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Wall Mounted Oil Boilers – 12kW to 24kW Output

These Wall Mounted Oil Boilers are meant to be installed internally in a building or dwelling. This range of wall hung oil boilers are all condensing oil boilers , that are rated either SEDBUK “A” rated for the standard or system wall hung boilers and SEDBUK “B” for the wall hung oil combination boilers. “SEDBUK” ratings have now been superseded by the EU commission regulation number 813/2013 which implemented directive 2009/12/EEC. Which now rates these wall hung oil boilers at both the rated heat output with a high-temperature central heating system, and also at 30% of rated heat output and with a low-temperature central heating system. This is now referred to as the “ErP” rating of the boiler. The smaller 16 kW output boiler is “B” rated, whilst the 20 kW is “A” rated and the largest model, the 24 kW model is “B” rated. These wall hung oil boilers are flued in a horizontal balanced flue configuration which is very flexible, ensuring easy installation suited to you site conditions .The other great thing, is unlike many other companies, this flueing kit for you wall mounted oil boiler is included in the price of the boilers.

With the Energy Related Products (ErP) Directive stipulating that from September 2018, NOx level emissions from oil boilers must not exceed 120mg/kWh, and these boilers comply with this regulation.

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Wall Mounted Oil Condensing Boilers – What are they?

Wall Mounted Oil Condensing boilers are very high efficiency appliances with combustion efficiencies around 95%. The extra efficiency compared with a ‘standard’ oil boiler is achieved by extracting virtually all useful heat from the combustion and flue products. So efficient, in fact, that the flue gases are cooled sufficiently to release their latent heat with the water vapour, which is produced in normal combustion, being cooled to the point of liquid condensate. A condensing boiler would have a typical flue gas temperature of 55°C compared with about 120 – 180°C in a standard boiler and about 300°C in many typical oil boilers manufactured about 20 years ago

  • How is this extra efficiency achieved? In our wall mounted oil boiler design, additional heat-exchanger capacity has to be created to extract out this additional heat that would normally be lost via the flue gases. As heat is increasingly drawn from the flue gases the greater, in inverse proportion, must be the heat absorption characteristics of the heat exchanger to extract the available heat.
  • What happens to the condensate produced ? In a condensing wall hung oil boiler as efficient as ours, condensate is produced continuously whilst the oil fired boiler is in operation, and this condensate has to be run off into a drain or the normal waste system of a dwelling or building. The condensing unit is remarkably free from narrow flue-ways and tubes. To gain access for inspection simply undo two wing nuts and detach the outer section of unit. This minimizes potential problems and maximizes the reliability of the condensing heat exchanger unit fitted in our wall mounted oil boiler combi version and our standard / system wall mounted oil boilers products

Wall Mounted Oil Boilers Models and Options

All of our Wall Mounted Oil Boilers, which also get referred to as Wall Hung Oil Boilers , are available in 3 different types, these being –

  1. Wall Mounted – Standard Oil Boiler ( or sometimes referred to as Conventional or even Regular oil boilers ) – These are oil wall hung boilers that don’t have an inbuilt pump or expansion vessel , that is required for a sealed system. These standard oil boilers are normally found in older heating systems, where the you have an oil boiler, linked to a hot water storage calorifiers ( more commonly known as a hot water tank ) fed by a cold water storage tank ( normally located in the loft ) and usually installed on what is called an open vent heating system , which was the prevalent or traditional way of installing heating systems up until 15 years or so ago. If you are looking to replace your old oil boiler, with one of our wall mounted condensing oil boilers, in an older existing heating system, then this is probably the replacement oil boiler for you. These condensing wall mounted oil boilers are a lot more efficient with a minimum efficiency of 90% , but if commissioned correctly you are looking at an efficiency of more like 93% , meaning that for very litre of oil burnt, around 93% of it’s useful heat is utilized, reducing your fuel bills accordingly. These oil wall mounted boilers are suitable for fully pumped, conventional open vented, or sealed heating systems. We do three models within this range , the smallest oil wall hung boiler – designated the “FHWS1” has a rated output of between 12 kW to 16 kW , which in Btu’s is 40,944 to 54,592 ,the mid-size unit – designated the “FHWS2” has a rated output of 12 kW to 20 kW -which in Btu’s is 40,944 to 68,240  and the larger external oil boiler model- designated the “FHWS3” is rated with an output of 20 kW to 24 kW, which is 68,240 to 81,888 Btu’s.


  1. Wall Mounted– System Oil Boiler – A system boiler is basically an standard boiler, which includes the pump and expansion tank that are built into the wall mounted condensing oil boiler which allows you to have a “sealed heating system”, which has become the standard over the last 15 years or so. The benefits of a ” sealed system” are that no feed and expansion tank is required in the loft or at high level ( another benefit is that these tanks were prone to freezing or bursting at the most inconvenient time , often cause major water damage or resulting in a failed heating system ), and that the oil heating installation is made a lot more tidier, easier, and simplifies servicing of the wall hung oil boiler, as everything is in one place , within the boiler casing. We only do the system oil boiler in the one version, designated the “FHWS-System “which has an output of 12 kW to 20 kW -which in Btu’s is 40,944 to 68,240.This oil boiler is suitable for sealed heating systems only.


