model; check the spec sheet for the size you are quoted.
On efficiency there is little to separate them: all four are A-rated and fully condensing. The real differences are the size range, how much hot water each delivers, what the heat exchanger is made of, and how hard you have to work to keep the warranty.
What size combi boiler do you need?
A combi heats water as you use it, so its headline kW figure is mostly about hot water flow, not heating. Bigger is not better: an oversized boiler short cycles on mild days and wastes gas.
| Home | Typical combi size | Hot water flow |
|---|---|---|
| Flat or small house, one bathroom, up to about 10 radiators | 24 to 26 kW | About 10 l/min: one outlet at a time |
| Three or four-bed house, one main bathroom | 30 kW | About 12 l/min: a shower plus a tap |
| Larger house, bathroom plus en-suite | 35 kW or more | 14 l/min or more, if your mains pressure keeps up |
| Two or more bathrooms used at once | Consider a system boiler and cylinder instead | A combi heats water on demand and can only share it so far |
The headline kW on a combi is mostly about hot water. Heating output is usually lower, and your heat loss decides what the house needs.
Your mains water pressure and flow also set a limit. A 40 kW combi cannot deliver more hot water than your incoming main supplies, so a good installer measures it before recommending a size. If your boiler is being replaced because it is old, my guide to signs your heating system needs upgrading covers what else to check at the same time.
Boiler by boiler: who each one suits
Worcester Bosch Greenstar 4000
Best all-rounderThe boiler most UK homeowners already know by name. It is a straightforward, well-supported combi that most heating engineers have fitted and serviced many times.
- For: up to 10 years’ guarantee through an accredited installer; strong UK support and parts availability; EasyControl smart thermostat works with it.
- Against: only 25 and 30 kW combis, so larger homes look elsewhere; usually priced above Ideal.
Vaillant ecoTEC plus
Best for hot water demandA long-standing favourite with installers, built in the UK, with stainless steel heat exchangers and the widest spread of outputs here.
- For: sizes up to about 40 kW with nearly 16 l/min of hot water; stainless steel heat exchanger; Quiet Mark approved; up to 10 years’ guarantee.
- Against: the widest and deepest of the four, so check the cupboard; the longest guarantee needs an Advance installer and the protection kit.
Viessmann Vitodens 100-W
Best standard warrantyA German-engineered boiler with a stainless steel heat exchanger and a touchscreen display, at a price close to the mainstream brands.
- For: 7 years’ cover just by registering it within 30 days; stainless steel heat exchanger; compact 400 mm width; weather compensation available.
- Against: fewer engineers know it as well as Worcester or Vaillant; miss the registration window and cover drops to 2 years.
Ideal Logic+ Combi2
Best for small spaces and budgetsThe value choice: light, slim and widely available, and usually the cheapest of the four to have fitted.
- For: only 278 mm deep and around 29 kg, so it fits most kitchen cupboards; 7-year warranty; 24, 30 and 35 kW sizes; works with Ideal’s Halo controls.
- Against: aluminium heat exchanger, which is less forgiving of dirty system water than stainless steel; a shorter warranty than the best of the rest.
Want a fixed price on one of these four?iHeat gives fixed online prices that include installation and VAT; check which of these models it offers for your home. Affiliate link: I may earn a commission if you buy, at no extra cost to you.
Get my iHeat quoteWarranties compared: read the conditions
A long warranty is worth having, but the headline number comes with conditions. Miss one and the cover can shrink to a year or two.
| Boiler | What you get | What keeps it valid |
|---|---|---|
| Worcester Bosch Greenstar 4000 | Up to 10 years | Fitted by a Worcester Accredited Installer, registered, and serviced every year; a system filter may be required |
| Vaillant ecoTEC plus | Up to 10 years | Registered on Vaillant Advance by an Advance installer, with the Boiler Protection Kit fitted, and serviced every year |
| Viessmann Vitodens 100-W | 7 years (paid upgrades to 10 or 12) | Registered with Viessmann within 30 days of installation, or it drops to 2 years |
| Ideal Logic+ Combi2 | 7 years | Registered after installation and serviced every year |
Some online installers add their own longer guarantee. Read who backs it: the manufacturer, or the installer.
Whichever boiler you choose, register it, keep the paperwork, and book the service every year. If the boiler fails within the warranty, the first question will be whether it has been serviced.
How much could a new boiler save you?
The saving depends on what you are replacing. An old non-condensing boiler, often with a permanent pilot light or a balanced flue on an outside wall, wastes far more gas than a condensing boiler that just needs a better set-up.
| Boiler | Seasonal efficiency | Gas used | Yearly gas cost |
|---|---|---|---|
| Old non-condensing boiler | About 70% | 17,143 kWh | £1,366 |
| Early condensing boiler, poorly set up | About 78% | 15,385 kWh | £1,226 |
| New A-rated combi, set up well | About 90% | 13,333 kWh | £1,063 |
My worked example: a home needing 12,000 kWh of heat a year, at the Ofgem cap rate for October to December 2026 (gas 7.97p per kWh). Standing charge left out, as it does not change.
