Condensing Boiler Guide: Best Condensing Boilers and Costs in 2021

Looking to replace your condensing boiler, and don’t know where to start? This guide has everything you need to know to make an informed choice.

We’ve analysed condensing boilers thoroughly and are ready to answer questions about how these devices work and whether they’re safe, reliable, and efficient.

Read on to find out more.

Looking to get a boiler installed? We recommend Heatable.

They’re a new kid on the block for online boiler comparison, so condensing boiler replacement costs here are about as good as they get.

Heatable new boilers

What Is A Condensing Boiler And How Does It Work?

Condensing BoilerA condensing boiler burns fuel to heat water and also collects heat from the escaping flue gases. It’s this function that helps it operate more efficiently.

A condensing boiler has 2 heat exchangers. One of them heats water as the fuel combusts, and from there, the water circulates through the radiator network. As the cool return water re-enters the boiler, it passes through another heat exchanger where it collects heat from the escaping flue gases. This process produces condensate — which gets drained — and earns the device its name.

Mind you, the boiler only condenses when the return water temperature is low enough. But the good news is that it still exposes water to a larger heating surface area, so it’s always more efficient than a conventional boiler.

How Does a Condensing Boiler Work to Achieve Efficiency?

Since condensing boilers extract the heat from the flue gases rather than letting it escape, they operate way more efficiently than their conventional peers. By capturing this latent heat, a condensing boiler can recycle anywhere between 20°C-150°C of temperature that would otherwise be wasted.

That’s going to lead not just to a smaller carbon footprint, but lower fuel bills too (whether that be gas, oil or LPG). Because of their efficiency, the UK government now requires all new boilers to be of the ‘condensing’ variety.

Energy Cost Comparison

But what do those numbers mean in terms of extra pound notes in your pocket?

Well, around 55% of your energy bills is spent on heating. If you’re looking to save money, your boiler is the place to start.

A study on the Energy Saving Trust’s website shows you could save anything up to £305 per year. That’s for a detached house, and a boiler with a low energy efficiency rating (G). Obviously, savings drop for smaller houses, and those with boilers that have a higher energy efficiency rating.

Old boiler ratingSemi-detachedDetachedDetached bungalowMid terrace houseMid floor flat
G ( < 70%)£200£305£155£175£95
F (70–74%)£140£210£105£120£65
E (74–78%)£120£180£90£105£55
D (78–82%)£120£180£90£105£55

Depending where you fit into the chart above, there’s a good chance a new condensing boiler installation could pay for itself in the space of just a few years (more on that below, see “condensing boiler installation costs”).

What Size Condensing Boiler Do You Need?

If you’re all set on upgrading to a new boiler, the next thing you need to figure out, is what size you need. Rather than rewrite something we’ve already written, there’s a quick guide to boiler sizes here. We’re going to assume you already know the type of boiler fitted in your property (a combi for instance).

There are two key takeaways from the above article.

The first is that no property is the same. One 5-bedroom house isn’t the same as the next. You need to get a boiler installation company to come and spec your property. Otherwise you could be overpaying for a boiler that has higher output than you need, or be installing a boiler that falls short in terms of hot water and heating output.

The second is a reference chart. You can get free quotes and an installer to spec your property here.

We can’t stress enough, use this only for reference. Get a Gas Safe boiler fitter to come and spec your property.

Property typeRadiatorsBathroomsSize of boiler (kw)
Smaller properties. Typically, apartments and 2-bedroom terrace houses.10 or less124kw-27kw
Medium sized properties. 2-4 bedrooms.10-15 radiators228kw-34kw
Large properties15-20 radiators235kw-42kw

Condensing Boiler Prices

So, how much do condensing boilers cost? The price (boiler only) is going to be determined by a few things:

  • Size (kw output)
  • Brand
  • Model

As an example, you could go with a 24kw from an unreliable brand that offers little or no warranty. That could cost as little as £500 upfront but is most likely not to be a cost-effective solution long-term. Boiler repairs aren’t cheap ☹.

We always recommend buying boilers that have a minimum of 5-years warranty; there’s plenty of choice. Below are some examples of condensing boiler prices, from brands (and specific models) that we’d recommend.

But first, let’s start with a few ranges we like and their warranties, rather than specific sizes and prices:

  • Ideal Logic Plus (7-years)
  • Ideal Max (12-years)
  • Baxi 600 (7-years)
  • Vaillant EcoTec Pro (2-years, upgraded to 5-years with Vaillant Advance)
  • Worcester Greenstar Combi (5-10 years)

The price chart below is for 24kw boilers only, that gives you an idea of how prices directly compare across different ranges. There’s a similar price difference when moving to larger boilers, such as a 30kw.

