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Genuine, unbiased consumer advice on gas boilers


22.5 million homes in the UK are on mains gas and it is by far the cheapest fuel available to heat your home. Around 1.6 million new boilers are fitted each year and the vast majority are wall hung combination boilers (which produce heating and hot water from single unit). Other types of gas boiler include warm air units and back boilers. In this guide we give an overview of gas boilers from how they work to what they cost to replace and direct you to more detailed guides where necessary.

Genuine, unbiased consumer advice on gas boilers

Balancing radiators

More important than you think for efficiency, David explains all


  1. Types of gas boiler
  2. How does it work?
  3. Sizing
  4. Efficiency
  5. Fuel costs
  6. Boiler installations
  7. Prices
  8. Open vent v sealed system boilers
  9. Brands
  10. Best gas boilers
  11. Building regulations for gas boilers
  12. Frequently asked questions

1) Types of gas boiler

There are three forms of gas-fired boiler:

Gas boilers

The vast majority of gas boilers work with radiators (known as a ‘wet’ central heating system). Some gas boilers are floor standing and sometimes incorporate a small hot water cylinder that boosts the hot water flow rates. Gas boilers are available as wall hung combination, heat only and system boiler models. For more on the different boiler types, see our Guide to boiler types. 80% of boiler sales in the UK are combi boilers so we have devoted a section of our site to this boiler type: Combi boiler help-centre.

Warm air units

Warm air units heat the home by blowing warmed air around the house via a series of vents. These boilers were very popular in the 1970s and are still manufactured today.

Back boilers

Back boilers were popular in the 1960s. They are a form of heat only boiler but the flue goes via a chimney breast and they are nearly always accompanied by a gas fire on the front. It was possible to replace a back boiler until fairly recently, although newer models were accompanied by an electric fire on the front. Whilst they are no longer available (because they cannot meet condensing boiler standards) they can still be repaired. See our Guide to back boiler repairs, replacements and servicing for more.

2) How does a gas boiler work?

Gas boilers burn gas to heat up water in your radiators. A pump circulates the heat water around radiators. For warm air units a fan blows across the hot heat exchanger and expels warmed up air via the ducts and floor vents. Combination boilers also heat up potable (drinkable) water for washing and showering via heat exchanger. Heat only and system boilers work with a hot water cylinder to produce hot water for washing and showering.

Boilers usually work with some form of thermostat which sets the temperature of the home. From a basic dial room thermostats to sophisticated advanced smart controls, the efficiency of the boiler is directly related to the type and compatibility of the heating control fitted.

3) Boiler sizing

A 6-10kW boiler will comfortably heat the vast majority of UK homes on a very cold day. However gas boilers start at 11-13kW for heat only/ system boilers and 16-18kW (and higher) for combi boilers. It is clear that even the smallest boiler will heat the majority of UK homes, however that comes with a slight caveat for hot water production.

Combi boilers often appear ‘larger’ because they show outputs of 24kW and upwards, but this is for hot water production. For boilers that work with a hot water cylinder, hot water production is a factor for sizing, so you may need a larger boiler depending on your system set up. For more help try our Guide to boiler sizing.

4) Boiler efficiencies

All condensing gas boilers have the capacity to be 92-94% efficient, which is equivalent to A-rated, however achieving efficiencies in the 90s depends entirely on how well the rest of the system is designed and controlled. Actual average efficiencies in the home are nearer to 80%. Getting the central heating system design right is just as important as which boiler you pick. See our Guide to gas boiler efficiencies.

You can also save 6-8% easily by turning down the flow temperature on your combi boiler. Read our Guide to turning down the flow temperature.

5) Fuel Costs

Mains gas is one of the cheapest fuels to buy for homes heating, at around 3p per kWh currently but prices are rising due to increasing demand. Fuel costs however are a factor of fuel price and heating system efficiency. Gas boilers can suffer low efficiencies of between 60-80% if the boiler is old, or new and set up poorly. For more on rising fuel prices see our recent blog on how Soaring gas prices expose the real cost of boiler inefficiency.

6) Boiler Installation

There are many things to consider when arranging a replacement gas boiler, starting with whether the time is right for replacement. We offer guides on every aspect of a boiler installation, from assessing the lifespan of the old boiler to arranging finance. But do you really need to change it? If you already have a condensing boiler you can make it much more efficient and avoid thousands £s in unnecessary replacement costs. More in our blog: Why a new boiler will not reduce your gas bills this winter.

