The 3 Most Efficient Combi Boilers

New boilers cost thousands of pounds in materials and labour. With dozens of brands and 100s of models, it’s not exactly easy to choose the best boiler for your property.

Below we’ll cover not just the most efficient combi boilers, but those paired with an extensive warranty from leading brands such as Ideal Boilers, Vaillant, Worcester and Viessmann. 

Here’s a summary of what’s included:

• This page at a glance
• A detailed guide to the 3 most energy efficient combi boilers
• A side note explaining the importance of warranty and the boilers we’d avoid
• Expected installation cost of an energy efficient boiler

The Short Version: The Most Efficient Combi Boilers 

Not got time to read this page in full? Bookmark it for a later date and skim through this summary instead!

An energy efficient combi boiler can save you £100s per year on your energy bills. According to the Energy Saving Trust, those in a detached property using a G-Rated boiler could save up to £415 per year by installing one of the A-rated boilers we’ve listed below.

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(Source: Energy Saving Trust)

The most efficient boilers in the UK run at over 90% energy efficiency. From a list of dozens of the best boilers on the market, we’ve picked our 3 favourites based on them all having a 10-year warranty and an incredibly high rating of 94%. 

The 3 best energy efficient boilers are: 

• Viessmann Vitodens 100-W Combi
• Worcester Bosch 4000 Combi
• Ideal Boilers Logic Plus

We’d choose to avoid combi boilers that are lower than 88% efficiency and those that don’t come with a warranty of 10-years. Simply put, whilst being efficient is important, reliability is too!

So which boiler should you choose? Well, this will depend on the exact specification of your property. 

Use our clickable form here, tell us about your property and we’ll give you fixed prices on-screen for the most suitable and energy efficient boilers.

Our Picks: The 3 Most Efficient Combi Boilers

To come to a conclusion on our top picks for the best energy efficient boilers on this list, we’ve considered a few things alongside efficiency: 

• Brand reputation and their after sales service 
• Warranty 
• Features incorporated into each of the boiler’s design

A side note here is each property is different. Heat demand, hot water demand and even the size of the installation area are just a few variables that could make one boiler more suitable than another. 

We only fit energy efficient boilers with a long 10-year warranty. 

To skip the detail and see which one’s most suitable, you can use our clickable boiler form here. Click a few options telling us about your property, and you’ll have a fixed price on-screen within 2-minutes for the most suitable energy efficient boiler.

1. Viessmann Vitodens 100-W

The Viessmann Vitodens 100-W is one of the most popular boilers we fit, you can read our Vitodens 100-W boiler review here. 

Viessmann offers the 100-W Vitodens in a range of sizes including: 26kW, 30kW and 36kW. This means if you have anything from a 1-bedroom flat through to a 4-bedroom property with 2 showers, Viessmann's energy efficient boiler will be suitable.

The big question is, how energy efficient is the Viessmann combi boiler? Viessmann’s spec sheet shows the Vitodens 100-W sitting at an ERP rating of 94%, making this one of the most energy efficient boilers on the market. 

Compare that to a boiler that’s a decade old or older, it could be rated at anywhere from D to G. D-rated boilers work at around 80% efficiency, whereas G-rated boilers could be working at less than 70%. 

So, what does that really mean for your energy bills? Put simply, if you are using a G-rated boiler in a detached property, you could save over £400 per year on your gas bill (assuming a new programmer, thermostat and thermostatic radiator valves are also fitted).

Not only is the Viessmann Vitodens 100-W one of the most efficient gas boilers on the market, it also has a long 10-year warranty too. 

Considering the Vitodens 100-W for your property? Use our clickable boiler form and get a price on-screen within a couple of minutes (no email or phone number required).

2. Worcester Bosch 4000 Combi 

Next on our list, we have the Worcester Bosch 4000 combi boiler. 

The Worcester Bosch 4000 combi matches the Viessmann Vitodens 100-W, with a 94% efficiency rating. This makes it one of the most energy efficient gas combi boilers available in the UK. 

