If you are one of the four million households across the UK who are not connected to the mains gas grid, then we understand that you will be looking to other means to heat your home. The most popular alternative to a gas boiler is an oil boiler. Whilst more expensive to purchase than electric and LPG boilers, they are cheaper to run.
However, just like the gas equivalents, the choice of oil boilers is vast. Due to this, making a decision on the best one for your specific needs can be a challenge. That is where we come in though, we are here to assist you in choosing the best oil combi boiler.
As an independent boiler installation company, we’ve done the hard work for you and picked out the best oil combi boilers available to purchase in 2021. The following guide has been developed with engineers and installers with over 40 years of experience.
To put together this comprehensive list, we have considered the following:
Awards and accolades
In addition to the above, we have also considered boilers that utilise blue flame technology and meet the NOx reduction levels legislation expected in 2022.
In this list, we have looked at oil boilers from the following manufacturers:
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Getting started with an oil boiler
If you are looking to choose an oil boiler due to living in a remote area, to get started with an oil boiler you will require an oil tank located outside of your property. This will require filling at least once a year, however, this depends on the size of your oil tank. The majority of oil boilers in the UK use kerosene oil. Once the tank is filled, you will be able to forget about it, with the fuel in the tank being stored until it is needed.
When replacing an oil boiler you are not legally required to replace your oil storage tank unless installing the boiler makes the tank non-compliant.
The cost of heating your home using an oil boiler
On average, the cost of heating a 3-bedroom home using an oil boiler will be close to £1,360 per year. However, this can vary greatly depending on the age and the efficiency of your boiler. By installing a new condensing oil boiler, you can expect to save between 30%-40% on your annual energy bills compared to using an older non-condensing oil boiler.
How do oil boilers work?
An oil-fueled central heating system essentially works in the same way as a gas-powered central heating system. Albeit by using oil to heat water rather than gas, beyond this though they work in the same way by heating your home via radiators and sending hot water to the taps.
Advantages of an oil boiler
Oil combi boilers are highly energy-efficient, and you can save fuel by heating up water on demand
The running costs of oil combi boilers are usually much lower than other types of oil and LPG boilers
Oil is a more efficient fuel than gas and LPG, so every unit of energy spent provides a better return.
All new oil boilers are now required by law to be condensing boilers and a as a result will run at an efficiency of 90% and above, compared with 60% for older boilers
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Our recommended oil boilers
Founded as Grant Engineering in 1978, Grant has been designing and manufacturing oil boilers in the UK since their formation.
Grant is known for quality and reliability, offering an oil boiler range that is easy to install and maintain. This quality has been recognised by Which?, with the entire Grant oil boiler range being awarded a Which? Best Buy award in 2017, 2018 and 2019. Whilst the efficiency of Grant oil boilers has been recognised with the Grant Vortex condensing boiler range being endorsed by the Energy Savings Trust.
Grant Vortex Blue Internal Combi Boiler
Price (Installed): From £4500 (inc. VAT)
The Grant Vortex Blue Internal Combi is available in the following sizes: 21kW, 26kW and 36kW
Vortex Blue Internal Combi
90.81% (Vortex Blue Combi 21), 91.71% (Vortex Blue Combi 26), 94.56% (Vortex Blue Combi 36)
Energy Efficiency Class
160kg (Vortex Blue Combi 21), 177kg (Vortex Blue Combi 26), 200kg (Vortex Blue Combi 36)
860mm (Height) x 614mm (Width) x 609mm (Depth) (Vortex Blue Combi 21 and Vortex Blue Combi 26), 900mm (Height) x 614mm (Width) x 609mm (Depth) (Vortex Blue Combi 36)
* When installed by a G1 accredited installer
We recommend the Grant Vortex oil boiler range as one of the best boiler ranges available to buy in the UK due to the efficiency of the range and the fact that the range meets the NOx reduction levels legislation expected in 2022. The range has some of the highest efficiencies in the UK, achieving gross seasonal efficiencies of up to 94.56% (SAP 2009). This has been recognised with the range being endorsed by the Energy Savings Trust.
Key to the efficiency of the Grant Vortex oil boiler range is the range’s use of the state-of-the-art Riello Blue Flame compact burner. The combustion process that takes place within a blue flame burner is much cleaner than in standard yellow flame burners. This is because the combustion gases within a blue flame burner are re-circulated through a specially designed longer blast tube which has a ring of slots within the combustion chamber. This allows the combustion gases to mix with atomised droplets of fuel and fresh air drawn in through the burner, to produce a clean blue flame that has much lower NOx emissions.
As a combi boiler, one of the clearest benefits of the Grant Vortex Blue Internal Combi is the fact that everything is contained within one unit. With the water heater and the central heating boiler combined, no hot water cylinder is required with this boiler.
Ensuring reliability, the Vortex Blue Internal Combi utilises Grant’s Vortex stainless steel heat exchanger. Unlike aluminium equivalents, this stainless steel heat exchanger is less resistant to corrosion and staining in a wide range of pH levels. This increases the boiler’s efficiency as corroded heat exchangers will become less efficient over time.