How long should a boiler last?

Boilers should last 22 years but are regularly swapped out half way through their lifespan and for little to no reduction in fuel bills for the household. We investigate the culture of premature replacement in the UK and provide tips for retaining your existing boiler until you can fit a heat pump. How long should a new boiler last before it needs replacing?

When to repair or replace?

When is a good time to replace your boiler? Many new boiler installations are ‘distress purchases’, in other words your boiler has broken down and needs replacing quickly. More often boilers can be cost effectively repaired and there are many good reasons to keep your old boiler going. For more guidance read our Guide to repairing or replacing.

Accredited installer schemes

Many boiler manufacturers run accredited installer schemes, whereby they provide a national network of installers who have undertaken additional training in their products. There are pros and cons to these schemes for consumers and they are not an indication of installer competency. We explain all in our Guide to accredited installer schemes.

Gas Safe Qualification

In all cases, gas boiler installers must be Gas Safe registered and experienced in boiler installations. We have a guide to the Gas Safe registration which steps you through how to check your installers in qualified for the work they do: Guide to Gas Safe registration.

Checking installer skill and competency

Fitting boilers is a different skill set to those that repair boilers so it’s important to find someone that specialises in this area. 99% of installers have not be trained to set up condensing gas boiler to run at their higher efficiencies. We have done a lot of work to set a new skill and installation standards, read more about our Elite Installer Network.

Paying for a new boiler

Cash, credit card, interest free or 'rent a boiler' schemes are some of the options to pay for a new boiler. We explore all the ways to pay in our Guide to boiler finance.

7) Prices

Boiler prices can be refer to the cost of the gas boiler or the cost of the whole boiler installation including parts and labour and it worth making the distinction. Sometimes it's difficult to compare quotes because parts and labour are not broken down. It is therefore helpful to also look at the cost of the boiler in order to understand where differences in installation cost might occur.

Boiler installation costs

Our Guide to boiler installation costs gives a great overview of boiler cost including labour and sundries for a range of scenarios.

For a bespoke installation price, go to our Boiler quote calculator. It’s completely free to use and no personal details are required to see the prices.

Stand alone boiler prices

If you just want to compare standalone boilers prices, we compare over 200 boiler model prices in our three comprehensive boiler prices guides: combi boiler prices, system boiler prices and heat-only boiler prices.

8) Open vent v sealed system

There are still a lot of heat-only boilers out there on 'open-vent' systems. We guide on on this topic and give you the pros and cons of moving to a 'sealed system. Central heating help-centre

9) Boiler brands

The mainstream boiler brands include big names including Worcester Bosch, Vaillant, Baxi and Ideal as well as some lessor known brands also doing great work, such as Ravenheat, Intergas and Viessmann. You can find out more about them in our Guide to gas boiler brands

10) Best gas boilers

There are a range of great quality, technically advanced, boilers for all budgets. The key thing we look out for is how low the boiler can go down in kW. The lower it can go, the more efficiently the boiler can work all year around. The short answer is to check out our Best boiler guide.

11) Building regulations for gas boilers

Building regulations cover many aspects of a new boiler installation, including its efficiency, heating controls, flue position, boiler location, frost protection and condense pipe drainage. Go to our Guide on gas boiler regulations for more.

12) Frequently asked questions

Where can gas boilers be fitted?

It is common to find your boiler in the kitchen, utility, bathroom and sometimes a bedroom. Boilers can also be located in an adjoining garage or loft, however this can be less efficient as many boilers lose heat via their cases. Where a combi boiler has replaced a boiler with a hot water cylinder, the boiler is often relocated to the airing cupboard. For more scenarios and pros and cons try our Guide to moving your boiler.

Which are the best boilers for 3 bed houses?

The vast majority of homes in the UK are 2 and 3 bedroom houses and often they have similar amount of floor space. We have given extra attention to 3 bedroom houses in our consumer guide: Which are the best boilers for 3 bed homes? The guidance will also guide owners of two bedroom homes.

Can I get an extended boiler warranty?

All boilers come with a minimum 2-year warranty, but most extend that to 3-5 years as a minimum. Most boiler manufacturers’ offer extended warranties but many do require that the installer is accredited with them. This means that the installer has undertaken additional training on their boilers and when they register the boiler you will get the benefit of a longer warranty period. For more on warranties jump to our Guide boiler warranties and guarantees.