The Best Oil Boilers in 2021 (Ultimate Guide)

Last updated: June 14, 2021

With an estimated four million households in the UK not connected to mains gas, an oil boiler is often their go-to when it comes to heating their home. While a little pricier to install, an oil boiler can actually be a cheaper alternative to electricity and can also be located either inside or outside your property.

So, if you are thinking of installing or switching to an oil boiler for your heating and hot water system, read on. As we dip into this alternative heating system in our best oil boilers guide.

What Is An Oil Boiler?

While gas boilers are currently the dominant source of heat and hot water for most domestic homes, an oil boiler is an alternative way to fuel a central heating system.

Overall oil is a cheaper fuel source for heating than LPG or electricity and are a good option to consider if your home is not connected to the National Grid and its main gas supply. So, instead of having your fuel supplied by gas pipe, with an oil boiler you need a tank outside to store your oil reserves and which will need to be topped up when you start to run low. This means you will need to find an oil supplier who can arrange deliveries to your property via a fuel truck.  The external oil storage tank can be unsightly and on the large side so you do need the external space to locate it. However, an oil tank can also be stored underground, if you can afford the expense for the groundwork.

Other than needing an alternative way of storing the fuel, oil boilers largely work in a similar way to a gas boiler and can supply sufficient heat for both your central heating and hot water needs.

One main difference to a gas boiler is that an oil boiler can be located outside or inside the home. They can also be floor or wall-mounted. However, when it comes to the mechanics of an oil boiler, it works in largely the same way as a gas boiler.

How Does an Oil Boiler Work?

An oil-fired boiler works in a very similar way to a conventional gas boiler.

Simply put, as cold water enters the boiler, the oil is ignited inside the combustion chamber to warm up the boiler’s heat exchanger. The heat exchanger then passes on all its heat to the cold water. The result is lovely hot water which is then used to supply the radiators in your central heating system, as well as all your home’s hot taps and showers.

When it comes to the energy efficiency of a working oil boiler, all oil-fired boilers in the UK require a SEDBUK – or Seasonal Efficiency of a Domestic Boiler in the UK – rating of at least 86%.  With oil boilers rated from A to G, this means the most energy efficient oil boilers should be between 89% and 93% efficient.

 The Cost Of Running an Oil Boiler

Like all other types of boilers, the more energy efficient an oil boiler is, the cheaper it will cost to run, so when buying an oil-fired boiler, go for the highest SEDBUK rating you can afford.  However, oil boilers are more expensive to run than gas, with estimates based on the standard UK home putting the annual running costs of an oil boiler at around £700 a year, compared to £550-£600 for a gas-fuelled system.

You also need to factor in the cost of its annual service, which is typically between £80 and £180, depending on its age and condition and is comparable to the equivalent cost for a gas boiler.

Types Of Oil-Fired Boilers

Just as with gas or LPG boilers, you get a choice of three system types available when considering an oil boiler set up for your home. The type of new boiler you need will depend on the size of your home and how much hot water you actually need.

  • Combi system

Oil combi boilers – or combination boiler – provides hot water and heating via a single unit, and so doesn’t require a large water storage tank or cylinder, so is a great space saver. It also heats up water directly from the mains water on demand for both your central heating and taps, so you don’t experience that dreaded cold shower when the stored hot water runs out. However, the potential downside to this instant water on demand is that you can lose pressure in the water flow rate if multiple people are using the hot water at the same time. This means a combi boiler is best for homes which don’t have excessively high heating and hot water requirements at any given time.

  • Regular

A regular oil boiler is also known as a conventional boiler and will have a separate cold water tank, typically located in the attic. The water in this tank gets heated via the boiler before being diverted to a separate hot water cylinder to be stored, ready for use. Unlike an oil combi boiler system, with regular boilers you don’t get constant hot water ‘on tap’ as when the hot water cylinder empties, you then need to wait for it to refill. While regular oil boilers take up more space, due to the separate tanks, the large size of the tanks means that these are the best oil boilers for larger properties with multiple occupants as they can handle high water demand at any one time.

