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Are External Oil Boilers a Good Choice?

Becky Mckay
By: Becky Mckay
Updated: 6th July 2022

Oil boilers are one of the most popular heating solutions for over 4 million UK homes that are not connected to the gas network.

For many, one of the biggest attractions is the fact that it's possible to install an oil fired boiler outside the home. But is an outside boiler really the best choice for you?


What is an external oil boiler?

The difference between an internal oil boiler and an external oil boiler is simply that external oil boilers are installed outside.

External oil fired boilers work in the same way as internal models, needing a tank to store the oil – which will be delivered by a supplier. This tank is connected to the boiler, automatically feeding it fuel as needed. The boiler will then heat and supply the water needed for your radiators and taps.

Benefits of an external oil boiler

While having an external oil boiler will help to save valuable space around the home, there are many other reasons you might want to consider installing one:

  • More space indoors: Moving the boiler from your kitchen or utility room can free up space and, in the case of old, bulky boilers, remove an eyesore.
  • Less noise: Some boilers are noticeably noisy when running with humming, vibrating, banging or pinging sounds a constant irritation.
  • No carbon monoxide risk: Although not a common occurrence, carbon monoxide leaks are a real risk which can be removed by putting the boiler outside.
  • Prevent damage to home: Occasionally, boilers can leak oil which not only causes damage but can smell.
  • Easier access for servicing: Moving the boiler outside means your home is not turned into a temporary workshop during annual servicing and can improve access making your engineer's job a lot easier when it comes to the annual service.
  • Heating for outdoor buildings: If you choose the house the boiler in a garage or outhouse, it will lose a certain amount of heat to the air around it which could be a positive if the area is used as a work area or for storage.

Best external oil boilers

The best oil boilers are manufactured by Firebird, Grant and Worcester Bosch:

  • Firebird Enviroblue
  • Grant VortexBlue
  • Worcester Bosch Greenstar Heatslave II

The ranges from each of these manufacturers include external oil boilers with different output ratings to give you greater choice when it comes to finding the most suitable heating system for your property.

External Oil Boiler Model Output Ratings (kW) Efficiency Warranty Potential Cost
Firebird Enviroblue Combi-Pac 20, 26 and 35 93% 2 years £1,300 – £1,900
Grant VortexBlue 21, 26 and 36 90.8% – 94.5% 2 years £1,285 – £2,605
Worcester Bosch Greenstar Heatslave II 18, 25 and 32 91% 2 years £2,200 – £2,425

Use our comparison tool to compare external oil boilers.

The potential costs of the best external oil boilers don’t include the price of installation, which you will also have to take into account. To get the best possible price on the installation, we highly recommend comparing 2-3 quotes from different oil boiler installers, that way you can be confident you’re getting the most competitive price.

To make finding quotes easier, complete our simple online enquiry form and we’ll match you with up to 3 local oil boiler installers who will each provide a free quote.


External oil boiler considerations

If you don’t have the room to spare for an internal oil boiler then you might not have an option but to have an external oil boiler installed. However, it’s still important to bear in mind the following considerations:

  • Outdoor space: properties with limited outside space might not be able to accommodate the necessary large oil storage tank. It needs to be located in a relatively easy to access place to make delivery easier, otherwise you may need extensive piping.
  • Heat loss:  All boilers radiate heat to the air around it to some degree, which is okay if this heat escapes into your home, but if the boiler is outside (and not contained in a building like a garage) this heat is completely lost, reducing energy efficiency. Heat is also lost from the pipes as it transports hot water from the boiler to the property which also makes the system less efficient. Insulation or placing the pipes underground can reduce this to some degree.
  • Outdoor maintenance: While external oil boilers are housed in a protective casing, the unit is still exposed to the elements and rough weather, which can increase the likelihood of rusting, freezing or other damage. If pipes are hidden from view underneath the ground you will not notice leaks as they appear. Pipes will also be susceptible to heat loss into the ground. Outdoor installation also means braving all the elements when you need to carry out maintenance.

What kind of oil is used in oil boilers?

There are two types of oil most commonly used in domestic heating: 28sec (also known as kerosene) or 35sec. They are differentiated by their thickness, i.e. how long it takes a ball bearing to pass through a set amount of oil: 28 seconds or 35 seconds.

Of the 2 oils, 28sec is the more popular as it burns more cleanly, producing less smoke and other deposits. Another way of differentiating these oils is their colour: 28sec is yellow and 35sec is red.

How does oil compare to other fuels?


While oil is a more expensive fuel than gas (per kWh), it is more cost effective than other alternatives on the market, e.g. LPG and electricity. A drawback of the price of oil is that it can increase in line with global supply and demand so can be difficult to predict.

Generally, it costs between £1,400 to heat a 3 bedroom house with an oil boiler for a year. Compared to between £700 – £1,000 for a gas boiler and around £1,300 for LPG, oil systems are definitely more costly. It is possible to combine an oil system with solar thermal heating or solid fuel to lower heating costs.

Oil boilers are in shorter supply than gas models and so tend to be a bit pricier by, on average, around £300 – £400.


Regulations introduced in 2007 stipulated that condensing oil boilers must have an energy efficiency of 86% or more. Modern day models exceed this delivering between 92 – 97% efficiency which is equal to the highest efficiency gas boilers.


While oil is not an 'install and leave' system like gas or electricity, it requires far less refuelling than a solid fuel system like burning coal or biomass. As long as you have oil in the tank, the refuelling is automatic. However, you will need to keep an eye on the level of fuel in your tank and reorder in good time as running dry can cause damage to the system.

The need for a large storage tank on your property can be less than ideal if you're concerned about appearances, but this can be remedied by installing it underground.

Carbon emissions

Burning oil produces twice as much carbon as gas does, but less than coal. Installing an A-rated boiler will reduce the carbon emitted as much as possible.

If you’re looking to reduce your home’s carbon emissions, consider a heat pump or solar thermal panels.

Is an external oil boiler right for you?

A lot of the concerns around external oil boilers stem from weather related issues e.g. rusting components or freezing in winter. In reality, modern external boilers have been built specifically with this in mind and many problems can be prevented through proper insulation and professional installation. In addition, you will gain space in your home and remove the potential for damage from oil leaks, noise, smell or carbon monoxide.

However, external oil boilers are more expensive and, depending on the layout of your home, can require significant piping and labour if you are placing the tank underground. For some, having a bulky boiler on view outside their home is not appealing.

High efficiency, external oil boilers are a perfectly viable option for many people and, if installed correctly, well insulated and maintained, it can be as effective as internal oil boilers without impacting on the interior of your home.

Need more guidance? We will put you in touch with up to 3 registered engineers near you who can advise you on the best oil boiler for your home.

External boiler installation

Whether you are installing an oil fired system from scratch, switching an internal for an external boiler or simply looking for a new oil boiler replacement, make sure you use an OFTEC registered engineer.

There are several regulations about the positioning on oil tank on your property and safety requirements to prevent fires in the home spreading to the oil tank. For this reason it's really important to do your research and consult with an OFTEC registered professional when planning the installation.

The cost of installation will depend on whether you are carrying out direct replacement or moving the boiler to a new location. They will also vary from installer to installer so it's important to get at least 2 quotes to compare. Boiler Guide can put you in touch with up to 3 registered professionals who can provide you with a tailored quote for your new oil boiler.

Becky Mckay

About the author

Becky Mckay

Becky is one of our home heating and renewable energy experts and has a wealth of experience writing about the world of heating.

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