External Oil Combi Boiler – 8.2 kW to 40.5 kW

External Oil Combi Boiler  – An external oil combi or combination central heating oil boiler is an innovative space-saving concept.An external oil combi boiler is both a high-efficiency hot water heater as well as a central heating boiler integrated, hence the name, within one compact unit. Consequently, no separate hot water cylinder is required, therefore conserving space within the property. In over two thirds of homes in the UK, a Combi Boiler is an instantaneous domestic hot water heater as well as a central heating boiler combined.Water is heated up directly by means of a thermal store when a tap is turned on on, saving not only on cost, but additionally on the space needed for a hot water storage tank. Supplying immediate access to hot water.

An oil combi boiler is especially suited to small, medium or large properties or homes with one or two bathrooms, and being installed outside also means freeing up space within your property. Our external oil fired combi boilers are housed in a robust cabinet that protects the boiler in all weather conditions.

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External Oil Combi Boilers from Flexiheat UK

How Does an External Oil Fired Combi Boiler Work?

An oil combination boiler system operates in a comparable way to its gas powered alternative, utilizing an oil fired boiler to heat water, which provides the central heating via radiators as well as hot water through the taps in your home.The primary difference between the two is that an oil-fired boiler needs a continuous oil supply, delivered and stored in an oil storage tank. Most oil-fired combination boilers have an internal plate heat exchanger to supply domestic hot water , which is the same system used by gas combi boilers.The appeal of oil fired combi central heating boilers is expected to increase in the UK since they have many benefits over various other kinds of heating systems.

Here is the process in full –

A central heating boiler is basically a furnace, burning fuel to produce heat, and an oil combi boiler works on a fairly straightforward principle. Various boilers have different configurations, yet they all make use of a device called a heat exchanger, which transfers heat efficiently and effectively from the burning oil to the water.

When you turn on the hot tap, that’s the boiler’s cue to wake up and start functioning. The water comes via the boiler on its way from the mains to your tap, and as it does so, it flows over a small sensor which informs the boiler to begin burning the oil. The heat exchanger inside the boiler gets hot; the water then flows through the heat exchanger and gets hot too and arrives in your bath nice and piping.

A similar thing occurs when you switch on the central heating. The boiler fires up its oil burner, and then a pump inside the boiler begins pushing water through the heat exchanger. The warm water flows out through a pipe into the radiators, making your home nice and warm, and after that back into the boiler where it gets heated up once again replacing the heat it has lost to your rooms, and so on and so forth.The water for your central heating system, and the mains water that you drink and wash in, come through two completely separate pipes.

The Benefits of an External Oil Combi Boiler – The Pros:

There are numerous great features that have made “combi” or combination boilers the most popular boiler type across the UK. In general, External Oil Combi Boilers are:

Much More Efficient

Oil Combi boilers are extremely efficient when compared to older oil boiler models. In fact, replacing your boiler with an external oil combi boiler could potentially save up to £300 a year on your heating bills.


External Oil Combi boilers are far less complex and complicated compared to traditional heating systems, which means that installations tend to be cheaper ans easier. It is also easier to find replacement parts if a problem may occur.


Without the need to heat up a water tank ,external oil combi boilers can provide an unlimited supply of hot water on demand. No waiting time for the hot water cylinder to warm up !

More Compact

As there is no need for a separate water tank, an external oil combi boiler system takes up much less space than conventional heating systems. This makes them ideal for homes that have limited room, and often a much easier install too !


The cold water from a combi boiler system is always safe to drink as it is fed directly from the mains. However, this is not always the case with traditional systems, which may store cold water in a tank before it is piped through the house.

Easier to Sell

As  oil combi boilers tend to be more efficient than other boiler types, this can be a selling point if you are looking to rent or sell your property. This will also show on the Energy Performance Certificate (EPC).

Much Less Likely to Experience Pressure Problems

External oil combi central heating boiler systems do not have the tendency to suffer low pressure or air locks, as the hot water is taken directly from the water mains


Central heating systems with a external oil combi boiler generally have less sludge build up, because the water is piped directly from the mains and not kept in a tank where it can rust and debris can build up

Easier to Service

The popularity of external oil combi boiler’s means that a large majority of boiler engineers are trained how to repair and service them. This makes it easier to find a qualified engineer in your area.