  • System

A system oil boiler is fed cold water directly from the mains, rather than a separate cold water tank, but unlike the combi boiler, system boilers then send the heated up water to a hot water cylinder for storage. While a system boiler doesn’t need a cold water tank which will take up loft space, it still requires a separate hot water cylinder so isn’t as compact as combi boilers. The plus for a system boiler is that you won’t lose water pressure when it is put under high demand, but you do need to wait for the hot water cylinder to top up again once all the stored water has been used. Overall, a system oil boiler is a good choice for homes with multiple bathrooms.

The 4 Top Oil Boiler Brands

With options for system, regular and oil combi boilers, there’s a wide range of products on the market. So, once you know which type of oil boiler you need, you need to decide which brand and model to go for. To help you choose, we take a look at four of the brands behind the best oil boilers you can buy.

Warmflow

This Northern Ireland-based company has been manufacturing oil fired boilers for the past 50 years and has a reputation for innovation and quality. They also currently produce the UK’s only triple A-rated oil boiler, the award-winning Agentis internal oil boiler range, which they say can save on average five litres of fuel per week, compared to other boilers and brands. The Warmflow combi, system and regular oil boiler range includes both internal and external boilers as well as boiler house and professional boiler systems.

Prices for a domestic oil boiler from Warmflow start at around £1,300 for a 21kW regular boiler (heat only), rising to just under £3k for a large 33kW. All their oil boilers in the Agentis range come with a five-year warranty as standard.

Warmflow are a good choice if your carbon footprint is a key driver to your purchase as they are market-leaders when it comes to A-ratings. Not only do they now offer a triple A, but they were also the first UK company to bring the precursor – the now widely seen AA rating – to the market.

The company scores an average of 4.4 out of 5 on Trustpilot, with their service and products considered excellent by reviewers.

Grant

Founded in 1978, Grant also has a long track record of manufacturing award-winning boilers, including with its Vortex condensing boiler range, which is endorsed by the Energy Saving Trust. Available in a range of outputs, from 12-70kW, an oil boiler from Grant works with most sized homes and are amongst the best oil boilers for energy efficiency on the market. The Vortex range currently features 13 different model ranges, including combi, regular, internal and external as well as wall or floor mounted.

Prices for a domestic oil boiler from Grant start from £1365 for a 21kW regular boiler through to around £2,350 for a 46kW regular external unit. All Grant oil boilers come with a warranty starting at two years as standard.

While not the most ‘budget-friendly’ when it comes to initial purchase and installation cost, the Grant oil boiler range is known for its energy efficiency as well as innovation when it comes to design.

Grant scores an excellent from its customers, with a decent Trustpilot average of 4.6 out of 5.

Worcester Bosch

A brand synonymous with the quality gas boiler market, Worcester Bosch is also known for their oil boiler range. A market leader in domestic boilers since 1962, the brand name also comes with a reputation for quality and build durability, reinforced by their Royal Warrant to supply boilers to the Queen’s Sandringham Estate.

The Worcester Bosch oil boiler range is currently manufactured at their Derbyshire factory site. Pricewise, the Worcester Bosch oil boiler range is at the premium end, but you do get quality and value for your money. And, as all their oil boilers are A-rated, you are getting a pretty efficient oil boiler for your money, plus the added Worcester Bosch reliability.

The bestselling Greenstar Heatslave oil combi boiler range starts at around £2,350, while their system boilers start at £1,750 and their oil regular boilers come in at a starting point of just under £1,500.  All the Greenstar range come with warranties of between 2 and 5 years, with some models offering the option of extending to 10 years.

Worcester Bosch consistently scores well with its customers – 79% of reviews score them excellent – and currently has a Trustpilot score of 4.6 out of 5.