In this example, replacing an old non-condensing boiler saves about £300 a year. Replacing a condensing boiler that is simply badly set up saves much less, and you may get most of that by turning the flow temperature down and fixing the controls instead. My energy-efficient heating guide shows how.
Rules for fitting a new combi
- Gas Safe: the boiler must be fitted by a Gas Safe registered engineer, who notifies building control for you. Check the card and the registration number.
- Boiler Plus (England): a new combi must come with time and temperature controls plus an energy-saving measure, such as weather compensation, load compensation or a smart thermostat with automation and optimisation.
- TRVs: when a boiler is replaced, radiators need thermostatic radiator valves, apart from the room with the room thermostat. See are TRVs worth it?
- Cleaning and protection: the system should be flushed, treated with inhibitor and fitted with a magnetic filter. Most manufacturers ask for this to honour the warranty.
- Back boilers: if your old boiler sits behind a gas fire, the job is bigger. See replacing a back boiler.
Getting the efficiency you paid for
A condensing boiler only reaches its rated efficiency when the water coming back from the radiators is cool enough for it to condense, roughly below 55°C. Many new boilers are left at the factory setting of 70 to 80°C and never condense properly.
- Ask the installer to set the heating flow temperature as low as keeps the house warm, often around 55 to 60°C.
- Choose a control that modulates the boiler, such as weather compensation or an OpenTherm thermostat, rather than switching it on and off. My guide to smart thermostats and controls explains the difference.
- Bleed and balance the radiators so every room heats evenly. See how to bleed a radiator properly.
- Book the annual service: it keeps the warranty valid and the boiler burning cleanly.
Boiler or heat pump?
A new combi is the cheapest and quickest way to replace a failed gas boiler, and for many homes it is the right call today. A heat pump costs more to fit, even with the £7,500 Boiler Upgrade Scheme grant, but it can cost less to run in a well-insulated home and avoids the next gas boiler replacement altogether.
If your boiler is still working and you have time to plan, it is worth comparing both. See heat pump or gas boiler: ten years of running costs and heat pump vs boiler.
Getting quotes
You can get a fixed price online in a few minutes, or ask local Gas Safe engineers to survey and quote. Either way, compare like for like: the same boiler model and size, the controls included, the flush and filter, any flue or gas pipe upgrades, and who backs the guarantee. My new boiler cost guide lists what a fair quote should include.
Compare a fixed online price before you book a survey.iHeat gives fixed online prices that include installation and VAT; check which of these models it offers for your home. Affiliate link: I may earn a commission if you buy, at no extra cost to you.
Get my iHeat quoteCommon questions
Combi boiler questions
Which is the best combi boiler in the UK?
There is no single best one. Of these four, the Worcester Bosch Greenstar 4000 is the safest all-rounder, the Vaillant ecoTEC plus suits higher hot water demand, the Viessmann Vitodens 100-W has the easiest long warranty, and the Ideal Logic+ Combi2 is the best value for small spaces. A good installation matters more than the brand.
What size combi boiler do I need?
Most flats and smaller homes with one bathroom suit a 24 to 26 kW combi; a typical three or four-bed house with one main bathroom suits around 30 kW. Homes with two bathrooms used at the same time are often better with a system boiler and a hot water cylinder.
Are all A-rated boilers equally efficient?
They are close on paper, and all will be far better than an old non-condensing boiler. In real homes the difference comes from sizing, flow temperature and controls: a well set-up boiler of any of these brands will beat a badly set-up one of another.
Is an aluminium heat exchanger a problem?
Not if the system is cleaned, treated with inhibitor and fitted with a filter, which should happen with any new boiler. Stainless steel is more tolerant of poor water quality, which is one reason some engineers prefer it.
How long does a new combi boiler last?
A well-installed boiler that is serviced every year typically lasts 10 to 15 years. Keeping the warranty valid means registering the boiler and booking the annual service.
Can I still fit a new gas boiler?
Yes. There is no ban on replacing a gas boiler in an existing home. The Future Homes Standard means new homes built under it from 2027 will not have gas boilers, but that does not affect replacing yours.
Final thoughts
All four of these boilers are good choices, and any of them will be a big step up from an old non-condensing boiler. Choose by size range, cupboard space and warranty conditions rather than the energy label, and put as much care into choosing the installer as the boiler. A well-sized, well-set-up mid-range boiler will beat a premium one fitted badly.