Features & Benefits of our Outdoor / Outside Oil Fired Combi Boilers

Energy Savings

Our Evolution range of outside oil combi boilers have the highest efficiency, not only is the Heating efficiency class “A” but also the Domestic Hot Water efficiency class is also “A” rated. This makes these oil boilers class leading, and guarantees the lowest fuel consumption in this sector. These figures are certified according to the European ErP directive, which all boilers have to comply with.


The high level and effective use of insulation on the boiler body and boiler casing, combined with an airtight burner ensures that this oil boiler range is one of the quietest on the market

Reliability and Longevity

The heat exchanger of these boilers is made of cast iron, which ensures long product life and reliability.


This boiler range ensures the reduction of contaminating CO2 emissions, contributing to the reduction of the greenhouse gases effect, thanks to condensing technology which is used in the oil boilers.

The condenser is specially designed to avoid un-burned particles from the combustion process sticking to the exchanger surface in the condenser and thus reduces maintenance costs and ensures longevity of the boiler

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A diagram showing the condensing system on our oil fired external combination boilers

Ease of Use

Ease of use has been a priority in the design of these combination condensing external oil boilers. As a result, these boilers come with a highly intuitive and easy to use control panel.

This electronic control panel not only ensures the control of the boiler, but also some vital safety systems. These are –

Anti-freeze system – When the temperature of the boiler goes below 6º C, the boiler is activated even if it is switched off, to avoid problems in the system due to freezing.

Multi Directional Flue Options – These boilers come equipped with a 45 Degree horizontal flue outlet terminal, that can be placed on either the left or right hand side of the boiler, or at the rear of the unit, to suit your site requirements.

Water pressure safety system – this avoids boiler break downs due to the of lack of water in the boiler

Pump over run system– It has a post circulation system to prevent the boiler from overheating and ensures long boiler life span.

Pump anti-blockage system– Which reduces the maintenance and potential failure of the circulation pumps

External Oil Combi Boiler Range / Options

We do 2 models within our range of External Oil Combi Boilers – first is the 30 Model , which will produce  30.2 kW Output , or  103,042 BTU’s when used on an 50º C / 30º C flow and return designed heating system ( common for underfloor heating )  or 28.7 kW ( or 97,924 BTU’s ) when used on a 80º C / 60º C flow and return designed heating system ( common for older type radiator heating systems )  , this external oil combi boiler would normally be suitable for semi-detached or terraced properties and bungalows alike , but please consult a heating engineer or plumber should you be in any doubt of what size boiler you require or get in contact with us, via email and we can send you over some sizing advice.

                                             External Oil Combi Boiler  8.2 kW to 30.2 kW Output
Height   943 mm
Width   500 mm
Lenght   1000 mm
Weight KG (dry)   160 kg

This external oil combi boiler will produce 12.8 Litres / minute, with a temperature rise of 30C, also known as a Delta T of 30C or 15.4 Litres / minute, with a temperature rise of 25C (We will explain more about these figures, and what they mean further on down the page)

The second model is designated the 40 all will produce  40.5 kW Output , or  138,186 BTU’s when used on an 50º C / 30º C flow and return designed heating system ( common for underfloor heating )  or 38.7 kW ( or 132,044 BTU’s ) when used on a 80º C / 60º C flow and return designed heating system ( common for older type radiator heating systems )  , this external oil combi boiler would normally be suitable for larger semi-detached or detached properties and larger bungalows and alike.

                                             External Oil Combi Boiler  12.4 kW to 40.5 kW Output
Height   943 mm
Width   500 mm
Depth   1,100 mm
Weight KG (dry)   190 kg

This external oil combi will produce 17.8 Litres / minute, with a temperature rise of 30C (Delta T 30), or 21.4 Litres / minute, with a temperature rise of 25C also known as a Delta T of 25C.

Download the external oil combi boilers brochure from Flexiheat UK

Whichever external oil combination boiler you require, we highly recommend that you use an OFTEC approved / trained installer who promotes excellence in the oil heating industry by setting standards for installation and maintenance of oil-fired appliances for the domestic consumer. Although this oil qualification is not mandatory.

