External Oil Combi Boiler – 15kW to 28kW

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 smaller sized homes with one bathroom as well as a shower/ en suite.

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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.

External Oil Combi Boiler Range / Options

We do 2 models within our our range of External Oil Combi Boilers – a 15 kW to 22 kW Output model , or  51,180 to 75,064 BTU’s, this external oil combi boiler would normally be suitable for smaller 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.

                                             External Oil Combi Boiler  15kW to 22kW Output
Height   990mm
Width   582mm
Depth   400mm
Weight KG (dry)   126kg

This external oil combi boiler will produce 12 Litres / minute  , with a temperature rise of 50C , also known as a Delta T of 50C ( We will explain more about these figures, and what they mean further on down the page )

                                             External Oil Combi Boiler  23kW to 28kW Output
Height   990mm
Width   662mm
Depth   400mm
Weight KG (dry)   137kg

Which ever 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.This external oil combi will produce 14 Litres / minute  , with a temperature rise of 50C , also known as a Delta T of 50C ( We will explain more about these figures, and what they mean further on down the page )

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 fired combi boiler flow rates 

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

As a 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. An example of this are –

Approximate Flow Rates in Litres per minute (ltrs/m) – and the maximum temperatures recommended

Sink / Kitchen Taps  –

Newer Monobloc sink mixer = 8 ltrs/m – Water should be at a temperature of between 46ºC and 48ºC to ensure thorough removal of grease

Old style ½” kitchen tap = 12 ltrs/m – Water should be at a temperature of between 46ºC and 48ºC to ensure thorough removal of grease

Basin Taps –

Modern monobloc basin mixer with 10 or 12 mm connections = 4 to 5 ltrs/m – Temperature – 41ºC  max

Old style ½” basin tap = 10 ltrs/m  – Temperature – 41ºC  max

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 – Temperature – 40-43°C

Standard shower head – 12 Ltrs/m – Temperature – 40-43°C

Drench Showers – Can be anywhere form 15 Ltrs/m to 67 Ltrs/m ( Depends to the manufactures figures ) – Temperature – 40-43°C

Ok, so with all this stated , our smaller external oil combi boiler will do at least 12 litres per minute , and our large external oil boiler 14 litres a minute , raised through 50C – so what does this mean in reality ? ( bearing in mind the average incoming water temperature for a cold water main in the south of the UK , is around 10C )

In the UK we like our domestic hot water produced at 60C , the main reason behind this is that at this temperature is Legionella – Legionella bacteria are naturally occurring organisms present in many water systems, and if exposed to human beings can cause the development of Legionnaires’ disease, this is particular the case when the water is in an aerosol ( mist )state e.g. showers.

To counteract this we produce water at 60C , because – Legionella bacteria will die at temperatures at 60ºC , and above

In a perfect world we would like to produce water at the stipulated temperatures of each domestic hot water outlet , but we can’t because –  Legionella bacteria proliferate between 20ºC and 45ºC, which is right in the zone that we require, thus we can’t do this

However, hot water temperatures that kill bacteria such as legionella will cause scalding – Scalding occurs well below the boiling point of water. Temperatures above 45°C can cause serious injury very quickly. With water at 70°C, partial thickness injuries occur in well under 1 second, and full thickness burns in approximately 10 seconds.At 60°C, similar injuries occur in approximately 7 seconds (partial), and 90 seconds (full thickness). Thus in the UK we use thermostatic mixing valves to prevent the risk scalding

The function of a thermostatic mixing valve is to mix the hot and cold water entering the valve to the temperature that has been pre-selected by the user.This is done automatically by a thermally sensitive mechanism within the valve that proportions the amount of hot and cold water entering so as to achieve the required blend. Thereafter, the mechanism automatically
compensates for variations in supply pressures or temperatures so as to maintain a safe temperature. Moreover, in the event of cold water supply failure, the thermostatic mixing mechanism will automatically “shut down”the flow so as to prevent dangerously hot water being discharged.For more details on this area, please download this leaflet below .

Flexiheat UK-TMV-Guide-Domestic

So we can now calculate how many how water outlets we can satisfy with our external oil combi boiler range , based on the split of hot water to cold water mix to achieve a mixed temperature of 40C , which is 60% hot water to 40% cold water –

Thus our 15kW to 22kW External Oil Combi Boiler will produce 12 litres of how water @ 60C , will give you 12 x 1.6 = 19.2 Liters of usable water @ 40C , then you can check this against the approximate flow rates table above, to see if it is suitable

Our 22kW to 28kW  External Oil Combi Boiler will produce 14 litres of how water @ 60C , will give you 14 x 1.6 = 22.4 Litres of usable water @ 40C , then you can check this against the approximate flow rates table above, to see if it is suitable

If they are not, then this isn’t a problem , you can just use one of our non combi external oil boilers , linked into a domestic hot water cylinder , for more information on this range , please see our products page.

So generally speaking , our external oil combi boilers are suitable for buildings with one bathroom , containing a bath and or shower, if you have two bathrooms ( which both could be in use simultaneously)  , or a large drench type shower , with high flow rates  , then you need to be looking at boiler and domestic hot water storage cylinder system as your answer.

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External Oil Combi Boiler installed over 10 years ago, and still looking good

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 – OFTEC OFS A100, European Directives BED 92/42 and EEC LVD EN 60335-1 and EMC 89/33C/EEC and are WRAS approved. 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.

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

Download Flexiheat External Oil Combi Boiler Manual