Condensing Oil Boiler – Commercial Oil Boiler – 15 kW to 1,500 kW

From – 15 kW to 68 kW Output ( For the Larger Commercial Oil Boilers 69 kW to 1,500 kW scroll to the bottom of the web page for the information)

Condensing oil boiler – With conventional oil boilers, waste energy, as a large portion of the thermal energy produced by the combustion, is thrown away, with the flue gas, which exits the boiler to thecondensing oil boiler, commercial oil boilers for sale uk,commercial oil fired boilers, environment at high temperature. Technology answers to this problem, through the condensation of the flue gas, by reducing the temperature of the flue gas until the condensation point. With this process, it is possible to exploit the flue gas latent heat, which is transferred to the water of the heating system. This way, we exploit the full potential of the fuel resulting to a significant reduction of fuel consumption and emissions to the environment.

A conventional oil boiler, may have an efficiency between 65% and 93%. With modern condensing technology,we may achieve up to 102% efficiency for the combustion of oil. This improvement of efficiency and the consequent reduction of fuel consumption leads to savings to your heating bill and protection of the environment in which we live in.This range of condensing oil boilers / Commercial oil boilers are produced according to the new directives of the European Union for energy products, in which very strict limits have been set, regarding efficiency and fuel consumption.Since April 2007 , the regulations state that all new central heating boilers that were oil fired must be high-efficiency condensing boilers unless there are exceptional circumstances.

High efficiency Condensing Oil Boiler

Modern condensing oil boilers are designed to use virtually all of the energy content of the oil – often referred to as the higher heating value. Compared to standard boiler technology, oil condensing boilers also recover the heat of condensation of the water vapour contained in the exhaust flue gas. Oil-fired condensing boiler technology therefore achieves the highest efficiency and very minimum emissions along with the lowest fuel consumption. It is important to know that to get your oil condensing boiler to work at its maximum efficiency, you need your return water temperature to be under 47C , which is the “dew point temperature” for heating oil .

Commercial Oil Boiler

Oil boilers ( which all now have to be condensing ) with an input of over 70 kW net for the installation are considered to be a commercial oil boiler. An example of this is if you had a 60 kW net oil boiler this would be deemed to be a domestic oil boiler, but if you had 2 x 45 kW oil boilers connected to the same heating system, then this would be classified as a commercial installation because the total net input is greater than 70 kW ( i.e 2 x 45 kW boilers = 90 kW in total )

This range of Condensing Oil Boilers has –

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Condensing Oil Boiler Range up to 68kW

The key benefits and features of these condensing oil fired boilers –

  • High Efficiency – typical of 93%  Net – Getting more heat from your oil and saving you money
  • Front access to the boilers –  The detachable front casing permits easy access for commissioning and future servicing requirements
  • Compact Dimensions – The 15kw to 41kW are only 410mm wide , whilst the 41kW to 68kW are 500mm, making these boilers easily fit through a standard door way.
  • Balanced Flue or Conventional Flue Options – With either flueing options all site conditions can be catered for, ensuring you can fit one of our Condensing Oil Fired Boilers
  • Multi Directional Flue Outlets – Again ensuring all site conditions can be adhered to, and making fitting the flue system to these oil condensing boilers so much easier !
  • All of this range of these high efficiency oil boilers, has pre punched pipe access points – Ensuring that the fitting of these oil boilers is both fast and neat.
  • These oil fired condensing boilers are suitable for installation into conventional open vented heating systems that have a feed and expansion tank or a sealed heating system that uses an expansion vessel , more information on these two different type of heating systems can be found here
  • These Oil Boilers can be used in a Multiple Boiler Configuration or set up – You can use 2 of these boilers in a reverse return boiler system , so you can one as duty, and one as assist, or have both oil boilers sharing the heating load depending on your oil heating system requirements
  •  They are extremely quiet in operation – a Maximum of 48 dBa

This condensing oil boiler range consists of two models ,firstly one that does heating only .This unit can also be coupled up to a domestic hot water cylinder via a 3 port valve , should you require larger volumes of hot water. and in the below chart , are the model numbers and outputs of each condensing oil boiler, it the heat only configuration. Please note all of these smaller condensing oil boiler / Commercial oil boilers up to 68kW are designed to burn on BS 2869 Part2, Class C2 28 Sec Kerosene only , not Gas Oil / 35 Second Oil.