  1. Wall Mounted Combination or “Combi” Oil Boiler – A “Combi” or Combination Oil Boiler are the most popular installed oil domestic boilers these days in the UK, as the provide both heat for the central heating system and Domestic Hot Water ( sometimes referred to as DHW) , for washing and bathing within a household. This makes the wall hung oil combi boilers extremely efficient as they only heat the domestic hot water when a shower or hot water tap is turned on, thus ensuring maximum efficiency. They eliminate the need for a domestic hot water cylinder or tank and the space required by it, not to mention the expense and also eliminate the need for a feed and expansion tank in the loft space. These Wall Mounted Combi Oil Boilers, are a complete package , or the complete wall hung oil combi boiler system , in one unit, being simple to use, easy to install and economical to run, with plenty of domestic hot water , for you to enjoy a bath or shower. The flow rate of these wall mounted oil combi boilers is 10 Litres a minute, with at least a 35 Degrees C temperature uplift or rise from the mains water temperature. This oil combi boiler has a heating output of 18 kW for the central heating, and a domestic hot water output of 24 kW. It has a minimum cold-water mains pressure of 1.5 Bar / 21.7 Psi and a maximum water pressure of 5.0 Bar / 72.5 Psi for the domestic hot water side of the combi boiler. This boiler is designated the “FHWS-Combi”.
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Wall mounted oil boilers from Flexiheat UK


               Wall Mounted Oil Boilers Technical Information 


Internal Wall Mounted Oil Boiler  12kW to 16kW Output – Heat Only

Model – FHWS1Standard Heat Only Boiler
Weight (dry)84kg


Internal Wall Mounted Oil Boiler  12kW to 20kW Output – Heat Only

Model – FHWS2Standard Heat Only Boiler
Weight (dry)96kg


Internal Wall Mounted Oil Boiler  20kW to 24kW Output – Heat Only

Model – FHWS3Standard Heat Only Boiler
Weight (dry)96kg


System oil internal wall mounted boiler option –

Internal Wall Mounted Oil Boiler  12kW to 20kW Output – System Boiler

Model – FHWS-SystemSystem Boiler
Weight (dry)107kg


Combination boiler option –

Internal Wall Mounted Oil Combi Boiler  24kW Output /

Model – FHWS-CombiCombination Boiler
Weight (dry)140kg

**The minimum domestic hot water flow rate for this boiler is 10.5 litres a minute,with a 35 Degrees C temperature rise.

Wall Mounted Oil Boilers – Flue Recommendations and Regulations

Balanced flue boilers can be judged to be more stable in burning conditions as they are less affected by flue draught and wind conditions. Air intake and flue discharge are more or less balanced. Also, balanced flue boilers take air for combustion from outdoors and, therefore, air change within the dwelling (itself a significant cause of heat loss) is reduced. Low level balanced flue wall mounted boilers may only be used with Kerosene. Gas Oil is not permitted. Flue terminals must be protected by a terminal guard if the flue exit point is under 2 metres. We can supply a stainless steel terminal guard which also doubles as a boiler protection guard for all our wall mounted oil boilers.

Balanced flue oil terminals must be 600mm from any opening door to window. Avoid positioning where flue discharge or noise could cause inconvenience or objection. Avoid narrow alleyways or confined spaces which could cause ingress of flue products, pollution, which could cause malfunction of the boiler. Also take care not to allow contact between the flue and combustible material.

These recommendations relate to basic safety requirements and should assist the installer in the siting of the wall hung oil boiler.

The principles are to :-

  •  Avoid combustion products from drifting into the dwelling
  •  To avoid possible combustion with materials such as plastic drain pipes, wooden eaves or other combustible material

Please see the picture below for the current regulations for the positioning of the flue terminal, for our wall mounted boilers range.

Flue Terminating Positions for Oil Fired Wall Mounted Oil Boilers

Wall Hung Oil Boiler Flue Kits – Horizontal 

These wall hung condensing oil boilers come with a horizontal flue kit as detailed below –


When using the Horizontal Balanced flue kit , the air for the combustion of the boiler comes externally into the boiler, and is considered “room sealed” thus no ventilation of that room is required. Our flue’s have been specifically designed to offer the smallest diameter on the market, meaning that they can not only be installed with the minimum of structural damage, but are also ideally suited for installations in listed buildings in which the minimum of structural damage is of maximum importance.

Wall Mounted Oil Boilers Horizontal Balanced Flue;Wall Hung Oil Boilers Horizontal

On the left, is the boiler external cover when installed. The right picture shows the internals of the boiler, which shows the burner, which is fully accessible from the exterior of the property, for servicing etc.



Which ever option 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.

It is important to note, that these oil boilers require a hole to be cut into the wall on which they are to be installed, so that can be installed. This ensures that the oil burner can be accessed from the exterior of the property, making servicing the boiler easy to service and also makes the boiler quieter, ensuring a more pleasant experience for the inhabitants of the home or flat. The additional benefit of the oil burner being used on the boiler, being external to the property ensures that no obnoxious oil smells are dissipated into the property.

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Wall hole dimensions for fitting the FHWS1 to 3 internal wall mounted boilers

Download the Flexiheat Brochure for FHWS Internal Oil Wall Mounted Boilers

Please see the dedicated Internal Wall Hung Oil Combi Boiler brochure below –

Download the Flexiheat Brochure for FHWS Oil Internal Wall Mounted Combi Boilers

Wall Mounted Oil Boilers – Best Quality & Price UK Wide

All of our wall mounted oil 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 these heard these External Condensing Oil Boilers called everything from Wall Hung Oil Boiler, Outdoor Oil Boiler, and the Wall Mounted Oil Combination boilers called Wall Mounted Oil Boiler Combi and Wall Hung Oil Combi Boiler – all we know is that we have a British built, high quality range of Wall Mounted Oil Boilers, at some of the best prices on the UK market, so should you require any more information on our Wall Mounted Oil Boilers , please don’t please don’t hesitate to contact our sales team on 01202 822221 or click here for our email contact form