Boiler Prices
Ideal Logic Plus 24kw£750
Worcester Bosch Greenstar 25I Combi£900
Baxi 624£710
Vaillant EcoTec Pro 24£850

As you can see from the boiler prices mentioned above, Vaillant and Worcester tend to be in the higher end in terms of cost.

Ideal Boilers are one of the cheapest of the high-end brands. And, although Baxi are generally up there with the more expensive, their new Baxi 600, is extremely competitive on price.

Best Condensing Boilers Installation Costs

I wouldn’t go ahead and choose a boiler just yet. As we’ve mentioned, it’s worth having an engineer come around and spec your property first.

Typically, we’d expect boiler installation costs for condensing units to start at around £1,800 for a cheap boiler, and upwards of £2,000 for something like a Worcester.

For bigger properties, the price is going to be £2,000 and beyond, even more if it’s an oil installation (oil boilers can be £500-1000 more expensive than gas and LPG units).

There are a few ways to get the best deal on your new boiler. The first, is to get a fixed price online, from a company like Heatable. You’ll get a Worcester 30I Combi installed for just £1,995, well below our £2,000 estimate mentioned above. And as a bonus, you’ll be able to get finance through Hetable too.

HEATABLE Worcester Bosch Greenstar i 30kW

But, if you prefer, you can get back-to-back comparisons from local fitters. By filling out this form you’ll be able to get up to 3 on-site visits and compare quotes.

Thinking about going with a national boiler installer like British Gas or Help-Link? We’ve created a detailed guide of what to expect in terms of price, in our guide to boiler installation.

Condensing Boiler FAQs

Are Condensing Boilers Safe and Reliable?

Yes, modern condensing boilers are both safe and highly reliable. 

While condensing boilers do produce a visible plume, this only happens because the water vapour leaves the flue at a lower temperature than it would from a conventional boiler. The lower temperature of the vapour is what makes it more obvious. Apart from the aesthetics, however, there’s absolutely no harm in this plume — at least no more than you’d get from the exhaust of a conventional boiler.

Another oft-cited issue is the acidity of the condensate. But measuring between pH 3.5 and 5 — which is comparable to the acidity of many fruit juices — the condensate can be safely disposed of with a proper drain.

Do Condensing Boilers Need a Drain?

Yes, condensing boilers need a drain. They have a second heat exchanger, which extracts latent heat from escaping flue gases, a process that leads to condensation. As the heat from the hot exhaust gets captured by the return water running through the heat exchanger, condensate forms and must be drained to prevent accumulation. Since the condensate is slightly acidic, your condensing boiler needs a special drain system that is not susceptible to corrosion. As all modern boilers on the market are condensing, you’ll have no trouble finding a suitable drain for your boiler.

Are Condensing Boiler Fumes Dangerous?

No, the fumes emitted by a condensing boiler aren’t any more dangerous than those produced by a conventional boiler. They look different, yes. And that’s because they escape the flue at a lower temperature, which makes the water droplets in the vapour larger and more visible.

Do Condensing Boilers Need a Water Tank?

It depends on the type of boiler you’re getting — condensing combination (combi) boilers do not need a water tank, whereas condensing regular boilers do. Essentially, being ‘condensing’ has nothing to do with whether a boiler needs to heat water and store it or is able to heat water on demand. All new, modern boilers in the UK — including models with and without a water tank — are now condensing boilers.

Do Condensing Boilers Need a Flue?

Yes, condensing boilers need a flue. Even though the exhaust gases get cycled through a second heat exchanger — which extracts their latent heat — they still need a way to exit the building. This is why all boilers that produce heat by burning fossil fuels (condensing or otherwise) need a flue.

At What Temperature Should I Set My Condensing Boiler?

Most condensing boilers have an indicator on their console, which shows the optimal temperature for the most efficient operation. Temperature matters because condensing boilers can only ‘condense’ — and extract flue gas heat — if the return water is around 55°C. From this point, the lower it gets, the more efficiently your condensing boiler will run. To lower the temps of the return water, you typically have to also make sure that the water leaving the boiler isn’t excessively hot either. This optimal supply water temperature depends on the size and type of your radiator network and the size of your home.


Thanks for reading out 5-minute buyer’s guide to condensing boilers. Hopefully that’s enough information for you to start getting quotes.

Got questions about condensing boilers, prices, the best brands, or installation costs?

Leave a comment below and we’ll get back to you as soon as possible.

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