Oil Condensing Boiler


kW Output

Boiler House Condensing Oil Boiler -Model 1
15 – 20
Boiler House Condensing Oil Boiler -Model 2
20 – 26
Boiler House Condensing Oil Boiler -Model 3
26 – 35
Boiler House Condensing Oil Boiler -Model 4
35 – 41
Boiler House Condensing Oil Boiler -Model 5
41 – 50
Boiler House Condensing Oil Boiler -Model 6
50 – 58
Boiler House Condensing Oil Boiler – Model 7
58 – 68

The other model in the this range of commercial condensing oil boilers, will do heating and domestic hot water production , via a plate heat exchanger, making them a combination oil boiler. Please see the performance of these ” Commercial Oil Condensing Combination Boilers ” versions as they are designated , below . These commercial oil combi or combination boilers are all ErP or Ecodesign Directive compliant, which came into effect in September 2015. All of the boilers are “A” rated Efficiency Classification for Heating, and  “B”  rated Efficiency Classification for Domestic Hot Water production, and have a maximum dBa rating of 48 Decibels, making these oil boilers extremely quiet. The models 1 to 4 have a  ErP Seasonal Efficiency of 92% , and the models 5 to 7 have an ErP Seasonal Efficiency of 90%, making these condensing oil boilers some of the highest efficiency commercial oil boilers on the market. The Domestic Hot Water efficiency ranges from 68% to 78% dependent on the model , and with rated Hot Water Load being XL for the first two models , then XXL for all the other size combi oil boilers.

Commercial Oil Condensing Combi Boiler ( Heating & Hot Water)



Domestic Hot Water  Production   (Lit/h) with a temperature rise of 40C 

Commercial Condensing Oil Combi Boiler -Model 1   15 – 20          110
Commercial Condensing Oil Combi Boiler -Model 2   20 – 26          140
Commercial Condensing Oil Combi Boiler -Model 3   26 – 35          200
Commercial Condensing Oil Combi Boiler -Model 4   35 – 41          200
Commercial Condensing Oil Combi Boiler -Model 5   41 – 50          230
Commercial Condensing Oil Combi Boiler -Model 6   50 – 58          350
Commercial Condensing Oil Combi Boiler – Model 7   58 – 68          600

N.B – Based on flow rates of 17.5 Litres per minute for models 1 to 4 inclusive and 27 Litres per minute on models 5,6 and 7. Extra Flow will reduce Hot Water Performance , Output flow has to be matched by input flow.

Blue Flame Low NOx Oil Burner Technology – Mandatory from 1st January 2018  on Condensing Oil Boilers

What is NOx ?

NOx is the term used to describe Nitrous oxide which is released during normal combustion.When combined with water and released into the atmosphere it produces acid rain, and results in significant long term damage to any structure that is repeatedly exposed to it.Besides being a single toxin, NOx combines with other substances and creates smog when it is exposed to sunlight. This creates a major health hazard for people that live in affected areas. NOx penetrates into the lung tissue and can cause permanent damage to lung tissue. Children, the elderly, people with asthma and other breathing difficulties, and people that work outside are especially susceptible to the negative health effects of NOx. The way to reduce that health risk is for companies to employ NOx reduction measures.

The new Low NOx Oil Burner differs to the older type Oil burner, with regards to the  following points:

  • New combustion head design
  • New air disperser design
  • New way of mixing air and fuel (turbulation)
  • Oil pre-heating for better fuel spraying
  • Ultra-violet photo-resistance (New!)
  • New fan for combustion air supply

It’s use, will be obligatory in the European Union from 1st January 2018! *The BLUE-FLAME burners, require heating oil with low sulphur content

Condensing oil Boiler Heat Exchanger 

Flexiheat supply a comprehensive range of Sedbuk A rated high efficiency oil boilers for commercial and industrial heating applications. Culminating from years of experience, on going development and our wealth of technical knowledge the specially developed “Y” section heat exchangers. which are fitted to all Flexiheat oil condensing boilers providing much improved performance and operating efficiency.

The unique shape allows us to start the transfer of heat to water, in the bottom chamber, before it reaches the baffles, as well as cleaner combustion, now that the baffles no longer have to be so tight in the exchanger to meet efficiency.

The unique “Y” section centrally located waterway design allows excellent heat transfer to take place,particularly in the lower combustion chamber. This greatly reduces the pressure build up normally associated with conventionally manufactured boiler types. The “Y” section design allows baffles to be a more open tolerance, providing for better overall running performance, smoother start ups and cleaner combustion. Baffles fitted to non central waterway heat exchangers, normally have to be extremely tight to attain required efficiencies, which can cause pressure build up and increase soot deposit in the boiler.This is one of the key benefits to our commercial oil condensing boiler range.