Firebird

A brand synonymous with the quality gas boiler market, Worcester Bosch is also known for their oil boiler range. A market leader in domestic boilers since 1962, the brand name also comes with a reputation for quality and build durability, reinforced by their Royal Warrant to supply boilers to the Queen’s Sandringham Estate.

The Worcester Bosch oil boiler range is currently manufactured at their Derbyshire factory site. Pricewise, the Worcester Bosch oil boiler range is at the premium end, but you do get quality and value for your money. And, as all their oil boilers are A-rated, you are getting a pretty efficient oil boiler for your money, plus the added Worcester Bosch reliability.

The bestselling Greenstar Heatslave oil combi boiler range starts at around £2,350, while their system boilers start at £1,750 and their oil regular boilers come in at a starting point of just under £1,500.  All the Greenstar range come with warranties of between 2 and 5 years, with some models offering the option of extending to 10 years.

Worcester Bosch consistently scores well with its customers – 79% of reviews score them excellent – and currently has a Trustpilot score of 4.6 out of 5.

Pros and Cons of Having an Oil Boiler Installed

Oil boilers may not be for everyone but for the right home, they work just as well when it comes to providing heat and hot water as boilers fuelled by other energy sources, whether that’s gas or renewables. To help you decide whether an oil boiler is right for you, we take a look at the pros and cons of having one installed.

Pros

  • Can be installed outside – which is a great benefit if your internal space is a little tight or you have limited loft space.
  • Ideal for people ‘living off grid’! If your home is not on the gas mains, then an oil boiler is an affordable way to ensure you have plenty of lovely hot water as well as a warm home.
  • Reduced internal fumes – by having your oil boiler installed outside, you can reduce the risk of a potential carbon monoxide leak happening inside your home.
  • Energy efficiency – compared to some other heat sources, such as electricity or LPG, oil boilers are more energy efficient, as long as you choose one of the A-rated models, which can also give gas a run for its money.
  • Less disruptive – this applies to externally installed oil boilers as they can be serviced, refuelled and repaired without causing chaos inside your home. And an external oil boiler, there will be less chance it will cause damage to your property if it leaks or malfunctions.

Cons

  • Installation cost – as you need an oil storage tank as well as the boiler, installing a new oil boiler on your property will be initially more expensive than a gas boiler.
  • Take up space – as you also need a separate external tank to store your oil, installing an oil boiler system can be tricky if you have limited indoor and outdoor space.
  • Fluctuating price of oil – depending on the global market, the price of oil you need to have regularly delivered can be more volatile than other fuel sources, so could well add to your budget.
  • You need to be organised! As you need your oil delivered, unless you have a reliable supplier and an organised diary you could run the risk of running out, especially during periods of heavy heating use, for example during the winter. However, this can be negated by installing a sensor in many of the newer oil tanks that automatically place a re-order with your oil supplier once the contents drain down to a certain level.
  • Adds to your carbon footprint – unfortunately, as a fossil fuel, oil is not the most environmentally friendly type of boiler to have in your home.
  • Flow rate – on average, the flow rate of an oil boiler is lower than that of a comparable gas boiler system. This means they are slightly slower at providing hot water to your home.

Interior industrial diesel boiler

Best Oil Boilers FAQs:

How much does oil boiler installation cost?

The cost of installing an oil boiler can vary, depending on the model and size, as well as whether you are installing it inside or outside your home. You also need to factor in the installation costs of the required oil storage tank, if you don’t already have one.

Depending on the size of the storage tank you need, you can expect to pay between £500 to £2000 for a new one, or you can opt to rent one from your oil fuel supplier, if they offer this service. And the cost of installing the oil storage tank can significantly rise if you are planning to get it installed out of sight and underground.

When it comes to your new boiler, you can expect to pay between £2500 and £4500, which includes the cost of the oil boiler and the installation work required.

When getting a new oil boiler installed, it is essential that the installation engineer is OFTEC (Oil Firing Technical Association) registered, which is the oil certification equivalent of a Gas Safe registered engineer you would use for the installation and service of a gas boiler. Once installed, you should be given a certificate by your OFTEC-registered engineer to confirm that the installation meets the current industry standards.