For those who want more information on domestic heating by oil: boiler systems – guidance  for installers and specifiers, this PDF has some great information on all the considerations.

Flexiheat UK Domestic heating by oil

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External oil combi boiler- floor mounted – internals picture from Flexiheat UK

External oil fired combi boiler flow rates 

Now for the question of ” is an oil fired combi boiler ” the right oil boiler for your property , read below to find out –

As an external oil combi boiler produces water on demand, the number of domestic hot water outlets, and the type that you i.e., taps, standard showers, drench showers etc and the flow rates they require are the critical criteria to know to ensure the combi boiler can produce the required flow rates. An example of this are –

Approximate Flow Rates in Litres per minute (ltrs/m)

Bath Taps –

Modern ¾” bath mixer tap = 12 ltrs/m

Old style ¾” bath taps = 15 ltrs/m

Showers –

Aerated low flow design – 7.6 Ltrs /m

Standard shower head – 12 Ltrs/m

So, as you can see from the flow rates above , our model 30 oil combi boiler , should cover all of the hot water demands , however if you are a larger user , or might just want to have the peace of mind or higher hot water flow capacity , then go for the model 40, as this has the better hot water performance.

Look out for Drench Showers – They can be anywhere from 15 Ltrs/m to 67 Ltrs/m ( Depends on the manufactures figures )

So generally speaking , our external oil combi boilers are suitable for properties with one or two bathroom , containing a bath and or shower, if you have two bathrooms and both are regularly in use at the same time, or if your home has a  large drench type shower , with high flow rates above what our external combi boilers can provided  , then you need to be looking at an oil boiler that is known as a heat only boiler and domestic hot water storage cylinder system as your answer. For more information on this range of heat only external boilers, please see our products page.

Internal Remote Controller

All of our oil-fired external combination boilers are supplied with an internal controller with temperature sensor. This is normally positioned in the same location that you would place a traditional thermostat.

This controller will not only act as a room thermostat, but you can also –

  • Program the operation time of the boiler, both the heating and the domestic hot water.
  • See boiler alarms / alerts
  • Modulate the power of the boiler
  • Adjust the working temperature of the heating flow and return temperatures
  • Adjust the Domestic Hot Water working temperature
  • Please note that this controller has to be hard wired to the boiler, it is not available in a Bluetooth or Wi-Fi version
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The internal controller, which is wired inside the property allows full control and monitoring of the outside / outdoor oil combi boiler.

Outdoor Oil Combi Boiler / Outdoor Oil Combi Boiler

Whether you call these oil combi boilers, an outdoor oil combi boiler or an outside oil combi boiler is you preference, and that’s fine with us. The trade (most heating engineers) will call these oil combi boilers an external oil combi or external oil combination boiler. However you wish to refer to them is fine, all we can tell you is that we supply a quality product , that is fully weatherproof and is made with quality branded components, ensuring a long and trouble free service life, with the benefit of having the boiler external to the property any associated smells of oil that you can have with an internal oil combination boiler are eliminated with the added benefit in that any service engineer / plumber doesn’t need access to your property to work on the boiler, which leads to less inconvenience and often less mess to clean up after.

External Oil Combi Boilers – Best Quality & Price UK Wide 

All of our external oil combi boilers have been designed and manufactured to the following standards – European Directives BED 92/42 and EEC LVD EN 60335-1 and EMC 89/33C/EEC. They are designed to burn Kerosene 28 Class C fuel – to British Standard Specification BS 2869 Part 2 Class C2 , European standard BS EN 2869:2010 Class C2, which are also referred to as Heating Oil, 28 Second Burning Oil or Central Heating Fuel. This grade of Kerosene has clean burning characteristics whilst maintain a high heat output and maximum fuel economy.

They will also burn 35 second gas oil to, which is often referred to as red diesel to BS2869: Part 2: 2010 classifications Class D.

We have  heard the External Condensing Oil Boilers called everything from Outside Oil Combi Boilers, Outdoor Oil Combi Boiler, and you can call them what you like, all we know is that we have a British built, high quality (we think it’s the best external oil combi boiler on the market) External Oil Combi Boiler, at some of the best prices on the UK market, so should you require any more information on our External Oil Combi Boiler range ,  please do not hesitate to contact our sales team on 01202 822221 or click here for our email contact form.