The additional surface heat transfer area created within the boiler by the “Y” section heat exchanger also allows Flexiheat to supply high KW output oil fired condensing boilers within typical domestic appliance size enclosures. Flexiheat UK is committed to offering the most technically advanced and extensive range of commercial oil fired boilers available.The condensing unit on our condensing oil fired boilers is made from 316 grade stainless steel. This high specification has been achieved through extensive research and testing to ensure our products achieve the highest quality.

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Oil Condensing Boiler, Commercial Oil Boiler up to 68kW – Heat Exchanger Set Configuration

  • Large heat exchanging surface
  • Removable for easy maintenance
  • Can be repaired if a leak occurs
  • Can be cleaned and re installed
  • Condensate drain pipe

Condensing Oil Boiler Flue 

Due to the fact that almost all of the useful energy is recovered from the flue gas, this means that we have a very low temperature of  circa 50C for the flue gases. This means that you can use either , Plastic flue pipe as on our condensing oil fired boilers up to 41 kW or a Stainless Steel balanced flue system for the 41 kW to 68 kW Output models. This stainless steel flue system utilizes an “0” ring seal and clip fixing system which allows all parts to be quickly assembled.

  • Flue gas exit – 120 mm Balanced Flue for the 15 to 41 kW output and 150 mm for the 41 kW to 68 kW commercial oil boiler models
  • The stainless steel low level balanced flue kit is designed to make installation simple with a telescopic system and seals to allow for a perfect fit each time. The kit is compatible with all  accessories and has a maximum extension of 575mm and a minimum of 355mm. The Flue kits are supplied with burner air hose/clips, lubricant, boiler outputs between 15-41kw ( OD diameter =  120mm ) and for outputs between 41-70kw ( OD diameter = 150mm ), a typical flue kit is shown in the below picture
  • The stainless steel high level horizontal flue kit is designed for simple installation with a vertical and horizontal adjustable extensions and seals which allows for on site adjustment to suit individual conditions. The kit is compatible with all typical accessories and can now create an offset system by using a 45 degree elbows  and flue pipe extensions ( again shown in the picture below ) 15-41kw ( OD diameter =  120mm ) and for outputs between 41-70kw ( OD diameter = 150mm )
  • The stainless steel 3000mm vertical flue kit shares the same accessories as the High Level kit and is designed for simple installation with seals to all accessories. By using our 45 degree elbows, extensions and adjustable extensions it is possible to create offsets and therefore versatility from the condensing oil boiler. The kit can be extended to a tested 6 metres and whilst traditionally ending with a terminal rain cap as an alternative a terminal discharge elbow can also be used – 15-41kw ( OD diameter =  120mm ) and for outputs between 41-70kw ( OD diameter = 150mm )
Oil Condensing Boiler Flue Kit Options,Commercial Oil Boilers Flue Kit Options

Oil Condensing Boiler / Commercial Oil Boilers Flue Kit Options – Pictures are indicative

Condensing Oil Boiler Warranty

Heat Exchanger warranty – 5 years
Burner and Controls warranty – 2 years
Electric and electronic components warranty – 2 years

LARGER COMMERCIAL OIL BOILER ( Condensing)- XC-K Range -Power Range – 69 kW to 1,500 kW
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Commercial Oil Boilers / Industrial Oil Boilers

This Commercial oil condensing boiler series, called the Flexiheat XC-K has 13 models covers a output range from 69 kW to 1,500 KW, in order to choose the most suitable boiler for the different requirements of the heating system. These commercial oil boilers can be be fitted with a relevant burner that either uses Kerosene or Gas Oil as the fuel to be burnt. They are two different burners,and your desired oil has to be specified at time of inquiry or order.The” XC-K” range of commercial and industrial oil fired condensing boiler from Flexiheat is suitable for all those applications in which, for installation reasons, it is necessary to use a large water content condensing boiler.The construction fully complies with the requirements stated in EN 303: Part 1.The components of the pressure vessel parts, such as steel plates and pipes, in contact with the smokes, are in stainless steel AISI 316 L and all the other pressure vessel parts are manufactured in certified carbon steel, according to the tables of  Euronorm 25 and Euronorm 28. These commercial oil boilers can be used in a cascade configuration of up to 8 boilers, meaning the highest output available is 12,000 kW or 1.2 Megawatt in cascade.