And, like the Vitodens 100, this is going to mean a huge saving on your energy bills; around £400 per year if you live in a detached house and have an old G-rated boiler. 

So, the energy savings here compare with the best boilers for efficiency on the market. But, is this boiler suitable for your property?

Like the Viessmann (and any boiler for that matter), without seeing a property and knowing specific details, it's hard to pinpoint which is the best boiler to install. 

However, as a rule of thumb, the Worcester 4000 boiler (30kW), would be perfect to fit in a 2-3 bedroom property that only has one shower. If you have more than 1 shower, a mid-range combi boiler like this simply won't keep up with demand. 

Alongside huge energy efficiency of 94%, the Worcester 4000 combi is packed full of features. It comes with a 10-year warranty (as well as a 12-month workmanship warranty when fitted by us), anti-frost condensate and smart fill.

One of the most important design features incorporated into the Worcester 4000 is the power modulation function. The most energy efficient gas boilers all now come with power modulation. 

Essentially, older boilers will be working at maximum capacity, even when they don't need to. Power modulation means the Worcester 4000 only uses the power (and energy) it needs to. Ultimately, this increases energy efficiency and a big saving when it comes to your energy bills.

For a more detailed guide to this unit, jump over to our Worcester Bosch 4000 combi boiler review. 

Or if you’d prefer to simply get a fixed price on-screen, use our clickable boiler installation form here. 

3. Ideal Logic Plus

Ideal Boilers is one the best energy efficient boiler brands in the UK. Matching the Viessmann and Worcester Bosch boilers mentioned above, the Logic Plus comes with a 94% efficiency rating. And on top of that, it’s endorsed by the Energy Saving Trust!

As with the other most energy efficient combi boilers we mention here, the power modulation feature is what really allows it to hit this level of energy efficiency. The Logic Plus will modulate power to exactly what it needs to consume, down as low as 4.8kW. 

The Logic Plus comes in 3 sizes: 24kW, 30kW and 30kW. The 24kW is suited to small properties (1-2 bedroom apartments), with the 30kW suited to larger 2-3 bedroom homes. However, both of these boilers will only power up to 1 shower a time. By contrast, the 35kW would be able to power up to 2 showers at a time.

Whilst the base warranty from Ideal is just 7-years, that's extended to 10-years when fitted by a Dwellow boiler installer. 

If this sounds like the boiler for you, jump over to our in-depth Ideal Logic Plus combi boiler review here. Or, use our boiler calculator to determine if this is the right boiler for you. You’ll have a fixed price on-screen within a couple of minutes. 

Combi Boilers We’d Avoid

Leading brands offer highly energy efficient boilers, inclusive of a long warranty of 10-years or longer. Whilst energy efficiency saves money, reliability is just as important. With installation costs generally in the £2,500-£5,000 range, you want to make sure that the boiler you buy is going to last for years to come. 

Essentially, we’d avoid combi boilers with a low energy rating (under 88%), and also those that offer a low warranty on their product. Our choice for the best boilers that give both high energy efficiency and a long warranty, offering the overall best value for money include: 

• Alpha
• Ideal
• Vaillant
• Viessmann
• Worcester Bosch

Looking for a more detailed guide to which specific boiler brand you should choose? Read our page here that covers the most reliable boiler brands in the UK, or get a fixed price for your new energy efficient boiler using this page.

Cost to Install An Efficient Combi Boiler

The older your boiler is, the more you’ll see in terms of energy savings. For those with a boiler that’s 20-years old or older, this could mean that the monthly repayments of a new boiler are almost covered by these energy savings!

But, how much does it actually cost to fit an energy efficient boiler? 

This isn’t a simple question to answer, as each property is different. But generally installations sit in the £2,500-£3,000 for an average sized property. 

For example, depending on the specific options you click using this form relating to your property, we fit the 94% efficient Worcester 4000 from £2,675.

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The quickest and easiest way to determine exactly what the cost of installation would be for your new energy efficient boiler, is to use our clickable boiler tool here.