Endorsed by the Energy Savings Trust
Meets the NOx reduction levels legislation expected in 2022
One of the most efficient oil boilers available in the UK
Comes with a 24-litre expansion vessel as standard.
Equipped with the award-winning Vortex stainless steel heat exchanger, ensuring efficiency
Quiet operation – 50.6 dB
All Grant oil boilers are available with a 7-year warranty when installed and registered by Boilerhut Ltd (subject to T&Cs)
Extended warranties with Grant Accredited Installers
Boilerhut works closely with Grant and is a G1 accredited installer. This means our installers are rigorously trained to meet the highest quality standards and complete installations in the safest way possible. By choosing us to install your Grant boiler you will also be eligible for Grant’s comprehensive 7-year guarantee, something which is only available to G1 accredited installers.
Whilst Navien are relatively new to the UK market – having only been operating here since late 2014, they have been producing boilers in South Korea since 1978. In 1988, they became the first company in Asia to develop and produce a highly efficient, environmentally-friendly condensing boiler.
Today, they are the name behind the best selling boiler in the US, operate in 30 countries worldwide and have ambitious plans for growth in the UK market.
Navien LCB700 Blue Flame Combi Internal Boiler
Price (Installed): From £4400 (inc. VAT)
The Navien LCB700 Blue Flame Combi Internal Boiler is available in the following sizes: 21kW, 28kW and 36kW
Navien LCB700 Blue Flame Combi
Energy Efficiency Class
91kg (21kW), 92kg (28kW), 95kg (36kW)
860mm (Height) x 600mm (Width) x 600mm (Depth)
When installed with three of Navien’s Smart Plus room thermostats, the Navien LCB700 Blue Flame Combi achieves an A+ efficiency rating. This makes it the UK’s first A+ rated oil boiler range.
Like the Grant oil boiler above, we recommend this Navien oil boiler due to these efficiency levels. Again like the above example, this boiler utilises blue flame technology which assists it in achieving this efficiency. The blue flame technology employed in the LCB700 enables the boiler to emit extremely low NOx emissions of 57mg/kWh and meets the NOx reduction levels legislation expected in 2022.
It also has a stainless steel exchanger which we recommend over the cheaper and ultimately less efficient aluminium alternatives found in other oil boilers. This will ensure resistance to corrosion which can affect efficiency after a number of years of usage.
In addition to its efficiency, we recommend this Navien boiler because it comes with a 10-year warranty which is great for peace of mind. This warranty covers the boiler’s components and protects you against issues caused by manufacturing defects. However, to ensure this warranty remains valid, you must ensure that you have the boiler serviced annually by a Gas Safe registered engineer.
As mentioned above, this boiler will achieve an A+ efficiency rating when installed with a Navien Smart Plus room thermostat. Using the Smart Plus thermostat with this boiler will enable you to control your heating and hot water anywhere and at any time providing you have an internet connection.
Beyond just controlling your system, the Smart Plus thermostat will also enable you to check the status of your heating system, see how much energy you’re using and importantly, set up your heating system to become more efficient. It does this in a number of clever ways, the first of these using the smart forecast check functionality. Using this, your heating system can check the weather forecast using OpenWeatherMap and adjust itself depending on the current outside temperature – this ensures you are only using energy when you need to.
Another great smart function which the Smart Plus thermostat adds to the LCB700 Blue Flame Combi is geofencing. Geofencing technology will ensure your house is only being heated when needed. It does this by registering your smartphone and those of others in your household and turning your boiler on and off based on your proximity to home.
Built into the LCB700 Blue Flame Combi boiler is an intelligent mode. Utilising self-learning, this will monitor DHW usage and adapt based on your typical usage routine.
Blue flame burner
Stainless steel heat exchanger
Compatible with Navien’s Smart Plus thermostats for increased control and efficiency
Built-in Intelligent Mode to monitor and adjust based on your typical usage
Full text-driven control
Self-diagnosis and error feedback provided by the display
Built-in usage function to show how much fuel has been used
*when installed with three Navien Smart Plus room thermostats
Founded by Cecil Duckworth in 1962 as Worcester Engineering Co Ltd, the company was acquired by Robert Bosch GmbH in 1992 and became Worcester Bosch. Since then, they have continued to be one of the most trusted heating companies in the UK and are the current market leaders.
Worcester Bosch offers an extensive range of boilers utilising a range of different fuels and suitable for properties of all sizes.
Worcester Bosch Greenstar Heatslave II Internal Combi Boiler
Price (Installed): From £4740 (inc. VAT)
The Worcester Bosch Greenstar Heatslave II is available in the following sizes: 18kW, 25kW and 32kW
Greenstar Heatslave II
Energy Efficiency Class
Up to 5-year warranty
124kg (18kW), 126kg (25kW), 128kg (32kW)
940mm (Height) x 640mm (Width) x 770mm (Depth)
The Worcester Bosch Greenstar Heatslave II is the UK’s most popular oil combi boiler and for this reason, has earned its place on this list. This boiler also comes recommended by Which?, having received their Best Buy award for oil boilers in September 2020.