Efficiency of our Commercial Condensing Oil Boilers –

102% at full load
104% at part load in condensation mode

Commercial Condensing Oil Boiler  – Advantages

Stainless Steel Combustion Chamber-  in stainless steel AISI 316L, entirely water cooled, above the tube bundle, so that the assembly could form a structure apt to favour the thermal exchange and the evacuation of the condensate.

Two Return Connections – allowing both high and low temperature heating circuits

Special “progressive” and Patented  Smoke Pipes in stainless steel AISI 316L with, inside, 6 special sectorial pipes and turbulators in AISI 304, that guarantee a very high heat exchange and resistance to the acidic condensate.Tube bundle slightly inclined towards the smoke chamber in order:
– to let the condensate to be collected over there
– to avoid the acidic wet deposits remain in the pipes
– to clean, thanks to the gravity, the exchange surfaces

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Commercial Oil Fired Boilers – Internal picture showing the stainless steel smoke pipes, perfectly suited to handling the acidic condensate

Central Heating Flow connection placed on the rear upper part and two return connections, for low and high temperature, in the lower part. For models 69 and 100 the Flow and Return connections are towards the back side; for models 250 to 1550 Flow and Return connections are towards the upper side (as shown in the picture)

The high temperature return connection is placed so that it doesn’t interfere with the low temperature return connection

The low temperature return connection is placed on the extension of the lower outer shell to that it can exploit at the maximum the heat recovery

These commercial oil boilers are equipped with a blind cylindrical furnace, in which the central flame of the burner is reversed peripherally towards the front. When the combustion gases have reached the front part, they are sent through the door into the tubes of the third pass to reach the rear flue gas chamber and then the chimney.

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The combustion chamber is always pressurised while the burner is operating, within the power range of the boiler.The chimney must be calculated so that no positive pressure is detected at its base.

– Robust & Quality construction.
– Reliability.
– Energy savings.
– Friendly to the environment

Commercial Oil Condensing Boiler Applications –

As to Standard EN 303-1 the Flexiheat XC-K Commercial Oil Condensing Boilers are intended for water heating (up to 100°C), for central heating installations, as well as for other similar use. The maximum operating pressure allowed is 6 Bar.There is also an option to have a higher working pressure of up to 8 bar if so required.

As well as benefiting from –

– Perfect design.
– Easy maintenance.
– Large heating surface.
– Three passes of the burned gases.
– Uniform thermal charge.
– Long lifespan without problems.

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Commercial Oil Condensing Boilers – Technical Data according to the ErP Directive 2009/125/CE – The models with an Output higher than 400 kW are not covered by the directive

Commercial condensing oil boiler technical data according to the ErP directive 2009/125/CE

Download Flexiheat Brochure for XC-K Oil Boilers

Easy Maintenance

The door of XC-K Commercial Oil  boiler opens to the left or to the right ( as standard we supply the boilers with the hinges on the LH side ), so that there is easy access to the combustion chamber for cleaning and is also equipped with a Pyrex covered hole for easy visual inspection of the flame during operation. The smoke chamber cover is fastened with screws and has a special cleaning opening,which also acts as expansion opening.

Control Panel

Our Commercial oil condensing boilers are accompanied by a control panel, which is equipped with all the necessary instruments and functions.The control panel includes: general switch, safety thermostat, burner thermostat, water pump thermostat, and thermometer and operation lamps.There is also provision for additional installation of all automatic components of a modern boiler facility for automatic function and energy saving.

Insulation Level

These commercial / industrial condensing oil boilers benefit from a heavy insulation of 100mm of fabric finished mineral wool around the boiler body. There is also insulation on the boiler’s door and smoke chamber in order to minimize the thermal radiation losses.The commercial oil boilers  door is constructed in steel with insulation in special super light refractory cement which is able to reduce by more than 30% the heat losses by radiation.


Our commercial and industrial oil boilers operate with heating oil, in accordance to DIN 51603,or can also be fitted with burners such as natural gas, or liquid gas according to specifications to DIN 4788.

Specifications / Standards of our Commercial Oil Boilers

Our boilers are manufactured following the German specification DIN 4702. The manufacturing process is based on the European Norm DIN EN ISO 9001, as to the equipment, the manufacture and the control of sub assemblies, as well as to specialized personnel and the design. These commerical and industrial condensing oil boilers have the CE approvals and quality mark.

Should you require anymore assistance or information on our Condensing Oil BoilersCommercial Oil Boilers, please don’t hesitate to contact our sales team on 01202 822221 or click here for our email contact form