What size of oil boiler is right for my home?

Choosing the right size of oil boiler means working out the appropriate output rating for your home. For a system and regular oil boiler, you need to look at the CH (central heating) output rating, whereas for combi boilers you also need to consider its DHW (domestic hot water rating.

An output rating will tell you the maximum power of the boiler unit, and it is measures in kilowatts (kW). The higher the kW output rating, the better an oil boiler will be at meeting higher user demands. Which is where your property’s size, as well as number of bathrooms and radiators you have, come into the equation when working which are the best oil boilers for your home.

For example, an apartment or terrace house which has up to 10 radiators, two bedrooms and one bathroom, will need an oil boiler with an output rating of between 9kW and 27kW, depending on whether you are looking at a system, regular or oil combi boiler.

A larger property, with up to 15 radiators, 3-4 bedrooms and 2 bathrooms will need a higher output rating, typically between 18kW and 34kW, again depending on the type of oil boiler you are opting for. Meanwhile a large home with four or more bedrooms and two or more bathrooms is going to need a boiler with an output of at least 27kW.

If you are in any doubt as to what output rating your new oil boiler needs, discuss your requirements with your boiler showroom or engineer before choosing the types of oil boiler systems you and your home really need.

Can my oil boiler run on diesel fuel?

It is possible for some oil boilers to run on diesel, but they need to be professionally converted first. And it is only system and regular boilers that can be safely converted, but with certain conditions attached. An oil boiler converted to run on diesel fuel must be installed outside and have a high-level flue, as burning diesel with a low-level flue is not legal in the UK.

Internal combi boilers cannot be converted to diesel and the fuel is not recommended for indoor heating systems.

If you are considering converting your external oil boiler to diesel, you should only ever use an OFTEC (Oil Firing Technical Association) qualified engineer.

What temperature should I set my oil boiler to?

Your oil boiler’s thermostat setting should be set in the same way as any other domestic boiler, with a minimum hot water temperature of 55-60°C as it needs to kill any legionella bacteria that may be in the water pipes. For your heating, this should be set for your preference as well as time of year and the different rooms requiring the heat. The recommended summer temperature for most homes is 18°C, whereas in winter it is in the region of around 19-21 degrees.

What are oil boiler alternatives?

While a good option for heating your home if you are not connected to mains gas, even the best oil boilers have their limitations and, as they use a fossil fuel, are not the most environmentally friendly when it comes to your home’s carbon footprint. However, there are alternatives to an oil boiler you could also consider:

Biomass

As a renewable energy source, biomass fuelled boilers are a cleaner and more sustainable alternative to oil. A biomass heating system burns wood pellets, chips or logs to power the central heating and hot water boiler. Biomass boilers can also qualify for the government’s Renewable Heat Incentive (RHI), meaning your heating system could also earn you money.

Electric heating

Electric heating, including underfloor and storage heating, can also be an alternative to gas, although installation can be expensive and the whole system can be costly to run.

LPG boiler

Producing less carbon than an oil boiler, LPG – liquid petroleum gas – is a natural hydrocarbon fuel that is efficient at powering a boiler. However, like oil, you will need an external storage facility to contain a regularly delivered supply.

Air Source Heat Pump

Working in the same way as a refrigerator, but in reverse, an air source heat pump is an external unit that uses outside air to warm up a refrigerant, which turns from a liquid into a warm gas, that is used as a heat source for your home and hot water.

Ground Source Heat Pump

This is similar in operation to an air source pump, but uses pipes buried in the garden to extract heat from the ground, which is then used to heat your home and hot water system. The length of pipe needed however, does depend on your central heating and hot water requirements as well as the size of your home.

Get connected

If gas is your preferred heating method, rather than an oil boiler, you could look into getting your property connected to the mains supply. But for off-grid homes, this can be an expensive option that if feasible will attract a high upfront cost to get it sorted.