With three output sizes available – 18kW, 25kW and 32kW, there is an option of the Heatslave II available for small, medium and large properties with one or two bathrooms.
Another oil boiler that is noted for its efficiency, whilst not quite as efficient as the other boilers recommended here, the Greenstar Heatslave II does have a respectable 90% efficiency rating and is A-rated as a result.
Designed to fit under a standard kitchen worktop, the Heatslave II can easily be installed in a kitchen alongside a washing machine or a dishwasher. Ensuring it doesn’t look out of place in your kitchen, the Heatslave II’s design features a drawer in which the controls are housed – this is also handy for families with small children, where the controls would otherwise be at the right height for little hands.
If you’re not keen on having the boiler on show, it can also be kept out of the way and housed in a kitchen cupboard. Alternatively, it is suitable for installation in a utility room, airing cupboard or garage, should you have the space available.
For greater energy efficiency, the Heatslave II is also compatible with green energy sources such as solar panels.
Another feature of the Greenstar Heatslave II to aid efficiency is the Eco hot water mode, this is an optional feature that enables you to turn off the preheat option for water – whilst this will take longer for hot water to be delivered, it will result in a marked difference when it comes to fuel usage.
Ensuring the Greenstar Heatslave II is equipped for the future, it is compatible with Worcester Bosch’s EasyControl smart thermostat. When used in conjunction with this device, you will be able to control your heating system wherever you are, if you have an internet connection. In addition to this, you will also be able to check your energy usage and when used with smart TVRs, you will also be able to control the heating in individual rooms. Again, should you have children – you will also be able to toggle the child-lock setting on and off, all from your smartphone.
The EasyControl smart thermostat can also automatically turn the heating down if you’re not in the house and can detect when you’re returning home, turning the heating up to your desired temperature.
Improving efficiency, the EasyControl smart thermostat enables you to view the performance of your Greenstar Heatslave II boiler and understand your gas consumption for heating and hot water, all directly from the companion smartphone app.
Cross firing burner
Concealed heating controls
Designed to fit under a standard kitchen worktop
Digital control offering precision heating and hot water control.
Front access for routine servicing and providing flexibility in where the boiler can be installed
Quiet operation – 56 dB
Compatible with green energy sources
Compatible with Worcester Bosch’s EasyControl smart thermostat
Established in Ireland in 1980, Firebird offers a comprehensive range of oil-fired boilers suitable for internal, external and boilerhouse application.
Renowned for their oil-fired boilers, Firebird is committed to manufacturing products in accordance with the highest quality standards including ISO 9001 : 2008.
Firebird Envirogreen Internal Combi Boiler
Price (Installed): From £3900 (inc. VAT)
The Firebird Envirogreen Combi is available in the following sizes: 20kW, 26kW and 35kW
Energy Efficiency Class
2-year standard warranty
167kg (20kW), 170kg (26kW), 173kg (35kW)
845mm (Height) x 595mm (Width) x 614mm (Depth)
Included here is a budget option, from £3900 installed, the Firebird Envirogreen Internal Combi Boiler is offered at a notably cheaper price point than the other oil combi boilers included within this list.
Available in 20kW, 26kW and 35kW options, there is an option of the Firebird Envirogreen Internal Combi Boiler for small, medium and large properties with one or two bathrooms.
One of the reasons we like the Firebird Envirogreen Internal Combi Boiler is for its compact dimensions. This means like the Worcester Bosch Greenstar Heatslave II mentioned above, it can easily fit underneath a standard worktop in the kitchen or utility room. Should installation in these not locations be suitable, it is also a good choice for installing in a garage.
With a 93% efficiency rating and an A rating for heat, the Firebird Envirogreen offers great efficiency which equals lower fuel bills for you.
This efficiency is greatly achieved by the use of blue flame technology. The blue flame technology employed in the Firebird Envirogreen enables the boiler to emit low NOx emissions which exceed the minimum stated levels.
The Firebird Envirogreen also offers fast, easy installation with multi-directional flue options, whilst all of the main component parts are undone with a single Allen key. This also makes servicing and maintenance much simpler for heating engineers using a 4mmm Allen key.
Excellent hot water performance – over 16 litres/min on 35kW model
Suitable for installation underneath a standard worktop
PCB controller using highly accurate thermistors.
Rapid re-heat after hot water draw-off.
Hard-wearing casings with a high-quality finish
Front servicing access
Concealed control panel
Twin circulating pump – no diverter valve
No flow switch
Fully insulated and acoustically designed for ultra-quiet operation
Multi-directional flue options
Navien LCB700 Blue Flame Combi Internal Boiler
Our recommendation for the best oil combi boiler for 2021 is the Navien LCB700 Blue Flame Combi Internal Boiler.
We have come to this conclusion based on the following:
Efficiency: The Navien LCB700 provides excellent efficiency of 93%
Warranty: With a 10-year warranty, the Navien has the longest warranty period of all of the boilers recommended here
Technology: Blue flame technology, a stainless steel heat exchanger, self-learning DHW water usage monitoring and integration with Navien Smart Plus room thermostats all contribute to the Navien LCB700’s excellent efficiency
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